Refresh my memory; why does an AC cool excessively?

I know dirty compressor coils can effect this. The system cant shed heat as well as it could and the refrigerant gets super cooled.

Are there any other reasons?

Symptoms of a Dirty Coil

Air Conditioner Basics

Hope that helps.

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You might want to re-think that, or re-word it at least.


Are you referring to inside coil icing up? Many reasons, often lack of air flow due to improper filter, closed vents, dirty coils. Beyond that it’s best to have an HVAC tech perform a check for proper charge and then go from there.


The most common reason for evaporator icing is low refrigerant. As Simon said there are others and diagnosis is not the Home Inspectors job. In fact it is to be avoided as it will get you into trouble.


I do not what you mean by “excessive cooling”.

Excessive cooling is the result of:
Oversized unit

These opinions will not:
Undercharged will not excessively cool.
Dirty coils will not excessively cool. Dirty condenser raises the refrigerant temp on both the high and low side of the refrigerant circuit. Dirty Evaporator freezes, reduces air flow, raises the temp. Dirt on the evaporator insulates the refrigerant from the heat (so does ice on the coil).

The cooling capacity of ice is nowhere close to the cooling capacity of the refrigerant changing state, so the presents of ice does not cool better. If the ice were to melt, it would absorb 144 Btu/lb (not much). But this would never happen if the A/C is still running. A/C’s absorb 12,000 Btu/ton/hr.


I know ill get plenty of critism for bringing up the topic, but here we go :grinning:

In Texas, we’re required to report when the AC temp differential is outside the range of 15*-22*. I use a temp probe before and after the evap coil, and then tape up the holes when I’m done.

Its relatively obvious what sort of things can lead to the differential being too low - dirty evap coil fins, low refrigerant charge…
What are the sorts of things that can cause an AC to affect a 27.5* differential, like the house I was on the other day.

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There are so many variables to consider when troubleshooting. The system should be examined from one end to the other before making a judgment. Often it is a combination of issues that are affecting the system performance. But many A/C service techs choose to add refrigerant when they find a low suction pressure: fill 'er up, collect the service charge, and head on down the road to the next victim. The outdoor temp in your part of Texas was probably 50 degrees or lower when you took your readings. (It’s been in the 30’s and 40’s here in Georgia for the past week.) Most good A/C companies don’t let their techs add refrigerant on cooler days. If they do, they will disable the outdoor fan in the condensing unit, in order to artificially raise the head pressure and the temperature of the liquid refrigerant in the line going to the evaporator coil. A cardboard box or a work jacket are sometimes used to cover the fan to get a similar effect. The pressures in the liquid line and the suction line should have a differential of at least 100 psi for the liquid refrigerant to even start to spray out of the metering device into the upper coils of the evaporator tubes at the right flow rate to pick up sensible heat. All of the visible problems with the unit need to be located and addressed before diagnosing a unit as being low on refrigerant. Even a few ounces of refrigerant, one way or another, will sometimes keeep a heat pump from operating correctly. Likewise, conditions such as fan speed and a failing fan motor capacitor in the air handler can affect the system’s operation.


If you really want to get a handle on A/C troubleshooting, get a copy of a smallish red book called Doolin’s Troubleshooter’s Bible. It was written about 60 years(?) ago by James Doolin of Garland(?), Texas. He wrote it in plain English, and even penciled the drawings and schematics in the book himself. He even teaches you the basics of electricity from the street through the A/C unit. Doolin has passed, but his book is constantly updated with new technology by his family.


Wow, that’s a big hvac book. Thanks, I’d like to see that.

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A slammed filter, a dirty evaporator coil, or too little or too much refrigerant in the system, are the main causes of an evaporator coil dropping below its optimal temperature of around 40 degrees.


There’s the client friendly info I was hoping for. That makes sense, thank you.


First, Texas HI is run by a Real Estate Commission. That about says it all in my book. Enough on that subject…

Now, how do you deal with this requirement?
I am sure that they do not dictate where these temperature differential measurements (there is a TAB Standard for that). You should be looking for how many duct widths away from the coil do you test, and what do you do if there is a duct fitting before you get that far away from the coil. This has to do with laminar/turbulent air flow.

Where do you come up with the coils by-pass factor which specifically effects this Delta-T stuff. It’s about how long and how much air passes through a coil at a given velocity. If this factor is low, air gets colder/hotter relative to the time is spends in the coil. Also effects the latent heat (moisture) your thermometer can not measure.

Also, Under what weather conditions are these tests performed?
As David pointed out, you can fake out the equipment in thinking it’s 95F outdoors. But does TREC set the standard as just how this is done?

If I worked there, I would take the measurements at the most convenient location possible, regardless (as they have not instructed you otherwise. No drilling holes in the duct!). I would report the conditions as required and say nothing more. Ever! Your client, agent, or who knows whom will ask about these reported measurements. My answer would be, “It is a state requirement. Have an HVAC Contractor take a look at it if your concerned about this”. BTW: a 20F Delta-T is normal. 28F does not mean it’s a problem.

I bet there is something in the TREC Rules that says you do not have to analyze stuff. Find it and keep it handy.

Texas has one of the worst climates to be playing this Delta-T Game.
If there is humidity, a thermometer is worthless.


Here’s the newly updated Texas SOP section on delta-t, taking effect in a week or two:

(b)Cooling equipment.
(1)Requirements for cooling units other than evaporative coolers.
(A)the inspector shall:
(i)report the type of systems;
(ii)measure and report the temperature difference between the supply air and the returned
air or report industry-accepted method used to determine performance; and
(iii)generally report extraneous factors or conditions, present on the day of the inspection,
that would adversely impact the temperature differential of an otherwise performing unit; and
(B)the inspector shall report as Deficient:
(i)inoperative units;
(ii)deficiencies in the performance of the cooling system that:
(I)fails to achieve a 15 degrees Fahrenheit to 22 degrees Fahrenheit temperature differential;
(II)fails to cool adequately as determined by other industry-accepted methods;

I do it under most conditions. Not if its too cold out. If its on the cooler side that day, I put an informational item saying something along the lines of “The temperature outside was cooler on the day of inspection. performance will vary depending on outside temperatures, the unit may not perform as well under hotter outdoor temperatures”

Yea, a comment along those lines is front and center whenever my “delta-t” is outside the 15-22 range.

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I would just use an infrared gun or other thermometer at one of the supply registers closest to the unit; along with the return air temperature at the thermostat or return grille, to get the Delta-T. That’s what the AC techs do to get a quick indication of what is happening. I sure wouldn’t go poking any holes in the ductwork or A/C housing.

I do that when I have to. Sometimes I slip my temp probe between the return air opening & the installed filter if its right under the furnace/evap coil housing, in a closet, for example.

what do you think the worst case scenario is? there are almost always previous holes in the same general location, with the same tape I put over the holes. i only poke through soft ductwork, i dont drill holes in metal ducts.

Unless TREC specified taking the temps at the air handler, I would just go with the “industry standard” that AC guys use. The thermostat on the wall should be near the return, so that tells you what the return air temp is. If you stick your temperature probe between the filter and the metal opening for the filter, you are increasing the volume of warm air coming in from the attic (in most cases100 to 130 degrees during the cooling season) entering the return plenum to mix with the return air from the conditioned space. Conversely, if you probe between the filter and the housing in an upflow unit in a closet, you are injecting cooler air from near the floor into the return air stream, again skewing your sampling results.That filter opening needs to be blocked with its factory metal cover: or taped over with the NASHUA metal tape.

Thanks for including that, I hate looking up state rules.

So yes, you must take measurements.

It does not address “How”, so even though the use of an IR Thermometer is inadequate for this purpose, your free to do so. HVAC guys using IR Thermometers are not operating within industry standards just because a bunch of them may do it.

I take “generally repor”’ as a one liner; “Low ambient conditions present during testing”. No need to discuss that it might not work when it gets hot outside.

So when you don’t get 15-22F Delta, you have to determine why!
How do you determine “deficiencies in the performance” when you get a low/high Delta-T?
Measure refrigerant charge against EPA Laws?
Measure airflow rates as required by the manufacturer?
Determine adjustment of refrigerant metering devices that controls super-heat?
Determine air duct leakage rates in accordance with ASHRAE/TAB standards?

Are you allowed to “recommend further evaluation by HVAC Guy” without knowing what the reason is for a low Delta-T? You first have to write your report and comply with "(A)the inspector shall: " requirement.

I love rule makers that make rules about things they know nothing about, and set up the Inspectors liability by requiring standards that can only be performed by other licensed trades. You’re required to guess at what is wrong, when it may in fact not be wrong. Someone is going to act on your wild ass guess and spend unneeded money on something that is just fine. Might that person attempt to recoup those losses? Do you want to be that person? Oh, I’m sure TREC will stand on your behalf if that should happen…

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We shut down a lot of crap when TN went to licensing:
Got rid of general contractors off the board.
Required contractors to comply with everything in the law except initial education.
Put Home Inspectors on the board to replace the “Majority” (Real Estate Agents).
NACHI also had a part in this.

HI’s do have a voice in these rules and changes to the rules. Do you not have someone to represent you guys?


I feel for you guys in Texas. Obviously, they should define the industry standard.

In a past post somewhere in a past post where an inspector had been fined $500 for not reporting the Delta-T adequately.

I know this is easy for me to say, but if the SOP does not define industry standard, I would take the liberty in my report and define it for them.

“Delta-T was determined by measuring the difference in temperature at the supply duct and the return duct using and IR thermometer according to industry standards. I observed a Delta-T of XXXXX.”

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