This is the HVAC school podcast episode number 33 about heat pumps, I'm just making it about heat pumps. I didn't want about defrost before I've talked about he bumps before, but today is just gon na be quickie on heat pumps. For those of you who haven't worked on many and you feel overwhelmed by them, hopefully we can ease your mind and before we do that, I'm gon na thank our sponsors carrier and carrier comm refrigeration technologies at refrig tech, comm, air lysis, air leases, comm /go. To get started with great american-made indoor air quality products nav a cat Navajo Bolcom you're gon na want to take a look at their new lineup for 2019.

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So, let's first start with the basic question of what is a heat pump. Homeowners will often say where's the heat pump. It does this have a heat pump in it. I don't know where the name came from.

I could probably look it up, but we know that an air conditioner moves heat around and a heat pump does the same thing. It just has the ability to also move heat from the outside in so maybe that's where it came from it's pumping heat in from the outside and we move heat around by creating a temperature difference. So hot goes to cold right. So if we have a higher temperature, we can move it into something of a lower temperature and that's what we do so we put the evaporator coil outside now, instead of being inside, and we have to have that of Aperta coil at a lower temperature than the Outdoor temperature to get heat into it, and then we move it through the compressor it that heat inside down the discharge line goes into the evaporator coil or what was formerly the evaporator coil now becomes the condenser coil.

So, on a heat pump, I tend to sort of call things by more generic names, so the indoor coil is what we traditionally call the evaporator coil the condenser coil that we would traditionally call it outside. I now call it the outdoor coil and instead of calling it a suction line, we call it a vapor line because it is vapor either way. The liquid line does stay. The liquid line, we're going to talk about that.

So, let's start with the basic refrigerant circuit. So you normally come out of the compressor or discharge line condenser out of the condenser liquid line to the metering device out of the metering device to the evaporator out of the evaporator down the suction line back to the compressor. It's the exact same thing in a heat pump. It's just that you use a reversing valve or a 4-way valve.

If you prefer, you use a reversing valve, that's generally located very near the compressor generally right next to it, and it sort of interrupts the discharge line in the suction line coming from the compressor, and it read averts that refrigerant either going to the outside coil or Going to the inside coil and then either coming back from one or the other, so it basically just switches which coil is the evaporator coil in which coil is the condenser coil. So that's how that works now reversing valve uses generally a pilot duty solenoid. So it actually uses pressure from the system in order to help shift it. It's very rare that you have reversing valve problems, they're pretty reliable when they do have problems.

It's usually because the system had some sort of contaminants in it. They got into that slide, or maybe it was overheating if the compressor gets very hot over a period of time, so your discharge line is very hot. That can damage the reversing valve, but reversing valves are pretty stable. There's a couple that had rub out issues on the little pilot lines.

You don't want to have to replace a reversing valve if you've never done it. The things are a nightmare because you have three lines down at the bottom that are very close coupled to each other. It's really difficult to get them off. If you do have to replace one, I generally recommend cutting it out further down and then taking the stubs off of the old one and then braising them on a bench in together, so that that way, when you're in the system, it makes it a lot easier To put together that's sort of another side now, but you don't want to have to replace a reversing valve but its job, it's just to slide back and forth.

It has a slide in there. It's like, I said the process is activated by 24 volts. So then, it actually uses the suction and head pressure to make that slide, move and the reason why you need to know that is because, in order for a reversing valve to shift the system has to be running. So if you don't have a functioning compressor or you have a compressor - that's not pumping, then the reversing valves not going to move.

So you can't shift the reversing valve with the system off, for example, the compressor off. That's how that basic refrigerant circuit works. Now one thing that changes is that you're generally going to have two different metering devices. Now there are some systems that are heat pumps that only function with a single metering device metering in both directions.

You may see that in some small water source, heat pumps, things like that, but generally speaking, you're gon na have to separate metering devices. You're gon na have the typical one on the inside. It would be that TXV or a piston or evie or a cap tube. Whatever it is on the inside by the evaporator coil again, this is on a split system and then on the outside.

You're gon na have a separate metering device on the outside that works for the heating mode. So we often will call that inside metering device the cool mode, TXV or metering device, and we will call that outside metering device, the heat mode, TXV or a metering device - and you don't always see TX T's they're, not always TX pays carrier, for example, uses a Piston on the outside a lot of times with heat pumps, you'll find accumulators, and that's just to prevent liquid refrigerant from going into the compressor during running on heat pumps, you're almost always going to have a crank case heater and that's because you're gon na be functioning. The system, when it's cold outside and you're more likely to have liquid refrigerant migration into the crank case when it's cold outside, because obviously that's where the liquid tends to condense. In that compressor crank case.

For those of you who believe that crank case heaters aren't necessary in warm climates, you are incorrect. They are necessary in warm climates if the manufacturer specified it, because it helps prevent that off cycle condensation of liquid refrigerant inside that compressor, and that can cause a lot of oil loss during startup. So they call that a flooded start. That's the reason why it's not people say oil migration, it's not oil migration that they deal with it's there to prevent liquid from making it into that compressor during the off cycle, condensing in there, and then the oil does leave the compressor when that all boils up Violently, when that compressor starts in that flooded, start condition, but it's not strictly oil migration and the way that we think of it anyway.

That's another side note, so you have these two metering devices, it's common for heat mode metering devices, especially TX fees, to have issues because it's a fairly high vibration environment. So most of the problems that we see with heat mode, TX fees or for the typical reasons they didn't flow - nitrogen, while braising or they're, overheated or whatever. But another reason is that they tend to shake around more because they're near the compressor. And so there's certain brands that have more issues than that than others.

The lennox is one that we've had a lot of issues with, because the TX e is kind of free hanging inside there and it shakes around. And then the cap tube breaks off from the bulb tube or the external equalizer tube breaks off or rubs out or whatever. So those can be issues with heat pumps from time to time, depending on the design, but obviously, depending on the initial install practices. It's important to follow proper, install practices, so that's something that you can visually look for.

I'm a lot of people want to know. How do you check the charge on a heat pump? I wrote an article on this, which is a very general rule of thumb. Sort of article, the main reason is you follow the manufacturer. Specs.

Some manufacturers will tell you you can only weigh in the charge. That's a bummer. Sometimes I'm never gon na. Tell you not to follow manufacturer specs, but I wish they would give you some guideline, because it's not always just for charging purposes.

It's not like I've got a flat system. I need to charge it well, in that case, of course, weigh in, but sometimes it's just a matter of. I think that I might have a slight problem with the charge and how can you know whether it's correct or not? So I give some guidelines: I'm not going to go over these now, because they're numbers rules of thumb and you can go to HVAC school and type in the word heat pump and search and you'll find that article but follow manufacturer specifications. You can, in some circumstances where it's not too cold.

You can sometimes use the feel piece makes a charging jacket you can put on the top of the condenser that helps restrict the air flow a little bit and then that can be used to charge it. Even in cooling mode by and large, those sort of the cutoff is 65 degrees. When it's below 65 degrees, you need to test the equipment in heating mode when it's above 65 degrees. You need to test it in cooling mode.

You can test either way just to make sure it runs and cooling your heating, if you're outside of those ranges but you're not really gon na, be able to check the charge. There's a couple rules of thumb, a really common one is check the discharge line temperature, the artist formerly known, as section line the big line. If you check that temperature, it's generally going to be about a hundred to 110 degrees above the outdoor temperature, that's a quick little rule of thumb, your suction pressures. Obviously, the saturation is going to relate to the outdoor temperature, the colder it gets outside the lower.

Your suction pressure is going to go and you can generally find a relationship with that. It's not always going to be the same unit 2 unit depends on the efficiency of the unit, the coil size after coil size. How much frost is build up on the coil? Is going to impact it, but you're generally going to see 20-25 degrees lower than the outdoor temperature on the saturation side? So saturation is your evaporator temperature. Your evaporator is now outside.

That's your outdoor coil and so that temperature has to be below the outdoor temperature. You know that it can't be too much lower, otherwise, that's an indication of possibly frost build-up or not enough air flow over the coil or possibly under charge or under feeding with heat pumps in heat mode. You got to kind of use, feel if you'll want to do it in a way. That's surefire.

What you can do is shut off your heat strips look at the actual capacity charts from the manufacturer. The nice thing about capacity, charts and heat mode on a heat pump is that all of the capacity is sensible because you're not making any water on that inside coil. So and of course, it's the inside coil that matters from a capacity standpoint. So you can look at those charts with the outdoor temperatures in the indoor temperature and you can figure out from the manufacturers expanded data.

What your total capacity is and then using the sensible heat equation backwards. You can calculate how your system performance is based on those charts. That's the surefire way. If you really are worried about whether or not a heat pump is performing up to specification, that would be a way to do it not generally practical for most texts in the field, which is why don't bother teaching it, because how many technicians in the field are Gon na look up the factory sensible heat chart every single time if, in heat mode, every single time they check a system, it's not really practical.

So I'm not gon na waste your time going over exactly how to do that here, but just know that it can be done, especially if you really want to have that level of detail now. The next thing is defrost, that's huge, obviously, the more ice that builds up on that outside coil. That's going to drive your suction pressure down, that's going to drive your compression ratios up and you have to get that defrost off at some point and so the way the defrost works in almost all situations there's different ways: they do it, but they have a timer In the board that periodically looks at some form of defrost sensor, it could be a thermostat like a clicks on which is just a snap action, bimetallic disc, which is what carrier generally uses on their more simple units, all the way up to using several thermistors that Check a delta t between ambient temperature and coil temperature, which is what Trane has done for years, but in some way that defrost board is looking at the temperature of that coil and then every so often that timer will look at that and say. Okay, do we need to defrost or don't we and we'll make that decision and go into defrost, I'm depending on the strategy used in many cases, once it gets below 32 outside you're gon na defrost, every single time that timer hits and those timers can be 30? 60, 90, 120 minutes, depending on the brand and obviously in areas that you have more moisture outside.

Maybe it's rainy or whatever you're gon na, probably want to defrost a little bit more than super dry, arid environments for defrost, isn't gon na be as necessary. So that's where you would set the timer a little higher up, but you basically want to have, as you defrost as you possibly can have. Defrost is very simple: when it's ready to go into defrost it shuts off the outdoor fan. Traditionally we call it.

The condenser fan, but it's not the condenser. Now it's the evaporator in heating mode shuts off that outdoor fan. It shifts itself into cool mode. So that way, it's pumping hot discharged gas into that outer coil to defrost it, and it brings on the auxilary heat again and that's in the case of where the auxilary heat is heat strips most commonly.

You also have a type of system called a dual fuel system, and that's where you use the heat pump down to a certain point, and then you start bringing on the furnace and in all cases you really want to set up some sort of strategy. So that way, you're not gon na bring on your auxiliary heat until it's either not going to keep up with the temperature requirements of the space and that's what we call balance point or you bring it on where economically, it makes more sense to use the secondary Heat then the heat pump and that's what we call economic balance point. That's where you have a dual fuel system and as it gets colder and colder outside heat pumps, most of them become less and less efficient, and so, as that you hit this point where it makes more sense to just run the furnace. Now I'm like a heat strip situation where you're running the heat strips the auxilary heat electric heat, in addition to the heat pump and a dual fuel system, it's either/or, so you either are running the heat pump or you're running the gas furnace and there hits a Point where it makes more sense to run the gas furnace than the heat pump and that's what we call economic balance point, whereas traditional balance point is just you hit a point at which you need additional heat in order to keep the space ten, and so it's Not an economic calculation, it's just a practical calculation as far as keeping the space comfortable, there's a whole lot more.

I could say about this, but two words I'll say is that, first of all, it's very simple and logical: it's not as complicated as it sounds. Obviously the controls there's more wires, because now you have auxilary, he potentially you've got your orange circuit or your blue circuit, which can be used to switch the reversing valve. One thing to know is that the condenser runs in both heat and cool, and so you have to have a whi call in both heat and cool. So traditionally Ruby would say why is cooling and a heat pump? Why is actually just the compressor, the outside unit and getting it to run, and then what switches it to heat and cooling in the first stage is just that reversing valve call.

Most manufacturers use an orange terminal. That means that it's energized and cooling de-energized in heating. Some use of B terminal and that's energized in heating, de-energized and cooling, again pretty simple low voltage stuff, but always look at the obvious things. Air filters dirty coils, all those things impact a heat pump in the same way that they would an air conditioner.

Just in Reverse, so when you have a dirty evaporator, coil in cool mode, you have low suction pressure, low superheat when you have a dirty evaporator, coil or a dirty filter in heat mode. You have a really high head pressure. So and the same thing opposite is true with the condenser coil, so also that gives you a little overview check. The obvious things like you always would, if you do, that, you'll watch for robots, you look for oil.

You check your air flow, make sure it's good, as somebody came up to me in a training the other day and said he was a Russian guy, I'm not gon na try to do a Russian accent, but he said and remember a BC missing a BC. What is that he says air conditioning is air and conditioning ABC is air before conditioning always check all your air flows. First, look for the obvious problems with air flow before you do anything else, and that's true with a heat pump just like it is with anything else. If you have a water source heat pump, when they you get a think about water flow.

In addition to that, are you providing it with enough water because that's gon na be a huge factor as well, so there you go. I know that was rambling. I've been told that I talked too fast in these short episodes. Sometimes that's probably true, but I'm trying to get a lot in and packing in the value.

So you may need to listen to it a couple times. I might sound kind of weird at half-speed, so I wouldn't necessarily go to that. But alright, thanks for listening, you.