Delta T and return location

This is a 5000 sq ft, two story, 2007 stick built house. Two separate HVAC systems for upstairs and downstairs. Very open layout. The return registers for both HVAC systems are in the upstairs ceiling. Its very open, plenty of room for air flow to the return from downstairs supply. I have one system running at a time. Delta T for upstairs is 20 degrees so that’s great. Delta T for downstairs is about 30 degrees so too much. The average supply temperature is about 43 degrees which is lower than usual.

Do you think the placement of the return register could be throwing off the Delta T, or the lower supply temperature is also indicating a problem for the downstairs system?

I also had one supply register that was 6 degrees off from the others in the upstairs system.

Let me know any insights and advice. Thank you

Are you repairing the AC or doing a Home Inspection? Performance evaluation is beyond the scope of a Home Inspection and can get you into some serious trouble.


Poor design the return for down is up.
I think it would be a little warmer upstairs.
Perhaps that’s why there’s a 30* split.

Thanks for your opinion Marc. Was thinking the same.

Too many variables involved even to make an educated guess. For example, changing the thermostat in a one-story house with one HVAC system and one central return would have too many variables to judge the adequacy of the system. For example, the thermostat was set at 76° when you arrived. You turn the air down to 72° then what? The AC has to cool the air, the walls, the floors, the ceiling, and the contents. The time needed for that to happen depends on the efficiency of the ductwork, the size and condition of the AC unit, are the interior doors open or closed, the heat load based on the time of day and outside temperature, and the location of the thermostat. I had a house where the sun was shining on the thermostat for a period of time due to the window placement. This is a shortlist, now multiply by a two-story open plan house with two HVAC systems.

So I dont follow the relevance of some things you mention here (temp of contents and so on). If you could explain that further I would appreciate it. I’m measuring temperature difference between return and supply. I typically let the system run for about thirty minutes, and doors are usually opened and closed during this time, although kept to a minimum and interior doors mostly open. I realize this is not perfect and beyond the SOP.

Mostly I was looking for experience/advice on evaluating multiple HVAC systems in a house using thermal imaging. I saw this as a somewhat unique situation.

Seeing the return for the downstairs at the upstairs ceiling is pretty common on larger 2000-era built homes. Sure, I admit it’s an odd design, but I see often enough.

As long as their hot/cold air flowing, that’s all the matters.

Let me answer your question more directly. That house was designed for two AC units to cool it properly. Running one at a time will not cool the house properly since one unit is undersized it will take a lot longer and depending on the heat load may never get the delta T you expect. I assume you were using an infrared thermometer to check your temperatures. With the returns in the ceiling, the connecting ductwork is in the attic plus the heat rising in the house will likely make the return grill hotter than one located in an interior wall. So you would have to run the AC units longer than your probably use to. In addition, the cool air from the 1st-floor registers has to travel farther through warm air to reach the return air grill, so that will influence the Delta T.

Ok thanks! I figured the location of the return is throwing the temperature difference off. I wasnt sure about testing one at a time or both, so perhaps my readings would have been closer to desired if I had run both systems, which is good to know. I was using a TIC. The supply registers on the first floor were about 10 degrees colder than upstairs (around 43 downstairs and 53 upstairs). Do you know if this is normal or does that tell you anything?

One should never rely on a delta T to derive or state any conclusion about the design or function of the AC system installed. The delta T can vary significantly and even when it shows you whatever “number” you’re looking to see, the system could in fact be undersized, oversized, undercharged, overcharged, can have leaks in it, etc… About the only thing you can tell when you run a unit is if it’s cooling or not unless there are clear signs the unit is malfunctioning such as icing, not responding, the inside coil is as warm as indoor ambient temperature :slight_smile: Measuring delta T alone is completely useless because you cannot rely on it alone to draw any concrete conclusion.

I agree with Simon. I don’t use Delta T. I typically take off the front panels on the furnace. On electric furnaces I use an AMP meter to verify the elements are working. Its not uncommon to find one or more elements not working. This is typically attributed to a bad sequencer. On AC units I will check the temperature of the A-coil if possible. It’s my experience after the AC runs 15 to 20 minutes in a occupied house where the AC is being used the coil temperature is usually in the upper 30’s. Registers closest to the A-coil are typically in the 40’s and the farthest registers are typically in the upper 50’s. Houses that have been closed up for weeks with the AC off in this hot weather will need to run allot longer. I always show up early to a vacant house to ensure the water is on then make sure the water heater is on then turn the AC down while I do most of the inspection before checking the AC unit. Once done I only state the HVAC was operational at the time of the inspection, or state the AC unit wasn’t working or was producing warmer temperatures than expected and recommend an HVAC technician inspect the unit.

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Randy and Simon,
I agree that Delta T alone is not a tell all. From my reading it appears to be a useful sort of standard. I got into Delta T because I was in a house, poor condition overall, and I was skeptical of the HVAC system. It was over a hundred outside and I think hotter in the vacant house. The air coming from the supply felt cool to me. I looked at it with my TIC and the temp was almost 80 degrees F. So obviously thats not going to cool a house to 70 degrees. I asked myself how cold is cold enough? Does it depend on outdoor temperature? and other questions. In that situation I recommended further evaluation and service by an HVAC professional.

So my question to you is what determines if it is actually cooling? There needs to be some sort of standard it seems to me. 80 degrees was significantly cooler compared to outdoor temp. If I were the buyer I would not want to be under the impression that the unit is working properly. I suppose the numbers that you mentioned are what you go by, Randy, and if it is different than that you recommend further evaluation. Sounds like you perform a very thorough evaluation.

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