A question on AC/Heatpump temperature differentials

Specific Inspection Topics Inspecting HVAC Systems
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Learned a new word today!

Let’s put this in a real-world context: When I use my IR gun to take temp readings at registers and returns and find a “differential” of less than 15 degrees in 10 houses and call all of them out for “inadequate differential”, how many of those ten houses do you expect to actually have a problem somewhere in the HVAC system? Do you think I would be misdiagnosing a problem?

I would expect most of them to actually have a problem somewhere. Regardless of “proper” techniques, I think its a pretty good starting point.

Again, I understand how this might grind your gears if you’re the type of person that’s really interested and invested in the nuances of HVAC work. But I wouldn’t mind an inspector doing this for a house I was interested in buying.

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22

I argue the opposite. They likely do not have a problem. Your not measuring air temperature, your measuring a surface that is being cooled. And what is that surface? Inside the vent on the duct? The vent louvers? What about diffusers or screens? How long has it been cooling? What is the Humidity? etc.

In a way, they made it easy for you. Shoot the vent and report it. Who cares if an HVAC person is actually required?

23

There would be an HVAC person involved, after I recommend it because the differential between the temperature of the register area and the temperature of the return area isn’t great enough. Maybe its because of ducting, maybe its because of the unit’s condition, maybe it’s because it’s not sized correctly. The thing that matters to me is how well the customer is being served by the HVAC system. Next step is an HVAC person comes by to do these things yall are talking about and get to the root of it.

Honestly, I probably wouldn’t make the note unless the differential I was getting with an IR gun was less than 13*. And there are usually other comments to compliment it, like old unit or dirty fins.

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24

Or the other variables I mentioned which could cause a bad reading on a perfectly good system. I’m not really disagreeing with you. The IR at register measurement is not really a great tool for such a precise requirement from your state. They are screwed up.

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25

Agreed! Standard pat phrase to be used for this debacle. Would you like to massage it some?

The cooling system temperature differential between the system supply and return has been measured outside of the 15 - 22 Degree Fahrenheit range as mandated by Texas Real Estate Commission (TREC) for a properly functioning system. As a result TREC has determined that this cooling system is not properly functioning and is deficient. If you have any questions regarding this determination I highly recommend you have your Real Estate Salesperson/Agent contact The Texas Real Estate Commission for actions that must be taken to ensure this deficiency is corrected. You may also contact TREC whose contact information can be found here Contact Us | TREC.

Notice there is no other recommendation for actions provided. :rofl:

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26

Do you think there are better tools for a home inspector to use, outside of punching a hole in the ductwork, etc, etc? How about a thermal image of the register? Do you think the low end of the temp scale on a thermal image would be more accurate than an IR gun reading?

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It is the same tool, basically.

Do not get me wrong, you can use an IR gun to draw an opinion in general terms. The state is screwing it up.

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28

My meathod-
I measure the differential with a thermometer probe as close to both sides (upstream & downstream) of the evaporator coil as possible. I carry foil tape in my bag (takes up less room than a credit card) & put a small square over the holes I make in the plenum.
Because I am (reluctantly) regulated by TREC, I report as deficient any temperature differential outside of their parameters (15-22 degrees F). Much like Manny, my narrative includes a portion that states that “the temperature differential is outside the parameters set forth by TREC & the inspector is mandated by TREC to report this condition as deficient”
I believe measuring air temperature at the registers is inaccurate. There are other variables that may affect the temperature differential measurement such as leaks in ducts, length of ducts, duct insulation missing, etc.
I also believe that IR guns are inaccurate as they don’t measure air temperature & have a degree of inaccuracy usually (2%-5%) that increases with distance. I think many inspectors “shoot” the vents with IR because it is easier & faster for them.

**I do shoot the vents with an IR gun or thermal camera as a method to look for gross variances between supply registers that may clue me in on deficiencies in air flow.

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29

There’s a chart in my older AHRI text Fundamentals of HVAC/R by Carter Stanfield

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that corrects Delta T for relative humidity.

There are others available on the Internet that require conversion to RH, but that’s easy enough. Here’s also an article from Honeywell claiming Delta T is the “Magic Number.”

https://forwardthinking.resideo.com/comfortzone/2017/08/why-delta-t-is-the-magic-number-for-proactive-diagnostics-and-problem-solving/

30

And there is a very easy method to determine WB so you can use a psycrometric chart, should you elect to learn how to use one, without any more test equipment. You need two points to determine six other properties of air coming and going (which would be called the “Apparatus Dew Point”). And you can actually determine the efficiency/capacity in Btu’s, and compare it with the unit nameplate.

First, most IR Thermometers do not have the ability to adjust the emissivity of the object you’re aiming at, other than Shiny/Dull. If you aim at the register, it’s painted. What is the emissivity of that paint? If you miss the register and hit the interior of the duct, you are hitting shiny bare metal or plastic flex duct. Do you know that IR can see right through plastics? So, now you’re measuring the fiberglass insulation that is not in contact with the air, so your not measuring air temps.

Point #1: So, you need a target. A piece of cardboard (not corrugated) and paint it flat black or use electrical tape which has a know emissivity of .95. Place it in the airflow and read the black spot. This is as close to the Sensible Air Temperature as you can get with IR. We call this “Sensible Heat”.

Point #2: Hang a clean damp rag in the air flow (SA & RA). Wait a minute and read the temp of the rag. This is your “Wet Bulb” temperature. It is measuring “Latent Heat”. It is always equal to (100% Rh), or less than Point #1 (<100% Rh).

Enthalpy (total heat): The sum of “Sensible Heat” + “Latent Heat”. If you’re not measuring “Latent Heat” you have no idea how the HVAC Unit is operating. The condensate coming out of the pipe is the “Latent Heat”. It represents the Latent Heat of Condensation which is 970 Btu/hr/lb of water. That is almost 8,000 Btu/hr/gallon. And you are just disregarding this work the HVAC is doing! ?

> For condensation, latent heat effects associated with the phase change are significant, similarly as for boiling, but in reverse. Latent Heat of Condensation – Enthalpy of Condensation

Change of HVAC Air Flow: (dirty filters/coils, improper duct design etc.)
When your air flow changes, for what ever reason, the Sensible Heat Ratio changes. Ie. the relative humidity value. When you have an airflow restriction (most commonly found by HI) you will get a great Delta T. But the unit is not operating properly. You could end up with a 1Degree F Delta after you change or remove the filter… Are you going to call out a 15F Delta for further evaluation? If not, you just walked away and left your client with a potentially expensive remediation. That OK with you?

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31

Whom are you addressing yourself to? Read the Honeywell article I linked to before pontificating on Delta T and what its findings may indicate.

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No one in particular. Are you narcissistic and think everything is all about you?!

I did. What;s your point?

  1. It does not mention anything about measuring with IR devices, which is what we are discussing.
    If you think this is an accurate method to test, please enlighten us.
  2. It does not address how the Delta-T is to be measured, nor does the BS TREC Reg address the specific method as prescribed by ASHRAE.
    I have discussed this method/requirements elsewhere in the past. Not going to beat a dead horse to death.
  3. TREC is the issue here. They are mandating a procedure not supported by HVAC science, just the HVAC Installers lazy old, “This is how we do it here” based on Old Wives Tales.

I could care less how you run your business or what pontificating remarks you wish to throw out there. You got some “Facts”, present them. I wast my time discussing things with inspectors like you, hoping some inspectors out there that can think for themselves, rather than blindly follow a self proclaimed HI Trainer, will take a second and think about it and look stuff up, then ask educated questions.

I had several service department managers over the years require their department to record Delta-T. My reports addressed Delta-H. When I was Called on the Carpet over it, I just handed them a Psycrometric chart and asked them what they planned to do with that information. “Just get out of here…” ! So I’ll just ask you to do the same. In simple English, put up or shut up.

Do you also subscribe to the square footage of the building to determine equipment size/capacity?

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That is the reality.

In a way, it makes it easier on the HI. But, it is not good for the HI industry.

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34

Brian - If you have to do it for TREC, it would nice if everyone used the right testers in the right location, don’t you think?

How is Ga. on this issue?

In Tn, we had a bunch of Realtors and General Contractors on the HI Commission as it started up. NACHI & Nick jumped on this and got better representation for HI’s here. I personally know inspectors in bogus complaints that the commission shot down in favor of the HI. So, that turned out to be a good thing.

35

Yes, of course.

We have no state controls. Wild wild west here.

So, I follow the NACHI SOP. I do use an IR gun for one thing…is the system producing conditioned air at the supply vents in each room?

Now, if I think the cooling is inadequate, based on my OPINION I will state that in my report and recommend service. I do not state how I came to that conclusion.

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36

Yes, it is an excellent tool for inspecting. as Clint Eastwood would saw “A man needs to know his limitations”… One of the first things you learn in Thermal Imaging Training Lvl I is how little it takes to creates a significantly wrong Delta-T.

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Real estate agents governing home inspectors? What could go wrong with that? :slight_smile:

I honestly don’t know how you guys in Texas do it. With the stories I’ve heard I’d be looking for another way to feed my family.

As for the topic, I use an IR thermometer just so I don’t have to physically get right to each register. I have put pictures of it in reports but usually only in pretty extreme circumstances. Most times if it’s only outside normal by a few degrees I’ll recommend the HVAC tech further investigate when the equipment is cleaned/serviced. The problem could be as simple as a dirty filter or could indicate the need for some repairs. “Further HVAC contractor attention is recommended for cleaning/service and potentially some repairs.”

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