High delta t during cold weather

Im an inspector in Las Vegas. Out here our temps range from really hot to quite cold. Our typical hvac systems are split systems, compressor outside, air handler/heater in the attic. Mostly gas fired but some heat pumps. Older homes have roof mounted package units.
I inspected a two story house the other day and the outside temp was about 45 to 50 with the wind blowing. The AC system was off when I entered so I fired up the heater and let it run while I completed the rest of my inspection. 2 plus hours later I shut the heater off and let it rest for 10 minutes. Put it in cooling mode and measured the temp at the air return at the ceiling of the 2nd floor. 84 degrees read by my digital thermometer.
10 to 15 minutes later the supply air temperature was reading 48゚. I then shut the system down.
My question is, given these conditions, would you call this out as a deffect of an extreme Delta T?

I know, don’t run if it’s too hot, don’t run if it’s too cold, but I’ve talked with multiple HVAC installers at every time of year and they all tell me they test their units, briefly, after installing regardless of the outside temperatures.

I would appreciate your thoughts brothers!

Just my 2 cents. Some manufacturers still set these temp parameters with the cooling system as noted in their manuals. I just follow the NACHI SoP parameters. If I had more HVAC training/certs then I would likely take a greater risk…but I see little reward in going outside the SoP. And the customers accept the explanation.


Hi Brian, thanks for your input. My Dilemma is how do I do the best inspections for my clients. If it was your mother or sister You’re telling me you wouldn’t run the AC system? Regardless of outside temperature? Knowing HVAC installers do it every day? I think the SOP needs changing.

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The best inspections you can do for your mother, sister or your clients is to get them to the experts that really know about the things that are outside your SOPs. HIs have a good basic understanding but we don’t know everything. HVAC is one of the places I’m personally the quickest to get bring in an expert.


All good points. Allow me to play devils advocate for a moment. Are we going to change the SoP over and above manufacturer recommendations? Keep in mind, we are free to exceed the SoP anytime we choose.

Let’s say it is 50 degrees outside and you bump on the compressor just to see if it fires up. Good, you can report that it powered on. Do you want to run it longer? You will likely get cool air at the supply, but that AC is not working very hard to accomplish this. Did you help your client? Maybe. But what if it breaks during operation when the manufacturer advised against it? Did you just buy a new condenser :man_shrugging:

Also, an HVAC expert will shoot 90 holes in your delta-T methodology. I just report the temperature variance between supply and return when I suspect there is a problem and report my concern as “this temperature variance MAY indicate a cooling/heating problem” and recommend evaluation. Sometimes I just say “In my opinion, the system was not heating/cooling adequately”

Gas fired furnaces, who cares…run them anytime.


It sounds like you operated a gas-fired furnace with an AC added. Even though the AC almost certainly has a scroll compressor, we should not operate the AC when outside temps are below 65F. HVAC guys can do whatever they want to do, but you could be paying for repairs to an AC by operating it when it is cold outside. Winter time temps create a limitation on inspecting and operating (don’t) AC. Obviously, air sourced heat pumps are a different animal, but those go to my next comments.
As for Delta T, proper or correct Delta T measurements are way, way beyond our SoP and almost no HIs do it correctly. Heck, most of us use IR or a laser thermometer which rarely can get accurate measurements for this purpose and never account for humidity, air volume, air velocity, system limits, etc. Tread carefully when thinking that you are doing split or Delta T measurements.


Most AC units work great when it cold outside.

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Try searching this here. We have beaten this horse to death many times. It is long, complicated and beyond the Home Inspection. There is no single answer that applies to each situation. If you’re up on your physics, thermodynamics, psychometrics, fluid dynamics, and uncommon sense, you can consider discussing this in your inspection report.

This is how an answer to a question like yours will go:

Ok, I’ll discuss this again…

You CAN NOT get a Delta T when the outdoor temperature is below design conditions and there is no indoor load! Remember reading, “Don’t run below 65F”? Well I don’t believe that either and I just got done arguing this point elsewhere here.

I know your idiot TREC rules want you to take them, so just take them and call it a day.

Try; Compressor Failure - #10 by dandersen

EVERYONE PAY ATTENTION: Good observation Gene!
This is what you find when you run the A/C when the outdoor air temp is below the thermostat set-point.

There is no load on the equipment (indoor air temp/moisture).
There is insufficient head pressure because of the OA temperature.
The unit was not modified to run in low ambient conditions.
There is a good chance of damaging the compressor running the A/C under these conditions for a long time, trying to get a Delta T that you will never get.

What you see here is called “Hunting”.
It is like your truck trying to go 70 MPH (80 MPH in Texas) with a bad fuel pump. No pressure at the Carb/Fuel Injectors and it will run just like this A/C.

There is a design refrigerant Delta P that is required for the system to operate properly. R-22 maxes out around 250 PSIG high side, 70 PSIG low side (180 Delta P). When it’s 60F OA, 101 PSIG high side, 40 PSIG low side (61 Delta P). Pressure drops 119 #.

When you starve the evaporator coil the pressure drops as does the refrigerant temperature. However, this temperature is not consistent across the coil so the coil gets warmer overall, Increasing temp and increases refrigerant pressure. On the outdoor condenser coil, as the load from the evaporator coil decreases, the temp of the condenser which is designed to run 30 degrees above OA Ambient Temp now runs colder (0 Delta P) lessens pressure to the metering device. Unless you increase the load indoors, or reduce the heat transfer of the condenser, it will hunt.

Understand? Your Sate doesn’t! They should if they make laws, no?

You can make this go away by wrapping the outdoor coil with something, or turn off the outdoor fan. This will raise the head pressure to design. You can run the heat up in the house to create a 95F load. Just don’t want to keep this up when the head pressure goes above 250 PSIG (R-22). It will trip off on a high pressure control.

It’s my guess you won’t want to try this, but that is what you need to do to satisfy the State Mandate.

One of the big differences in your location is the amount of latent heat in your air. Thus, the number of swamp coolers you guys use there. Your answer will again change from any other question answered here, so just try to grasp the concept, not the “numbers” you’re trying to calculate.