Air temp differences and flows in different rooms

Hi all. Looking for some HVAC guidance. I just inspected a brand-new construction home. High efficiency gas furnace (American Standard) located in attic, as was all the ductwork (flex duct). I didn’t notice any obvious excessive bends or pinch points, but also could not access about half the attic due to the ductwork and general physical constrictions of the space.

As I inspected each room, I used my FLIR thermocouple to check the air temp at each register (all in the ceiling) after the system had been running 5-10 minutes. One bedroom had air that was 10 degrees cooler, and the overall flow felt lower as compared to the other registers (but I realize that sensing the actual flow is, to a large degree, subjective). My question is, in terms of putting it in the report, what recommendation would/should I make? Is there even a rec to make - meaning, is 10 degrees - and a possible decreased air flow - in one room even anything to be concerned about in brand new construction?

For those that have their fingers dancing over the keypad just waiting to get snarky or condescending, please go have a coffee or go for a run instead. I’m looking for actual helpful guidance here. Thanks in advance!


In heating mode, one bedroom air supply vent was inadequate such as air flow felt low and temperature was much cooler than other supply vents. Recommend further evaluation for corrections as needed by a qualified HVAC contractor.

(now, you can choose to disclose your methodology but I do not think it is necessary and may open up another can o worms)

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Thanks Brian. I like that approach. And just curious…why not mention methodology? Do you think using a thermocouple is a bad idea?

No it’s fine. I just do not like for other “tradesmen” to start shooting holes in my methodology, the quality of my “special tools” etc. I find it works just fine to state my concern and move on, let the HVAC guy sort it all out.

Edit: However I do keep notes of my methodology and findings.

Noted. Thanks again!

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Just for further consideration, there may be temperature drops across long runs or other issues such as duct air sealing etc. My concern would be comfort for the occupants, that is why I try to keep it as simple as possible. You could even say “I have concerns this room may not be comfortable for the occupants or temperature may be difficult to control etc.” if you want to expand on consequence.

Yep, I had already put a line to that effect. I generally always try to explain the ‘real world’ consequence of an inspection item. Thanks again Brian.

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What was the temp difference after 15-20 minutes? Did you have the AC running prior to turning on furnace? Was this bedroom farther away from furnace than other rooms?

Are you there as a Home Inspector, or a HVAC Tech performing Troubleshooting diagnostics?


Is sufficiency at registers even part of SOP



But, I exceed the SoP often…it is a judgement call for the inspector. I was not there so I have no opinion on the OP’s specific situation and it may or may not make it to my report.

(note: Texas does require more such as temp readings at registers to include a flawed Delta-T reporting)


When the HVAC tech shows up and proves your concern wrong you will not get a referral from that customer.

Just did a Richmond American 11 month inspection. All the insulated ducts in the attic were “jump” ducts.

Never test for efficacy unless you were hired for that specific purpose. As Chris implied you didn’t run the system long enough to come to any conclusions.

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None. Absolutely none.
Overall ambient temperature: The adjective ambient means “relating to the immediate surroundings.” Also sometimes referred to as the ordinary temperature or the baseline temperature, this value is important for system design and thermal analysis.

Temperature at forced air floor registers and returns is for another set of measurement calculations all together.

I am going to defend the OP for a moment. New construction and he observed low temps and minimum air flow. Maybe the system was not running long enough, good point. But we act like a HVAC person has never forgotten to properly attach the duct to the vent or left some other seam open. It is his opinion that matters in the end and it certainly may be reportable. Are there risks? Sure. There always are on both sides of this equation.


And his job is to report that opinion of an issue being discovered, then to report it for diagnosis and repair by qualified people. Period.


My first observation is that you question your method and tools. You do not appear to have what it takes to make any determination of a significant defect with the system in this case. A thermocouple is the correct tester to measure fluid temperatures (air & water) as you asked. However you need more than temperature to determine anything here. We need to know enthalpy. This must be calculated, not measured.

You do not test air temperature at a room defuser to determine if the unit is operating properly. It is tested four duct diameters from the equipment plenums. You can test for heated air in each register to determine if the ducts are hooked up, that’s about it.

Air flow and temperature do not need to be the same. TAB (testing, adjusting, and balancing) analysis determines the heat/cool load for each room. Some rooms require more/less to maintain design temperature/humidity. Not that I’m saying any general contractor uses this practice unless demanded. If the duct is long and through an unconditioned space like your attic in this case, you will always have cooler/warmer air. If the airflow in this same duct is low, it has more time in the duct to cool down or heat up in summer. Maybe that is what is needed for TAB, you don’t know without testing the room.

You are looking at the supply duct. It might be the fault of the return duct design!?
You must remove air from a room to adequately supply conditioned air to the room (but you didn’t say anything about the conditions in the room served). You can’t condition room air if you don’t get it to the furnace to condition it. This promotes air stratification where it could be 40F at the floor and 120F at the ceiling. Where do you sit in a room? Comfort conditions are affected by a whole lot more than air temperature. Could it be that this low flow room is lower than the rest of the house, or does the rest of the house have too much flow? Do you get my drift? This stuff is beyond 90% HVAC technicians out there, so don’t take it personally.

BTW; if we do in fact need more air in that room, how do we get it? We rob Peter to pay Paul. We damper down ducts that have more than they need which increases lower volume, higher static pressurized ducts.

Do not say this unless you are a TAB Technician and know this for a fact.

Exactly. :+1:

Michael- Comfort is the hardest thing for ASHRAE engineers to determine. Comfort is not the same for everyone. Age, gender, race, psychological, physical conditions, of people are not the same. Ask your wife. So as Brian said, “may not” be comfortable.

The same person can be comfortable if in a room at 64F @ 56% RH or 80F @ 18% RH because enthalpy is 23.07Btu/lbDA versus 23.4Btu/lbDA.

Yes, but there is no available test for a home inspector to determine this except for visual. If he can not see it, he is not responsible for it as if falls under inaccessible. Except if there is no Delta-T at all, or no air flow at all. I measure all ducts for this Delta-T.

Hope this helps Michael


I generally only comment on vents that have no air coming out, which upon further investigation, is usually due to disconnected ductwork. No air when the others are blowing is fairly objective and doesn’t leave much room for interpretation. Beyond that, I feel there’s just too many factors to make a call based on the few hours we have to inspect the whole house. Calling something out if you’re not absolutely sure it’s wrong can make you look bad. Know your limits if exceeding the SOP.


Like this!

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That’s pretty wild! Bet that guy owns stock in AA batteries.