AC Air drop test

How long should a AC unit be on prior to performing a Air drop test on the AC?

Are you referring to temperature testing or pressure drop? For temperature testing at the register and returns, usually 15 mins +/-.

Temperature testing. If the AC unit has not been turned on for the season, it will give you good results if you just turn on the AC if the temperatures are above 60 degrees and you let it run for 15 mins? Or does the AC need to be on for a minimum amount of time? (overnight, 12 - 24 hrs)?

Would be nice if you guys really understood what your testing…

You are not going to normalize the IA in 15 min.
And you can’t test the equipment with a thermometer.

15 mins is not a hard and fast rule, hence the +/-. I live and inspect in the southern California desert where people tend to run their a/c throughout the year. I typically run the a/c, until it’s noticeably cooler then check the SA & RA temps. This will give you an idea of what’s happening in the rooms, what people typically care about. You can also check the temperature difference across the coil. Anything more than that is going to require a EPA license and specialized tools that most inspectors don’t have.

Any ideas on what is too cold for an AC unit? At what temperature is it possible for it to damage system components. e.g.; I noted a -12 F before my digital thermometer error’d out. And it seemed like it could have gone on for a few more degrees below that.

I noted it and recommended HVAC Tech check for deeper concerns. But it would be nice to know more from others here.

Duffy Cavanaugh
Solid Ground Home Inspection
Salem, Oregon

What were you measuring?

Sounds like a thermal reflection of the sky.
What kind of ‘digital thermometer’?