Howdy there this is a short episode of the HVAC school podcast, and this short episode is some common temperature measurement mistakes. Very common, actually see it all the time, but before we get into it, I want to thank our sponsors and their sponsors. Our carrier and carrier comm, we are both, see fad dealers and ductless dealers. The new carrier ductless program were part of that at kalos, and we appreciate working with carrier.

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Am I talking about one of the more common ones - and I was actually just talking to my service manager about this, and he gave a really good example, and that is that he was testing the system check the superheat and sub cool on the pressures and everything And it was all checked out really good, checked out to spec and the customers complaining that it just wasn't keeping up 78 degrees in the house equipment running. He was starting to think. Okay, it's just not quite meeting the expectations of the customer, but it's running fine and then he puts a air temperature probe in the duct and it's one of these applications where it's hanging horizontal in a garage and so there's not much duct for him to get His probe in and he puts his probe in and he's measuring about a 19 degree, delta-t drive, Balbir temperature split and he's thinking. Okay, this is normal, but then he goes through and he just throws an amp clamp on the heat strips and the heat strips are actually running, so he's measuring a 19 degree split and the heat strips are actually running and on the face of it.

That sounds crazy, like how could that be, but he strips not a heat pump, which is what this was their small heat strips just five kilowatt heaters. They go right into the center of the Airstream, and so the heat that comes off of those heat strips is gon na go right down the center of the duct and I've tested this so many different ways. This actually comes up a lot when people are trying to do the heat rise. Calculation, using heat, strips they're, trying to just run heat strips and calculate CFM of air based on the temperature rise, is that it's really difficult to measure the air temperature when you're running heat strips, because that hot air tends to go down that center part until it Has a real chance to mix so you've got to go quite a bit of ways down the duct or even to the first register closest to the unit to measure in order to get an accurate reading so sure enough.

He goes down and he measures and he's only got a 12 degree split, so a pretty big difference there and that obviously ended up being the problem. So you have to really take care when you're working with a system that has heat strips that, if you're trying to measure the air temperature with any accuracy. Whether with or without the heat strips, you got to make sure that you're measuring either further into the center of the duct, which has its own drawbacks. Because then, you're gon na potentially be affected by the radiant heat which we're going to talk about in a second or go further down the duct, where the air has a chance to mix you're shielded from the radiant heat of the heat strips or maybe even that.

First register closest to the unit which can be a pain in the butt. I know that, but that's really the best way to do it. The other option is to measure it a couple different points and then take an average that can also help you. If you measure a few more points average it out that's going to give you a better sense of what the actual temp sure is.

Now, if you're working on a typical fan, coil or a heat pump unit with an air handler - and these are just running in cool mode and you're - not running E strips and you're - not testing it. That way, then, generally, it has enough space to mix because it's gone through the coil, then through the blower, then up through the duct and now they're gon na have a little bit more reliable reading. But the other side to that which is probably more common for most of you around the country as you work on gas furnaces with coils on top either case you're on case coils, and when you put a probe in the duct above the unit. You're often very close to that coil and that coil, if it's looking at your probe, that's the way I like to think of it.

It's in visual contact with your probe, it's going to absorb heat from your probe via radiant heat transfer, and so a lot of people think that that only applies to hot objects like there's radiant heat transfer from the Sun, but it also works the other way. So when there's an object, that's lower temperature, like your evaporator coil, is it's going to absorb heat from your probe, the radiant heat transfer and that's actually one reason why probes are often less shiny because it helps reduce that a little bit but you're still gon na Get some of that radiant heat transfer and so you're gon na tend to read a lower temperature if it's exposed to that coil. The other thing that impacts it is because now you're measuring much more closely to that coil. The air doesn't have a chance to mix before you measure so you're much more prone to get an incorrect reading on a furnace with a coil on top.

Then you are with a fan coil where the coil is below the blower, and so it has a little bit more chance to kind of mix together before it gets to the point where you measure and that's an important thing to realize, and that's where you can Either average take a couple different measurements: you can go a little further down the duct if you have access a lot of times, if you're working in a crawlspace or a basement, or something like that, you have a little bit more access to the duct, even in An attic, but if you're working in a garage or a closet or something like that, where the duct goes up and then kind of disappears, you may need to go to that first closest register in order to get a more accurate supplier temperature reading. That's something to watch for it's difficult to know, obviously exactly how much of effect it's having. So you have to kind of just have a good sense of whether you're very close to the coil or whether you've got some space in general, you'd like to read at least a couple feet away from the top of the unit, and even more than that. If you can it's even better, if you can get a in there, so that way, it's got a little more chance to mix before it hits your probe.

Another thing to think about is this comes up also when you're measuring temperatures outside so when you're measuring after temperature. I see guys do this all the time. They'll say it's 95 degrees outside and while they're measuring by the condenser, with their probe exposed to the Sun, and so you want to get that probe into the shade away from the discharge, air of the condenser or dryer vent or anything else. You want it to be in the shade, not affected by anything else in order to measure that outdoor temperature, when you're gon na calculate your appropriate condensing temperature over an ambient there, you're gon na, maybe use a super heat calculator or something you want to make sure That you're measuring that in the shade, it's very important that that's where you get it so you're not affected by those radiant gains and even sometimes, if you're, in the shade, if you're in the shade, but it's affected by something else.

That's hot, like as an example. We work on a lot of air conditioners that are very near gas pool heaters, and so maybe that gas pool heaters working to heat up the spa and that pool heater shell is hot or warm. And now my probe is exposed to that. That can even be a radiant heat gain, so you want to think about what surfaces are looking at the probe and the same thing is obviously true in thermostats one of the first things you should learn in school literally, one of the first things is that the Thermostat shouldn't be located in a place, that's affected by sunlight or a door that could open or other hot surfaces.

So it's not just a matter of that thermostat not having sunlight shine on it. It's also is that thermostat in a location where it's going to be exposed or its sensors, going to be exposed where the mass of the thermostat is going to be exposed to another surface, that's going to be a significantly different temperature than the room temperature, the actual Air temperature in the room, because that will either drive up in the case of hotter surfaces being exposed to it or drive down the temperature. If it's colder surfaces being exposed to that thermostat because that's radiant heat transfer, radiant heat can be transferred through space without even needing to heat. The air in between example, always use, is put your hands in front of your face.

When you're looking at a campfire, you can feel that just shielding your face from that campfire cools down your face and that's just shielding your face from that radiant heat and now instead, that radiant heat impacts, your hands, that's doing the shielding and that's the same concept With whenever we're trying to measure temperature in an accurate way, we have to shield from radiant heat, which means we got to think about that induct out adduct firmest at outside. You got to think about whether or not radiant heats gon na be affected. You have to think about air mixing, especially when it just came through a coil or off of a burner of a furnace or whatever. You want that air to have a chance to mix like I said.

The worst case is that you're gon na see this they're going to be on cases where you have electric heat strips, because they tend to sit right in the senator that air stream they tend to really have some huge variations. Depending on where you measure, you got to be really careful with that, especially if you're doing that temperature rise calculation for air flow, it's much more difficult than it would seem on paper. We have a calculator in the new app for HVAC school for Android and iOS. That you can download now that allows you to do that calculation, but word to the wise.

It is not as easy as it sounds on paper. You got to take multiple samples in most cases and you really got to leave that unit off for a long time. So that coil has a chance to dry out before you attempt to do the temperature rise calculation, some of you you're, like I have no idea what you're talking about. That's fine, don't worry about it, but those of you who know what I'm saying this accurate use of temperature measuring equipment thermometers is extremely critical when it comes to getting an accurate result when you're doing these more detail type of measurements.

The final thing I want to mention is something that we don't think about a lot, which is that, when we're using line temperature clamps, we often don't think about how clean the copper is, and this has always been true - whether you're using a k-type thermocouple or a Thermistor proper contact to that copper is very important, but now there's a new product on the market. A field piece makes it: it's called the rapid rail system and it actually uses the copper as part of the conductor to make the circuit, and it just helps it measure a little more quickly, but it's much more sensitive to the copper needing to be clean. So if you've got a dirty section line, dirty liquid line and you're trying to measure with that new technology, that new field piece, clamps, you're, gon na measure incorrect readings - and so it's becomes more critical that you keep that copper really clean, although it always was. If you had a bunch of crap on your copper and you're trying to use a clamp on it, it's not going to measure as accurately, then, if you're on the bare copper so keeping a little bit of Emery cloth or maybe a steel, bristle brush or a Brass brush or something like that in your go bag is gon na, be a good thing to have, so you can clean it up when you take that measurement and get a little more accurate reading, when you're measuring superheater sub-cooled on critically charged systems even a degree Of difference can make a big difference in the operation of the system, so you want to make sure that you're getting those accurately.

So those are the three think about radiant heat, think about air mixing and think about cleanliness of surfaces when you are measuring temperature and by golly, if you're still using one of those laser infrared thermometers to measure temperatures, you really need to understand how that works. That's not what this podcast is about. I've written articles about it if you're using that day in and day out to measure Delta, T's and things you might want to rethink that or at least make sure that you've done your research on how that works and we'll talk about that again. Another time all right, hopefully that helps we will talk to you next time on the HVAC school podcast.