Thread: A/C Duct Output Temp vs. CFM Air Flow

I always check each output duct with a laser thermometer to check the differential temperature from the return. Typically all temperatures are within a few degrees. Last week I found two ducts where the output temperature was the same as the return and so noted in the report. One duct was in an eating area next to a kitchen and the other in a utility room. The temperature at the kitchen duct output was about 62 degrees F while the eating area was 76 degrees F. These ducts were only about 12' - 15' apart.

An HVAC technician serviced the system and then measured airflow instead of temperature. There was a 30% difference in output from the formal dining room and the eating area and 15% difference between the kitchen and dining area. The utility room is a much smaller duct. Is this much variance normal? Are these tests (temp vs. air flow) apples and oranges? Should I offer to go back and retest temperatures? My client is wondering if the problem was resolved.

Below are the numbers provided by the HVAC technician

4) Check / Measure air flow:

- Dining: 235 cfm.

- Kitchen: 201 cfm.

- Breakfast: 167 cfm.

- Utility: 57 cfm

AIR FLOW MEASURES GOOD. NO PROBLEM FOUND.


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Plus, minus 200 cfm for a standard, typical, normal house room is pretty much in the right range barring major construction variances or sizes.
From what you posted it sounds like the Seller is providing documentation that is designed to make it look like something was resolved. However, my guess is it hasn't been.
There will always be some variances. 14 degrees in temp is clearly a bit much. Especially for ducts that are only 12'-15' apart. My guess is the warmer supplies have a leak in the duct run. Or are running along a hot wall, above the hot water tank flue or something else. It also seems odd that the tech didn't include temps.
If you want to help the client, go verify temps at the registers. If not, fine. I wouldn't tell the client it seems to be resolved though. You could also recommend they go back and see if the rooms get cool or not.

An iR thermometer measures surface temps, not air temps to my knowledge.

I wonder what temp variance would have been noted had a actual thermometer been used?

Originally Posted by Raymond Wand

An iR thermometer measures surface temps, not air temps to my knowledge.

I wonder what temp variance would have been noted had a actual thermometer been used?

True. However, the IR thermometer will measure the register cover, which should be about the same temperature as the airflow over it. I would take measurements from near the center of the register, and be wary of registers that were made of different materials (wood, cast brass, sheet metal).

I also would like to test by putting a digital probe in the airflow and simultaneously measuring the cover with the IR.
I suspect you will find about as much variation between the two as you could find by moving the probe into different portions of the airflow.