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In today's live stream, Eric Mele and Bryan talk about dehumidification and reheat on the residential and commercial sides of the HVAC industry.
Recently, Eric diagnosed a commercial RTU that wasn't reaching the set dew point. In the hydronic system that Eric troubleshot, the analog output was flipped; it ran the valve for reheat in reverse. These hydronic systems use waste heat from the refrigeration rack to heat water in the system; that water then circulates to the unit in lieu of refrigerant circulation. Dew point is a critical element of psychrometrics from a moisture/condensation standpoint, especially in commercial spaces.
Air that reaches the dew point temperature will begin to condensate on a nearby surface. Higher airflow can reduce condensation, and condensation may be a slow process, but it will happen after you reach the dew point.
Reheat results in dehumidification, but it's not because we warm the air. Instead, the A/C runs longer without overshooting the setpoint. Longer runtimes improve dehumidification. Overcooling causes condensation, so we don't want to overshoot the setpoint. If you still need to drop the temperature of a space, then reheat probably won't be necessary.
Some common reheat controls are of the modulating variety; these have a variable signal depending on DC voltage or milliamps. You can control fluid within the reheat coil or control the amount of air moving across the reheat coil, though these utilize different strategies; you use a damper to control the air or divert hot gas with a valve.
We have two main dehumidification strategies. We can use mechanical cooling, which is when we use our A/C units or some sort of cold coil to dehumidify the air; in this strategy, the equipment cools the air below the dew point, removes moisture from it, and reheats the air. Another strategy is desiccant dehumidification, which treats the air by removing moisture with a desiccant. A desiccant is a medium that adsorbs moisture from the air.
Your typical stand-alone dehumidifier will have an evaporator and condenser in a reheating cycle. The air leaving the dehumidifier is typically hotter than the air going in, so the dehumidifier increases the sensible heat load. That higher sensible heat load can have a negative impact on your existing HVAC equipment. However, oversized systems can actually benefit from a higher sensible load because the greater load will increase the equipment runtime and pull even more moisture out of the air. But overall, while proper equipment sizing is important for dehumidification, it can only do so much to help with longer runtimes and latent removal.
If you discharge that high sensible heat from the dehumidifier into the return, you will likely derate your equipment's latent capacity. So, you'll be better off discharging the high-sensible dehumidified air into the supply duct, but you have to take static pressure and backdraft possibilities into account.
The best ways to improve your air conditioner's dehumidification efficacy are to keep moisture from entering a space in the first place, have a longer runtime, and use supplementary dehumidification.
Poor dehumidification can result in vents sweating and other moisture problems in the conditioned space. However, sweating may also occur on the ducts in the unconditioned space. If your ducts are sweating, you can either heat the ducts or decrease the dew point of the air in that unconditioned space. You can only control the air in those unconditioned spaces via encapsulation or ventilation.
Humidity in the conditioned space can come from ventilation; the home may pull humid air in from the outdoors. Pulling air into a space can worsen the humidity problem if you pull the air in from an undesirable source. Sealing your home will help, as will using a ventilating dehumidification strategy that filters the fresh air.
If you need to increase humidity, such as in a wine cellar, the best solution is to make sure you run a warmer evaporator coil. The cold coil is the key to dehumidification in compression-refrigeration systems. Evaporator pressure regulators (EPRs) and automatic expansion valves (AEVs) can help regulate the conditions of those evaporator coils.
Eric and Bryan also discuss:
Condensation and defrost on grocery equipment
Building pressure in commercial settings
Balancing pressure in residential spaces
Reducing setpoint for over-dehumidification during favorable conditions
Energy recovery ventilators (ERVs)
Suction pressure changes, compression ratio, and system capacity
Low vs. auto fan usage and dehumidification
Duct leakage
Ductless mini-splits and poor latent load removal
Caveats of overcooling and airflow for dehumidification
Check out information on the 2022 HVACR Training Symposium at
Read all the tech tips, take the quizzes, and find our handy calculators at

We're going to be talking about um, dehumidification reheat, we're going to cover the residential side, the commercial side again, mostly just going to be a conversation. I've got a psychometric program pulled up here. If you want to follow along and actually download an app there's, a lot of different apps that you can use for this munters has an app that you can use the one that i like most is called psycro degree f. I actually don't know if i even have it on my newer phone, because i went to android like a weirdo.

Let's see if i can pull it up cycle. No, i don't even have it anymore, but yeah munters has one and the one i've got is here from greenheck um and it is a computer-based um program application. Whatever you want to call it. So those of you who hate psych psychometrics we're not going to talk about it from like a supermathy standpoint, because nowadays you know you're mostly going to use an app for this sort of thing.

Anyway, um we do have eric melly here say hi to the audience. Eric melly hi, hi uh eric actually just recently had a service call on a reheat system, and i actually wanted to. I think i want to start by talking through that actually um, just kind of use it as a context for it and the video is going to be going live on youtube here. I think tomorrow.

So if you want to watch the video i think it'll be tomorrow, but um just go ahead and walk through that um. That was a seasons 4 unit, so a big rooftop unit, but just to kind of talk through what the complaint was and then what you found and then we can talk through how that actually works. So the com, the complaint about it, was the store. Dew point was not at set point now: they're, not um, referencing humidity in this case, like some of them just, are trying to hit a store, humidity, they're, actually trying to hit a stored dew point to keep the refrigerated fixtures working properly and not condensate all over.

The place okay, so pause. This is what i like about using real service calls to discuss. So we don't think a lot about dewpoint, but we really should you can plot dewpoint on a psychometric chart and we'll and i'll kind of do a little bit of that. A little bit later, but um to eric's point when you're dealing with a supermarket or a convenience store or anything like that, where you've got a lot of refrigerated appliances, cases that sort of thing inside the store, maintaining a dew point makes a lot of sense by The hvc equipment, because you're trying to keep the air to a point that it's not going to begin to condense on surfaces.

So you want to keep that dew point sufficiently down, meaning the temperature at which it will begin to condensate. Because if you allow dew points to float up, then even things that aren't so cold are going to condensate. If you force your dew point down, then you have a little bit more wiggle room as far as how cold you know, doors can be or frames or that sort of thing before they start to condensate right anything. I missed there, that's pretty much it i mean some.

Some controllers will reference dewpoint itself and some will just reference humidity. So most of the refrigerated fixtures i see for indoor use are 75 degrees and 55 humidity or below okay, which is i'm gon na. I'm gon na go ahead and plot this on my little chart here, which you know if you can see yet, because i'm not gon na i'm not gon na bore you with that and uh. Actually i won't.

I won't do that in real time, because now i'm going to uh now i'm going to really ruin our our show here, but uh go ahead and keep going. I can pull it up if you want, i'm i'm looking at it right now: 55, relative humidity. At 75 57.8 plotting it uh. Is that what you got 57.8? I've got 75 degrees and 55 humidity, but i'm using an app.

So who knows how uh it could could just yeah yeah what i'm looking it's much yeah it's much harder: okay, yeah there! It is there, it is yep yep there you go, that is correct, so dew point is 55 degrees. This is that what you said, 57.8 57. temperature is 75 and my humidity is 55.. Oh 55, there it is yeah.

I was that i was at 50.. Okay, that makes sense yes, 57.8. You are correct. So that means that anything that gets to that 57.8 point or more specifically the air around it gets to 57.8.

So it's not just the temperature of the surface, but but the air actually around it you're going to start getting condensation. So as soon as you get a surface that is at that level or lower you're in the danger zone. Now it's not always going to condensate instantaneously, like i said, if you have higher airflow over it, that's going to reduce condensation because that's going to um kind of disallow, the air from getting to that dew point temperature. A lot of people get confused between the fact that it's the they think that it's the surface temperature that causes the uh the condensation, but actually it's the air temperature getting to dew point that causes the condensation anyway.

So keep going through your diagnosis. Now, before i interrupt you so once i got down to it, the main thing i found was um. The analog output was flipped, so it was running the the valve for reheat in reverse, and this was a hydronic reheat coil. So it had a three-way mixing valve um on it, if i remember the orientation correctly, and that was a problem so when we say hydronic reheat for people who aren't familiar with what that means.

What is what is that? It's basically water, in this case water with a little bit of solution in it to prevent it from freezing. Although that's kind of ridiculous with where it's located but they're, using um using heat from the refrigeration rack to put that heat into the water and circulate that to the unit, instead of doing it with with refrigerant, which is probably a better way, because sometimes when they Do it with just straight up refrigerant from the rack, and it goes in reheat it really screws with the liquid levels in the rack now. Are they also using um that uh that excess heat from the rack for the domestic hot water for the store, as well, yeah? Okay, which is pretty typical when you have that configuration so they're, going to use it in tanks for domestic hot water and then they're? Also going to use it for reheat um, so as we go into this, i think a lot of people miss why reheat results in dehumidification. So what about heating? The air makes the store humidity, or in this case the stored dew point go down.

Well, it's actually not really about heating, the air that does it and i think that's what confuses people. People think that warming, the air up is dehumidifying the air. What you're allowing to happen is your air conditioning to run longer without overshooting the set point, because you could just keep running the ac until it gets colder and colder? I mean you're gon na have other consequences of doing that mostly uncomfortable people, but then you know, if you keep going from there, bad things will start to happen too, but essentially you're. Just warming, the air back up to not overshoot your comfort, cooling, set point and just maintain run time on your unit, so your evaporator can keep doing its thing and dehumidifying yeah.

I also want to address a common misconception and there's a common misconception that over cooling is only bad because the occupants don't like it, so a lot of people think well. The reason we don't want to over cool is because people get uncomfortable when it gets too cold and obviously that's one reason. But another reason is you: don't want to over cool a space for the very same thing that we're trying to prevent here, you don't want to overcool the space, because if you start over cooling the space too much then you're going to start getting condensation anywhere. That space communicates with the doors or the ceiling space or anywhere where the air is of a higher dew point.

So you're going to start getting sweating in walls, you're going to start getting sweating and drop ceilings. All kinds of you know: duct work is going to start sweating. You can run into all sorts of consequences of running a space um too cold, meaning a space. That's not designed to be run cold.

The reason why we can run a freezer or a cooler, so low temperatures, because it's so well insulated and so well sealed by design, but a something like a grocery store or a house or a commercial building. You don't want to run it too cold. I mean again, if you're in a super dry or arid type of desert environment, it probably doesn't matter but in markets like ours, where anywhere that there's connection in between that condition, space and the outdoors or anywhere that there's even a conduction between those two spaces. If you run a space too cold, you can start to get sweating and then the corresponding growth that we want to avoid.

So, in summary, we don't want to over cool the space because it's occupants don't like it, but we also don't want to overcool a space because we can cause other condensation issues. Does that sound right? You look very blank-faced about this. That's pretty much! That's pretty much! It right there that sums it up. Okay, so we don't want to over cool a space and we want to have sufficient run time.

So it needs to run long enough with the air going over that evaporative coil long enough, that it can drop the moisture out of the air and the reason that that happens. The reason that we pull moisture out of the air on a cold coil is because, presumably or at least by design, that cold evaporative coil is going to be below the dew point of the air traveling over it, and so, as that air travels over that coil. It gives up its heat to that evaporator coil, the air itself reaches dew point and then it begins to essentially rain out or give up its water to the cold surface of that evaporator coil. That water runs down the drain uh it goes outside and then we reheat it again so that way we're not over cooling the space.

Now there are circumstances in which you wouldn't reheat, and that would be cases where you need cooling. So if you need to drop the temperature in the space, then you you may not reheat, but if you are at temperature or near temperature, the you know algorithm and the control decides okay. We need to start to bring some of this reheat capacity into place and in this particular case, this actually had some modulation to that right i mean it wasn't a strictly on or off reheat was it yeah? This was a modulating setup, so they could. You know keep it to where they would have pretty much neutral air there's also multiple steps of cooling on the unit.

So you could you know, however, the control algorithm is exactly set up. You could keep the air neutral most of the ones i've seen set up. They do like you were saying they do prioritize comfort or not comfort, like sensible cooling over over reheat, but you could, you know, have it set up either way depending on what's more important to you, yeah and in the case of something like a grocery store, where We already mentioned where you have these cold appliances, these cold devices inside the space. Now we have another reason why we want to maintain humidity levels more than just you know, comfort or trying to prevent growth uh.

We also want to keep things from sweating and keeping things from sweating is helpful, because, obviously you don't want slip and falls, but also you don't want things to rot out. I mean we just don't want water running down inside the space, so it becomes really imperative and these sorts of structures that we control are either humidity or dew point. However, you want to track it, but we ultimately for that purpose. We're tracking dew point when we think in terms of human comfort, we often say relative humidity, but in terms of like keeping things from sweating, it's more about dew point.

It's all about dew point. It also affects you know open air cases. More than more than closed cases, because that additional latent load can cause coils to ice up faster, it might even affect uh cooling yeah, the actual ability to do the job right. Good point: yeah: if you have an open case where that case, air is communicating with the space air hey.

I just made a rhyme um it matters to control the latent load and by layton we just mean how much energy is going to go towards changing water. Vapor in the air to liquid water in the drain pan in terms of refrigeration. Obviously, in certain cases we don't want the case or box um humidity to get too low depending on the product, that's in it, but we certainly don't want the evaporator coil to have to deal with excessive latent loads, for the reasons that eric mentioned um with defrost. Being a huge part of that, we, you know the more moisture we expose that evaporative coil to the more defrost we're going to have to do in defrost is is a huge problem in terms of product.

Defrost isn't good for product, but defrost also isn't good for energy consumption. So another reason why, in in these sorts of cases, we want to control um the dew point of the of the ambient air in the space. It is actually - and this is actually kind of a an interesting thing for grocery or market refrigeration technicians. They tend to focus a lot on the refrigeration equipment, but making sure that the air conditioning equipment is working properly, especially in a high latent market like ours is a really big part of that um.

So being able to assess is the equipment working properly? Specifically? Is the dehumidification or reheat portion of the equipment working properly is um is actually kind of a big deal so anyway i'll let you continue with your diagnosis, eric what you found in this particular case. Well, that pretty much you know, sums up the call of why it was a problem. It was acting erratically because the 1 to 10 or 2 to 10 dc signal was interacting. With that modulating valve the opposite way, it should have been okay, so i mean so it was.

It was a programming it was. It was a programming main programming issue with that, okay got it. So that means that when it had the highest demand, it would actually close the valve uh or you know, disallow um, reheat and when it didn't need it, it was allowing more reheat or having more heat, got it and it would get there slowly because it would Modulate - and you know have no effect, so it would actually keep closing down the reheat as it's trying to hit that set point and then doing the opposite when it's not trying to reheat got it and on the topic of commercial kind of reheat and reheat controls. I think it would be helpful because some people, when they hear about you, know these dc signals and all that they don't they're not familiar with that.

So just give us a quick overview of are the most typical control strategies that you see and and how those how those work and because it really isn't that complicated. But if you've never heard about it before it may seem a little foreign. If you have any sort of modulating reheat where the valve can be or damper can be, you know somewhere between zero and a hundred percent. You'll have some sort of variable signal, usually it's two to ten dc, one to five dc or four to twenty milliamps.

So your actuator will get that signal and interpret it into a valve position. Yeah. So at one end, it'll be zero percent and the other end will be 100 of what that valve is allowed to travel right, yeah and go ahead, and i was going to say you can do it with an actual valve in the fluid and control the flow Of the fluid to the reheat coil or you can do it with a damper and control the airflow across the reheat coil yeah. We'll talk about talk about that a little bit because we actually did a podcast about this.

A while, a while ago, some of the different strategies that that you commonly see - and this is you know specifically to like generally going to be rooftop equipment, rooftop units that have it uh built in there - is only it's only a couple. Companies doing it with a damper, morganizer and train is now doing it on their equipment. I don't know if they just took it over or what i've seen them use that strategy on splits as well as rooftop units, but most of the other strategies are just using hot gas and diverting it from the condenser to a reheat coil. That is downstream of the evaporator, so that's one key thing: you'll see in the equipment is you're heating, the air up after it comes off the evaporator, because you want to be able to you don't want to affect your load on your evaporator and its ability to Dehumidify so you're warming, the air up after it passes over the evaporator yeah, and you also want to have sufficient um even space between the coils.

I mean this is all an engineering thing, but you really don't want to affect the operation of the evaporator coil with any sort of reheater dehumidification strategy. And we talked about residential i'll talk about that but um. But that is a mistake that you can make and you'll see that people will design systems where you attempt to dehumidify or reheat prior to the evaporator coil and that just derates the equipment makes it the equipment not work as well. Pretty much and carrier has an interesting humidimizer unit where they can either divert hot gas or liquid refrigerant to their reheat coil.

So you can either have some warming back up or a lot of warming back up of the air. I guess it would depend on conditions and how much cooling you're running it's usually on a larger unit. That's multiple cooling speeds and they have variable speed, condenser fans, so um a lot of the other ones like resner and aeon. I've either done the on off.

With a vat with a diverting valve or a modulating hot gas valve yeah, so when we say hot gas, we're saying either you know right out of the compressor or somewhere in the condenser. When we talk about liquid, there is an advantage to using liquid for reheat. If you can do it because now you're simultaneously using you know using the energy, that's still in your liquid in order to raise your air temperature but you're also acting as a sub cooler. So because that air is coming off your evaporator coil, it can actually drop your liquid sub sub cooling or increase your liquid sub cooling.

I should say um significantly more than you can just through a normal, a normal type of strategy. So there is some advantage if you can just use the additional heat off of liquid and sub cool it. You can get some efficiency out of that munters. They are more like a ventilating dehumidifier commercial unit from the ones i've seen those those ones, at least the ones i've seen taking a lot of outside air and have like powered exhaust and stuff on them yeah.

But that's for a specific customer. So that's why i've been seeing like the same one yeah. Well, a lot of those are custom. Builds, you know, so they have a lot of um.

You know you can add in the ventilating dehumidifier you can add in traditional reheat, and this is a this - is a big distinction to make a lot of people confuse reheat in terms of um reheating for dehumidification with you know, ventilating, uh, dehumidification or utilizing something like An erv or hrv setup or like a big enthalpy wheel. You know those of you who work on big rtu's you've seen these before, where it has this gigantic rotating wheel with, like a like a you know, almost a paper media on the inside of it and there's a transfer of energy between the two airstreams, the airstream. That's being discharged and the air that's being brought in so that way you don't have as much energy wasted kind of a different kind of a different strategy, and then you can use um, you know excess heat or gas in order to you know re-energize or to Drive some of that moisture out of the out of the wheel, so there's again a lot of different strategies, um out there for this type of design. That can you know achieve dehumidification, but it just depends on you know what exactly you're trying to accomplish.

Are you just trying to run the equipment longer? That's a typical strategy that we're going to see in florida with this type of reheat that we're talking about here, because we need to get additional dehumidification. We need to run the system longer, even beyond what the comfort, cooling or sensible requirement is when we say sensible. That just means the need that you have to to cool the space um to actually change the temperature in the space. When we say layton, we mean then the additional energy that it takes to change it from water vapor in the air to liquid water in the drain pan.

So anyway, isn't it a good also to you know, keep your building positively pressured. You know you people coming in and out: it's not gon na you're not pulling in outside air as much yeah in a commercial strategy. Um, that's going to be typical to try to keep a little bit of positive pressure um in a and again this depends on the market. Um.

You will have cases where uh in like northern markets, for example um. They don't want positive, pressurization, um they're, really just looking for balance pressurization uh, because uh you can get condensation in the outer um layers of the home structure. So it really just depends on. You know where, where you're worried about condensation and whether you're in a a cold climate, where it's more likely for you to get condensation on the outside of the structure or whether you're in a warm climate, where it's more likely to get condensation on the inside layers Of the structure, but in terms of like just preventing outside air, from from coming in the building from a humidity standpoint, it is better to condition and dehumidify the air and then have slightly positive pressure and again, the only way you can slightly positively pressurize.

The structure is by bringing in outdoor air and the only way that that's any better than having it come through. The doors is, if you're, bringing in that outdoor air and you're treating it in some way, whether it's through you know, dehumidification whether that's a desiccant type dehumidifier, which we talked about a little bit, there's the desiccant style wheels as well um. That was what i was talking about, um before when you're dealing with that that type of strategy, or whether you're um just processing it through through typical um. You know mechanical style refrigeration in order to dehumidify, so either way.

The only way that that's that strategy of positive pressurization is better is if you're treating it in some way, yeah and now, if you think about things like balancing, when you have uh kitchen equipment or hoods, that's a whole different animal um. You know traditionally, the old school way of doing that would be. You would have a you'd have a hood that would exhaust air and it's a crap ton of air, and then you would have them. That's it.

That's a technical measurement, one crap ton and then you'd have makeup air that would bring in a balanced amount of air or slightly greater um than what's being exhausted, and that would just balance the pressures in the building. So that way, you didn't get cases where you couldn't, you know open the door or you know those extreme pressure imbalances, but that alone didn't deal with the latent issues. It didn't deal with the condensation and so in our market. That's why you know.

Historically, there would be all these issues with condensation in kitchens and all this you might have balance pressures, but that doesn't mean that you're conditioning that outdoor air, that you're bringing in through makeup nowadays with modern codes, they're actually requiring us more and more to treat that Outdoor air, as we bring it in and um and that helps to uh, you know to keep the light and load under control and control dewpoint so that we're not getting condensation on surfaces as well as all the other things that we talked about. Comfort and everything else, but to that point, though, if you're noticing, when you enter a structure, that there's air moving a lot of air moving one way or the other yeah um that has to be dealt with, because that can quickly start to overcome cooling equipment and Start to cause problems like brian was saying with the kitchens. That's like a kind of like a containment strategy of we're gon na put put unconditioned air in right here and take it out right here, so it doesn't. Hopefully the idea was to not have time to you know, mix in the space with the rest of it, but if one or the other doesn't work right, it'll quickly, overtake the cooling in the space and you'll you'll notice, like doors, will either be hard to open Or they'll you know, come flying open and they'll be big rushes of air, so you know the best way to test that is to have it's kind of hard.

Sometimes, if it's real busy, you got to have somebody like everybody, not open doors and you kind of crack one open and hold a paper towel or some toilet tissue and see if it's getting sucked out or blown in with a lot of force and then try To see why is there an exhaust fan down? Is there a makeup air fan down? You know something like that: that's not good or use a precision manometer. That would be the the more technical way of doing that, but um. Typically speaking, you don't want uh, and this is a kind of a residential rule, but it's it's a good general rule. You don't want more than three pascals of pressure differential between internal zones and so and you'll see this even within in you know, even within a structure, let alone between the structure and outside.

You don't want to see these huge uh pressure variances for the reason that that eric just said and i'll give you a perfect example of this. I've talked about this before so those of you who've heard this story before sorry to bore you but um. There's an indian restaurant in claremont that i like to go to quite often in fact, eric lives literally a stone's throw away from this place, and we would go in there and i would notice a lot of odors like like strong bathroom nasty type odors and, and You could see they were trying to overcome it because they have like all these glade plug-ins all over the place, and it wasn't always that way, but it would definitely get strong, sometimes enough, so that you would smell it in the dining room. But then, when you went back towards the kitchen and towards the bathrooms, it would be like almost overwhelming, and it tells you how good the food was because i kept going back.

But i noticed one time. I noticed it and i was just paying attention and i noticed that the door was really hard to open, so i'm gon na pose this to you eric. I don't know if you've heard me tell this story before what do you think was going on uh? In that case, the door opens out uh. The door opens out yes, and it was very hard very hard to open, probably make up air was down.

There was at least an imbalance, and so the odor was being caused. By do you want to take a stab at what that was being caused by, because i didn't think about it at first, i couldn't really tell drawing odor through like floor drains and stuff yep. That's what it was so it was dried out floor drains and it was drawing odors out of those out of those floor drains and as soon as i so i ended up because i was like kind of embarrassed like i know the owner and we talked, we Talked in the past, so i ended up sending him an anonymous facebook message and uh it just kind of telling him hey look. You know i'm in the business and you may want to get your hood folks out, and i think that would take care of some of your problems and sure enough.

He responded that they did and um and it fixed it. So so whether it was the makeup air being completely down or the makeup air just having something wrong with it, i mean who knows, maybe the belt's really loose, or you know somebody you know adjusted the um, the shiv on it wrong or who knows you know What it could be um, probably not that, but you know something was causing there to be a significant amount, more air being exhausted out of the structure than being pulled in and that was causing odors to be pulled through floor drains. So it's just another reason why we want to pay attention to um zonal, zonal imbalances within a structure. We don't want there to be a significant pressure imbalance within a structure.

That's where you get cases where you know, we've all experienced it. We have a bedroom or something that doesn't get proper air flow because there's not enough return in the space positively pressurizes um, but even more extreme in commercial cases where you have these gigantic fans that are moving air around um definitely have to pay attention to that. Let alone this is a whole nother factor. That's universal residential and commercial um.

The sealing of the structure to the outdoors is also a really big factor: um that affects uh the amount of humidity. So that's not a dehumidification question. It's more uh. How much communication is there between the inside space and the outdoors, and that's why we do tests like blower door tests to see is the space appropriately sealed and where are the gaps and cracks or the pressure? You know forces that are driving moisture from the outside into the space in the first place, so in many cases dealing with a humidity problem isn't necessarily throwing more equipment added or more dehumidification.

It can be a matter of just keeping on the outside outside where it belongs and you'll see this. If you work in a market that has a lot of attics you'll, see around can lights or around the boots, you'll see dust or growth around the edges and when you pull them down, you'll see those gaps and cracks through the attic and that's where air is Escaping in and it's bringing you know, dust with it, but it also can condensate there as well, because you have very high dew point air in attics. If you've ever measured the dew point of aeronautics, i mean it can be 80 85 90 degrees. In some cases it can be the dew point of attic air.

So when that air comes in around those cracks it immediately, condensates condensates gives up some of its moisture as it achieves dew point. So those are a lot of reasons why you want to control pressure imbalances and you also want to control space sealing in order to maintain dehumidification appropriately any thoughts eric none whatsoever, wow wow. Why did i bring you on as a guest? You have no thoughts. That's your number one rule of having a guest is: is thoughts of the guests, um, all right, so uh.

Let's talk about a little. Let's talk a little bit because somebody mentioned economizers um. That's another thing that folks get confused. Sometimes we've talked about a lot of different things.

Here. We've talked about desiccant dehumidification, we've talked about um, you know bringing in fresh air through some sort of ventilation strategy, uh, whether it be energy recovery or heat recovery ventilation. We've talked about reheat, but an economizer is really a totally different thing. Do you want to take that at all? We don't have a lot of those in our market.

I mean we have them, but they don't really do a lot. They're smoked on jobs, probably because it's an architect that draws them all over the country and just like yeah, throw an economizer, essentially you're you're, when conditions are right, outdoors you're going to bring in air from outside mostly would be during shoulder seasons, so you don't have To run as much cooling or heating or whatever it is so you're gon na try to bring in some outside air to save some money on cooling or heating, mostly cooling. It's going to be it's going to typically be cooling yeah, because it's going to be conditions in which the outdoor air bringing in outdoor air becomes stage, 1 cooling and so you're going to have to have significant indoor heat gains. So this would be primarily a commercial thing where you're generating a reasonable amount of heat inside the space, and so now the outdoor air conditions, um.

The humidity and temperature is to a point that it makes sense to bring in outdoor air as stage one of uh stage, one of cooling, but a lot of them, though they just throw the economizer on, but they really don't have all the other stuff. They need to deal with it like a powered exhaust of some sort to try to you know not affect space pressure. So, ideally, you want powered exhaust and you're going to get rid of your return air to the outside and you're going to bring in outdoor air. In because, at that point, your outdoor air is better than your return air, so you know a lot of them.

I see around here is just like here's, an economizer accessory, but they're they're, only using it as a motorized damper, really they're, not even using it. For the built-in onboard controls that are on it, that can do all the other stuff yeah i mean, and that's so any all of this stuff that we're talking about has to go with an appropriate control strategy. That's dealing with pressure, that's dealing with um, something that decides whether it's modulating or on and off, but i think it decides appropriately when this when it's needed, which is why i like, in terms of the ventilation side of things, it's why? I really like demand ventilation where you're tracking co2, uh carbon dioxide or and or vocs in a space, and you are bringing in outdoor air in order to dilute that when that's a when that's a problem rather than just bringing in outdoor air all the time. That's especially a florida thing, because we really don't like our outdoor air very much.

It's not it's, not the greatest outdoor air. It's very unfriendly! You know it's not! It's not good. At parties uh, it's awkward, uh lots of things to dislike about our outdoor air. It's like soup, you go outside and you are breathing in the soup.

It is. It is yes, yes, indeed um. So anyway uh i don't know where i don't even know where i was going, because i just said so many so many disparaging comments about our outdoor air but um, but having control strategies that that actually look at that. So that way we're only bringing in outdoor air when there's actually a compelling reason to dilute the indoor conditions when you're talking about the dehumidification side.

That's where something like dew point makes a lot of sense, where the primary concern is making sure that you're not going to have condensation on the apparatus or you're not going to have conditions that adversely affect the internal machines or apparatus. That's where dew point makes the most sense, if you're doing it dehumidification for the occupants and for growth and that sort of thing then relative humidity may make more sense. Ultimately, all of these things have correlation. So if you keep your dew point, low you're, also keeping your relative humidity low.

So long as your you know, space conditions are still under reasonable, sensible, um type of conditions and again humans are only comfortable under a pretty small bandwidth of uh of temperatures. So you know whether we're doing it based on if we're talking about a space that has you know humans in it um or whether you're doing it on dew point or relative humidity. There's not going to be a huge, a huge variance there anyway, rich says: florida's water stinks more than the air um. That does.

That is true, specifically where i live. That's not true everywhere, but specifically where i live um we have very stinky water, and so you will not want to visit us rich um. Many many many many different uh things to keep our water from being stinky hey cory. Is there a reason you unmuted yourself, did you have something to add, or did you just want to make a bunch of noise? Am i unmuted right now? Oh i've been playing with my new smart watch and i was trying to reply to a text.

Okay, somebody's a rich kid. Somebody got a new watch yeah, it's it's actually a fitbit, it's not even a smart watch. I just wanted to sound rich yeah. I wanted to sound rich yeah, never mind poro, all right, so uh anything else.

We want to cover on the uh commercial dehumidification side, um we're talking about reheat you're, going to use heat either fro waste heat from somewhere. It can be internal to the equipment, so you can actually take waste heat off of your compressor discharge. It can be external refrigeration system that you're taking discharge gas or liquid directly out of the refrigeration equipment, or you can utilize a secondary fluid like in the case of a water system that you worked on a few days ago, and then you have modulating systems that You know modulate the amount of reheat that you're getting or you have systems that are just fully on fully off, and then we also have systems that do it. Based on modulation of air flow over the coil, rather than the amount of uh flow through the coil yep um, one thing i wanted to add: i'm not sure how this fits in it's kind of a service tip, but if you're, if your reheat equipment's, not keeping Up because normally what happens, this is the scenario i ran into quite a few times is, since sensible is priority in the heat of the day, it would just be doing sensible cooling for long periods, so you can lower your set point when conditions are more favorable.

So, in this case, they wanted to maintain like 55 humidity, but i lowered that set point to 45. That way, you know when conditions were favorable it over dehumidified the space to some extent to help it keep up in the afternoons when dehumidification would not be enabled, because sensible would be prioritized. So it's just something to keep in mind. Is you know if you're not keeping up when the space is occupied during high load? You could try to get your humidity down within reason when you're, not at peak load or when you're unoccupied yeah, and i would also say, in those sorts of conditions, look at any other possible causes for high moisture load.

So you know i've seen everything all kinds of crazy stuff things where you have you know: adapter curbs or roof curbs that are enormously leaking to the outside of the building and there's lots of things that can cause your humidity to be high. So as much as possible, try to assess what those are look at the ventilation. Look at those pressure imbalances, anything that could be causing a significant moisture driver and then, beyond that, i get your point um. You know you can drive the set point down and then it will um it over to humidify in order to keep up.

I've had an extremely undersized return on a unit as well. That was just pulling in way too much outside air, even though the the damper was in its quote, fully closed position, because even when they're fully closed, they're still usually significant air gap, and somebody had added too many splits to deal with the effects that were basically All caused by the undersized return, and it still wasn't enough so once i sealed up the outside air better, then this problem went away yeah and that's because high static pressure on the return. When you have high static pressure on the return to an undersized duct return, make more sucky, that's the technical term and suck in more outdoor air, so undesirable. In this case, and again, a lot of people in the messages are saying, like you know like well, engineers are the ones who design for this.

It's true. They do, but also just keep in mind that a lot of times um when things get specified, especially on um structures that are just using a lot of kind of like stock things, they're just kind of throwing stuff at it. Uh, it's up to us to still commission it set it up properly and actually look at how it operates once it gets up and running and making sure that it is maintaining what it's supposed to maintain um. You can't just trust that an engineer's gon na design it properly got it.

We'll just tell the customer that that the engineer come out and fix it and we'll charge them for a service call and we'll leave and we'll see if that customer ever calls us back again, yeah uh. One of the questions somebody said is: can the heater add latent heat? No, the heater a heater only adds sensible heat. So when we run the air over an evaporator coil, that's below dew point, we reduce the latent heat of the air by absorbing that latent heat into the evaporator coil. It goes into the evaporator coil because the energy is used in order to change it from water.

Vapor in the air to liquid water in the drain, pan there's an energy exchange there, and so that heat ends up in the refrigerant, but it's latent heat, because it's a change of state. Once we get past that evaporative coil and now we reheat it. That's all sensible so that that coil, regardless of the medium in the coil when you're adding heat to something, that's always going to be sensible heat so unless, unless you are actually doing a phase change or something in a fluid. But in this case it's air and we're just adding sensible heat to it unless you've got a leaking hydronic coil right right.

If you have a leaking water coil and it's squirting water all in it, then yes, then you may be adding latent heat, but then but then you have a different problem on your hands. I think so good stuff uh all right. So i want to talk a little bit more if we've covered the the commercial side and again, this is obviously just a summary glance at it. Um i wanted to talk a little bit more about the residential side, because there are some common mistakes that are made on the residential side.

It's all the same basic idea um, but there there are some um things that we found and folks, like ultra air talk. A lot about um, all of it is the same. You have two basic strategies for for dehumidifying i mean there really are basically two one is some form of cooling. The air via mechanical means generally below dew, point pulling moisture out of it and then reheating it.

That's a standard way that almost every dehumidifier residential grade humidifier that you get out there is going to do it. Then you have a desiccant dehumidification, which actually uses um a media that adsorbs moisture out of the air. So it's sort of like you, know those effort. Those damp raid packets that you put into your closet, you know where the little the little packets that absorb moisture that are in, like basically everything that you buy like your shoes or whatever they actually use a um.

You know chemical reaction that adsorbs or pulls that moisture out of the air, and then we use in the case of a regenerating system. We use heat to desorb it again and to get that moisture back out and to drive it outside. So that would be a uh all of a sudden. My mind just went completely blank a desiccant dehumidifier and those are the two standard ways that you dehumidify either mechanical means using the cold coil, whether you're doing it uh.

You know a direct expansion of aperture coil or some sort of other cold medium um, that's using a chiller or whatever you're, using a cold coil and then you're using a hot coil. So we call uh cold deck hot deck. So you have a and you could even just use it for heat. So if you shut off the cold coil and you just use the hot coil, you could just use it for heat if you're, using both at the same time, it's always going through the evaporator coil first or the cold coil.

And then it's going through the hot coil michael housh has done some pretty groovy things where he's designed systems that are, that use a hot deck, cold deck type system, using a chiller and a buffer tank for hot water and cold water. And then you can use that as the medium, so it doesn't have to always be an evaporative coil in the equipment. But you got to have something that gets cold below dew point and something that gets hot and then desiccant dehumidification uses chemical reaction and ads orbs desorbs using heat. So those are your two different typical strategies for what we're doing um the most common would be a super simple setup where you have an evaporator, coil and then a condenser right after it.

That's what your typical dehumidifier is going to be your ultra air, your um sanitary, i'm, sorry, it's santa fe now stand up. Ultra ultra series by santa fe is the new. The new name. Your april airs those sorts of brands.

Those are all going to use that basic um strategy, so the air goes over the evaporator coil back over the condenser, so it cools it drops the moisture out, reheats it so that it doesn't overcool and almost all of them. The air coming out of the dehumidifier is going to be significantly warmer than the air going in the dehumidifier, which means that not only does the dehumidifier pull out moisture, but it also adds load to the space. So what does that mean for a dehumidifier? What does that mean? If you take a house, let's say your house eric and you take a dehumidifier where the air coming out of it is hotter than the air going in it, and you put it and just slam it in your house. What's the result going to be from a sensible standpoint, you're increasing your sensible load, which potentially could do what be adverse on how your equipment runs or locally right next to the dehumidifier.

If you're talking a standalone - and you put it in a corner locally around - it is probably not going to be comfortable exactly so, it can affect it can affect the loads. It can affect the sensible loads of the equipment. So if you have a house, that's barely keeping up in the summer and you say well yeah, but it's also not dehumidifying. Well, we have a moisture problem and you slam a dehumidifier in it, whether it's one that you stick in the corner or whether it's a santa fe and you you know discharge that into the equipment.

You run the risk of it, not cooling the house, even worse than it did before, because from a sensible standpoint that dehumidifier is adding sensible heat to the equipment. Now there are systems, specifically the only one i know of is the santa fe ultra series sd-12, which is the one i put in my dad's house and that actually has a separate condenser. It looks like a ductless condenser and then it has a separate air handler on the inside and it can control how much of the heat is being used for reheat, and so that way it actually is sensible, balanced or actually cooling. Slightly generally, it's going to actually cool slightly, so it's still going to add some sensible cooling or it's going to have some sensible cooling effect, but it's going to have far more latent because it's still discharging some of that heat to the outside.

So in cases where you're really close on your loads - and you don't want to throw more sensible in it - and you don't want to you - know - resize your ac equipment, then that would be a good solution now. Luckily, most houses actually have oversized equipment, and so adding additional sensible load is actually a good thing, because if you take a device that is pulling latent out, meaning it's pulling moisture out of the house, it's dropping the dew point. It's dropping the relative humidity. It's doing all that stuff right.

You stick that in the house and then in addition to that, it's blowing hot air. That's also going to increase the sensible load which is going to do what to equipment run time increase it increase it. Sorry, i'm just asking you questions, sometimes because i get bored of talking to myself so much it's going to increase it, which then causes the equipment to also pull more humidity out. But there's a problem and i'm going to ask this of one of the apprentices.

So chad, jessica or corey, i want one of the three of you to answer this okay, so i want you to imagine you're taking a dehumidifier, it's pulling air in it's pulling some moisture out of it, but then it's also adding sensible heat. So there's more heat coming out of it and you take that and you dump it into the return of your air conditioner. So now going into the return of your air conditioner is drier, hotter air. What does that do to your equipment's performance, you're, going to decrease the system performance yeah? I was going to say yeah you're, going to you're, going to potentially increase its sensible capacity because now you're giving it more heat to grab but you're definitely going to derate its latent capacity, because now you're putting hotter drier air into the return.

So you're taking a piece of equipment, that's already been functioning as a dehumidifier, granted, not a perfect one. We'll talk about this in a second but but now you're making it operate worse as a dehumidifier by putting a dehumidifier in. So that's not an ideal strategy. Taking a dehumidifier and dumping hot, warm or whatever dry air into the return is not a good strategy, because you de-rate that evaporative coil's ability to function and again it goes back to the same strategy right when we want to reheat.

We run the air over the evaporator coil and then over the condenser. We don't heat air before we put it over the evaporator coil. If our intent is to dehumidify, that's not the best strategy. You may still get some improvement, but you're not going to get the maximum improvement.

Now answer me this brian: do you think it would be hack or genius to discharge that dehumidifier air into the blower compartment of the air handler so you're now downstream of the evaporator? But you're not in the supply duct, so you're still in a negative pressure yeah that would probably that would be fine. I mean you'd have to do some. I don't think i've ever seen. Anybody do it cut a whole side of the unit, no yeah.

I could just imagine somebody else walking up on it and being like what the actual heck is this well and that's and that's that's sort of a fan. Coil specific strategy too, like you know, obviously that wouldn't work on a gas furnace, because the evaporator coils above the blower, but if it was the fan coil, you know air handler setup yeah. You could certainly do that and that would work just fine. It would work better, actually um the the standard strategy that uh the you know, ultra air santa phase of the world recommend is that you discharge out of your dehumidifier into the supply duct and then, if your blower is running, then that's going to help circulate it.

But but you have to then recognize and design for the fact that your dehumidifier is pushing against significant static pressure when the system's running, and then you also want to have a backdraft damper. So that way, if either or isn't running, it's not going to just re-circulate the other direction. So that's a you know, that's another another thing you got to deal with, but that is the ideal and recommended method when you're using a separate standalone dehumidifier and even when you know you're using a portable, just think about the fact that you're going to add sensible Heat to that room, and is that something you want to do you, may you may want to add sensible heat so that way the equipment equipment runs more. It may not have an adverse situation, but you'd want to be very careful about doing that in a space like a bedroom.

You know where that dehumidifier is going to run at night. It's going to potentially be noisy, and then it's also going to make it uncomfortable for the occupant. So you just think you know think very carefully about that before you before you add it into it. Just discharge that air right onto the thermostat, the ultimate for the ultimate run time: yes, yes, maximum run time.

Equipment runs like crazy and you get the best of both worlds because again, in order to dehumidify well and we're talking about just the equipment side here. In order to dehumidify well, you need a cold evaporator coil and you need a lot of runtime you're not going to dehumidify. Well, if your evaporator coil is not cold, lower temperature, the better without freezing and you're not going to get good dehumidification. If you don't run a lot like a lower blower speed, well, okay, that's a good question.

I like this question. This is actually one of the areas i wanted to go to. So, let's think about what happens. Actually, this is a really good thought experiment.

So let's do this, i'm going to take this all the way. Okay, so you take a piece of equipment and where it doesn't matter the time it doesn't matter, the the cfm we're just going to say you lower it. So we go from 400 cfm per ton down to 300 cfm per ton. Okay, what happens to the equipment? Let's go through it one at a time what happens to our suction pressure, lower the airflow blower airflow? What happens to your suction pressure lower goes lower right.

What happens to your evaporator temperature gets colder. It's colder right. What happens to your head pressure? Yeah it'd go lower, yeah and not necessarily as much because it because you know you kind of have this limiting factor of outdoor air temperature, but it would. It would generally go lower.

That's what you're going to see if you re reduce airflow. Do it sometimes pull the cover off of an air handler and run it um and see what happens to your to your head pressure? It's going to go down right um, so that means that your compression ratio, uh with lower airflow, is going to tend to actually go up a little bit and that's just because of the relationship between a head and suction. When you, even though they're both dropping suction changes in suction, actually impact compression ratio a little bit more, which means that your system capacity is going to drop a little bit when you run lower airflow, your system capacity is going to drop a little bit total system Capacity right so so that's and again, there's actually some that isn't actually always the case. It does depend on apparatus dew point a few things, but typically that's what's going to happen right, okay, so we've decreased our system capacity a little bit we're running a lower evaporator coil temperature right.

What's going to happen to our discharge, air temperature, the air temperature coming off the top of that uh fan coil or furnace after we decrease the airflow, it's gon na get colder. It's gon na get colder right. Okay! So now, what's gon na happen when that air comes out of the vent and mixes with the air, you know right around and we're going to say it's florida market right mixes with that air.

14 thoughts on “Dehumidification and reheat talk”
  1. Avataaar/Circle Created with python_avatars Rob Webber says:

    Residential reheat is well designed with Lennox Humiditrol. Staging the condenser fan based on oat to receive the highest potential heat. Service area Barrhaven??

  2. Avataaar/Circle Created with python_avatars Bruce D says:

    My living has two air vents coming from one air duct that has a splitter. I want to replace them. I dont know the duct size. My living room is 300.8 cfm I believe and from the air handler its 17 feet to the split the 4 ft on the left and 5 ft on the right. I seen that you know and teach very well Are you in Nepean ?

  3. Avataaar/Circle Created with python_avatars Brian Mcdermott says:

    Super great info. in your discussion and great guest. Thank you Bryan. Keep up the great work.

  4. Avataaar/Circle Created with python_avatars Jay Miller says:

    Is there a company that services Plant City, FL that follows the standards taught on this channel. I am having humidity as well as other issues. I can't seem to find anyone that can figure it out.

  5. Avataaar/Circle Created with python_avatars Tien Yeow Siew says:

    Great sharing.
    Getting the fundamental right is so important to understand all these HVAC situations.

    Cooling coil below dew point = dehumidification effect
    (Latent and sensible cooling)

    Cooling coil above dew point = sensible cooling

    Thanks for your sharing!

  6. Avataaar/Circle Created with python_avatars Nathaniel Crum says:

    Is rhyming air with air really a rhyme?

  7. Avataaar/Circle Created with python_avatars Nathan Hurst says:

    Thank you Brian for all these you are consistently putting out here for us. At this game for 12 years and learn things and widen my perspective to other areas with every one you post. 👍🏻👍🏻👍🏻

  8. Avataaar/Circle Created with python_avatars Chris Cermak says:

    I work at a research library in Chicago were they want 65 degrees and 45% humidity year round. We use constant volume air handlers and reheat coils. So we run are steam boilers and chillers year round

  9. Avataaar/Circle Created with python_avatars Matt Florence says:

    I just wanted to thank you Brian for putting this on. I currently have a customer with a two-story house with very high percent relative humidity. I am trying to resolve the issue one step at a time and this was very informative. Thank you again sir.

  10. Avataaar/Circle Created with python_avatars Jason Johnson says:

    I have a building that uses face and bypass for dehumidification on the large hydronic air handlers.

  11. Avataaar/Circle Created with python_avatars Jason Johnson says:

    This topic along with airflow is a forgotten part of HVAC. Not a large portion of the trade thinks about DP/RH.

  12. Avataaar/Circle Created with python_avatars Ian Fawcett says:

    hi guys, rookie tech tuning in.

  13. Avataaar/Circle Created with python_avatars jimmy moreno says:

    Hey Are you in Orleans ?

  14. Avataaar/Circle Created with python_avatars FunTech# says:


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