A question on AC/Heatpump temperature differentials

First off, I just searched the forum and found a conversation about proper and improper ways to measure the delta-t of the HVAC system. Lets ignore that conversation for a second…

A proper delta-t for an HVAC system’s AC is 15-20 degrees is a standard we’ve all heard before.

But what about if the system has been running all day? It has cooled the house to ~72* and is maintaining that. In doing so, it has removed much of the humidity from the home.

Then a home inspector comes in and wants to test the air conditioning system’s operation and delta-t, so they crank the AC up for a target temperature of 60*.

In this situation, is it still reasonable to expect a good condition AC system to produce a 15*-20* differential? It was my understanding that the effectiveness of the AC system relies on taking humidity out of the air,and efficiency drops with humidity.

You are correct in considering the moisture factor in this conversation. There is 970 btu/lb of water the HVAC system has to deal with before it changes the sensible indoor air temperature. This is a lot.

Your answer is not cut and dry.
There are numerous factors involved in the operating conditions that can not be analyzed with just a thermometer. You need to determine the Btu/hr, not the temperature change (Delta-T).

How can you get a 15* ^t on 72* indoor air by setting the t-stat to 60 when 15* is 57*?
To get 15* ^t on 72* air, you have to operate the A/C outside it’s designed operation.

ASHRAE sets the design OA Temperature for every city in the US. If that design temp is 95*, when the outdoor air is above 95*F, it is too small to handle that BTU Load. What happens is that the BTU/hr entering the building is greater than the HVAC capacity. In this event, the unit never shuts off and because it is operating below its load, it’s Designed Sensible Heat Ratio changes. It continues to remove Latent Heat (moisture) lowering the Wet Bulb temperature which increases evaporation of your perspiration, and you can essentially be freezing at 85^F indoor air. If this condition is occurring at the time of your inspection, you will not get any Delta-T. Does this mean the unit is not removing 12,000 Btu/hr/ton? Absolutely not.

Can your Thermometer tell you if any of this is occurring? Absolutely not…

From the other perspective; if the HVAC unit is oversized (therefor wrong), you can easily get your 15-20 Delta because it’s designed sensible heat ratio works on temperature, not moisture. The result is a cold, damp cave with mold growing all over the place. But your Thermometer said everything was just Oky Dokey! That is, until an industrial hygienist stops by about the mold and determines the HVAC is wrong.

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The real question here is why are you stepping outside the home inspection SOP? The reason that comes to mind is a thought that this added layer of service will distinguish you from your competition and increase your demand. The end result of such thinking is an added layer of liability that could easily land you in debt up to your ears. Are you qualified through licensure or training to measure delta-t in HVAC systems?


Exhibit A: a snippet from the Texas Inspection SOP:

The inspector shall report as Deficient:
(i) inoperative units;
(ii) inadequate cooling as demonstrated by
its performance;

Exhibit B: Past TREC violation where an inspector failed to call out inadequate delta-t

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IMO, what got this inspector in trouble is that he reported the Delta T. Since he showed his methodology, he was held to the 15-20 Delta-T standard. Which as we all know, can be very misleading.

What would have happened if had never reported the Delta T?

Interestingly, since he reported a Delta T, he had to own it and chose not to fight.

Yea, that’s what I was thinking. I’ve gotten in the habit of taking a therm gun photo of the registers and returns, but I guess I could just show that there’s cool air coming from the registers to show that there is cool air being produced and just keep it at that, huh?


Or, you could just say in your opinion the units were producing adequate cool air at the time of inspection.
You could just click off inspected.

If you bring in an IR image, your methodology would be up for critical review.

Unless the SOP defines special tools or methodology, I would steer wide.

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how about for your own CYA purposes? Say the client comes back and says “the AC isnt working!”

Well, you say it was and that is why you marked it off inspected.

Think about all the things you mark inspected with no comment.

You could and probably should keep those IR photos to yourself until absolutely needed.

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Yea, I think ill continue documenting it though for my own purposes. it doesn’t have to hold up in court/ in front of the committee but it might make the client realize i did my job thoroughly and stop them in their tracks when trying to get me to pay.

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Exactly right. I take thermal images of the registers when running heat and AC. But I don’t put them in the report. They would be used exclusively to calm an irate client. If we would end up in court, well it’s a toss up from there. Just be consistent in what you do is the best policy. If you can show the judge that this is your regular unscientific method as a generalist, the judge might be more lenient on you.


Right, there will be no “gross negligence” at that point.

And for the record, I think TREC were being complete a**holes. A written warning over 3 degrees would have been completely reasonable. We are all adults, right? A $500 fine is ridiculous. People are trying to feed their families, sheesh.

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Yea, it makes me wonder how the rest of the client-inspector relationship was. I bet many times, once it makes it to the committee, there was already a heated back-and-forth and the client was out for blood.


That device is not measuring air temperature to start with. It is measuring the conductive metal, wood, plastic, or what ever the registers are constructed of. It is measuring the effects of the air temperature. It is also only measuring an apparent temperature because it has not been corrected for emissivity, temperature reflect, distance, atmospheric temperature and moisture of the air. These must be corrected separately for each the supply and then the return air.

Next, your measuring Delta-T. When you do not accurately correct with the above perimeters, your Delta-T is 100% Wrong, and by a significant amount.

Almost all State HI rules allow you to talk about things not required in their SOP, BUT you must be qualified to make those statements.

This is a very easy electrical IR scan correction. Note the Delta-T’s between auto settings and corrected. HVAC Air corrections is way more complicated.

A thermometer or thermistor is the proper tool for measuring fluids. Once you have this, you need to know where to take the Delta-T measurements in the HVAC System…


This ^^^^^^^

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Yea, that’s what I was thinking. I’ve gotten in the habit of taking a therm gun photo of the registers and returns, but I guess I could just show that there’s cool air coming from the registers to show that there is cool air being produced and just keep it at that, huh?

Yea, that’s what I was thinking. I’ve gotten in the habit of taking a therm gun photo of the registers and returns, but I guess I could just show that there’s cool air coming from the registers to show that there is cool air being produced and just keep it at that, huh?

Have you received the notice of the new Rule changes ?

(b)Cooling equipment.
(B)the inspector shall report as Deficient:
(ii)deficiencies in the performance of the cooling system that: [inadequate cooling as demonstrated by its performance;]

     (I)fails to achieve a 15 degrees Fahrenheit to 22 degrees Fahrenheit temperature differential; or

     (II)fails to cool adequately as determined by other industry-accepted methods;

This should work out well shouldn’t it. Another great change coming our way!

There we go. Thank you sir!

Another Real Estate run Inspection Board ignoring the Science.

I under stand your perspective, but I also understand the commission’s perspective.

There are 6 Brokers and 3 Public members of the TX commission.

Keeping in mind that I, as an inspector, am not an expert in HVAC, it makes perfect sense to me to use the “industry accepted method” of using an IR gun at registers and returns to provide a rudimentary and initial glimpse at what kind of result the HVAC system is producing. I get there are more professional and accurate methods of testing the system to find the real culprit of the performance issue - which I fully support the client in pursuing after reading my findings. This sounds to me like a perfectly reasonable course of action, though.

So now that we have it from the horse’s mouth, Please don’t speak disparagingly about our fellow inspector’s practices! We Texan’s are required to do these things. (I didnt mean to direct that last line towards you, David. I’ve seen many people talking about how silly it is to use IR guns for heat readings.)

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Agree, but in my brief reply that hardly scratches the surface of the Laws of Psycrometrics the Commission’s Directive does not even come close to being accurate (especially in the Texas weather zone). What works for Larry in the UP of Michigan will not work for you.

Just because you are not an HVAC Expert does not oblige you to use an incorrect testing method in my book. I am sure the Industry Standard you described does not specifically instruct how to do this testing (with an IR Thermometer etc.). I am almost certain that they do not even know how that device works. It doesn’t even measure air temperature for starts.

Whatever… your stuck between a RE run Commission and your Client’s expectations. One of them have the potential to bit you in the ass.

If you get in a jam, you can call me as an expert witness.

These things don’t lie like you thermometer…