Best way to inspect A/C

My own personal central A/C started acting up: freezing A-Coil, not cooling much etc. Some cool air exits the floor register but not enough to provide more than 2 C temperature difference between inside and outside after a number of hours. Fixed now…Problem was a my refrigerant level was low.

Here come the question: If this was a home inspection, how would I know the system is not working properly? After-all, I would not be there long enough to see water running, or ice forming, and anything greater than a 2 C temperature change. I would typically only be on site for 2-1/2 hours average.

The situation you describe is a latent defect. It would be hard to determine during the limited time (typically 3 hours) of a home inspection. My typical procedure is to note the status of the HVAC on the thermostat and take a picture. I will return it to these settings when I leave. In the summer months I turn the AC on and lower it to about 65. I verify operation of the outdoor unit and during my travels I check air temperature at each of the registers with my infrared thermometer. I am looking for air delivery (all habitable spaces should have a register) and temperature. Before I leave I run the heat to make sure the burner lights (I already checked air delivery). In the winter I do the opposite and run the heat first. Boilers are slightly more complicated and need some pre-checks before operating.

A similar situation to what you describe is an electric WH that has been off for weeks. There is simply not enough time to bring the water up to temperature. This is noted in the report.

Your contract should exclude latent defects (as do most Home Inspection laws).


I don’t really care about the temperature difference between the inside and outside of the house, but the difference between the intake and the registers (as the difference between the inside and outside will be greatly affected by how well the home is or is not insulated, types of windows, etc, not to mention how many times the agents and buyers leave the door open while I’m testing the system).

I take photos of the temperature coming out of the registers.

If the system has problems, it usually makes itself pretty apparent with-on 20-30 minutes.

And any AC older than 15 years gets a warning the system is older and may need repairs at any time, even if it’s working fine that day.

I also do a quick check of the refrigerant lines for worn or missing insulation, and the condensation lines for flow, signs of rust, etc.

I had a house yesterday, and I was getting plenty of cold air flowing. But the house itself wouldn’t cool down because it had uninsulated open beam ceilings in the living room, almost no insulation in the rest of the rooms, older wood windows, and the buyers keep leaving patio doors open the entire time.


Measure the temp at the return where the air is pulled into the system. Now measure the temp coming out of the supply vents. Many hvac guys are looking for 17-20 degree farenheit difference between them. Some may have a slightly lower tolerance like 15 degrees farenheit. If it seems like it is not working properly it may have an underlying issue and make note of it in your report

…being sure to use the appropriate measuring device (as most inspectors don’t) to get an accurate reading. You need to be measuring the temp of the actual airflow, NOT the register or duct materials themselves!!


Hey Jeffrey are you referring to the clip on style

Drew, if you are referring to a “wet bulb thermometer”, yes. Measuring temperature is a lot more than shooting a laser at it! How do you account for RH? Everything is “relative”!!

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Yep wet bulb is what I meant. Unfortunately in school we were advised to use a standard temp gun. I had my air condenser worked on last week and my hvac guy was using a wet bulb style to get his readings. (Wasn’t sure of the exact name.) I checked my readings against his and mine were 3 degrees higher showing how inaccurate a standard temp gun can be

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The laser cannot measure what it can’t “touch”, ie. air. The laser is reading whatever it is actually pointed on… register grate, ducting, carpet/flooring, etc.

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Can any of you guys recommend a good wet bulb thermometer? The two I have are a pain to get them to stay in place, i.e the magnets are lousy, and have one with a clip that is almost useless. Lost the two I used most and can’t seem to find where I got them. Thanks!

The laser on an infrared thermometer doesn’t measure anything. The laser is merely a pointer. Those thermometers were available long before somebody decided to add a laser. The unit measures temperature exactly the same way the infrared cameras do, it simply lacks the resolution or mechanics to present a picture. The laser only provides a general direction as the infrared thermometer is more accurate the closer you get to the source. This is indicated by the sensitivity diagram that is either on the unit or was supplied with the device.

The OP’s original question had to do with a latent defect. Home inspectors that try to diagnose the performance of AC systems have gone well beyond the SOP and may be setting themselves up for a lawsuit.

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You can forget all the wonderful advice above and simply grab the suction refrigerant line (the big one) and see if it is the temperature of a cold beer. Is it sweating? If it’s frozen there is nothing you can analyze until you get rid of the ice.

This assumes it freezes during the inspection, it may not.

This assumes it was frozen before you got to the inspection…

The OP’s original question had to do with a latent defect.

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You said it was a latent defect.

He said “Problem was a my refrigerant level was low.”

Taking Psychometric readings will tell you nothing about refrigerant levels. There are too many variables that produce the same outcome.

Any HI can grab the refrigerant lines and determine the status of the refrigerant charge (without gauges) if they were so inclined to learn. And it wouldn’t be a latent assessment outside of Home Inspection.

No, he said it was a latent defect, he just didn’t use those words. In fact you quoted him

You must not read well. He specifically asked how would he know there was a problem if he was not there long enough for it to occur.

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No he didn’t, and No I didn’t. How can he say it, and not say it in “so many words”.

Ok, now you need to read well.

Do you want to learn something or do you want to discuss Latent Vocabulary?

If you run the A/C (if it was off when you arrived) until you get the house into the 70’s and you grab hold of the refrigerant lines (knowing how an HVAC unit works) you can detect a low refrigerant charge before an HVAC guy could with gauges.

There is one unchanging law in HVAC Design, the refrigerant capacity of the unit must protect the compressor. You can feel this with your hand.

Want to know where I learned this?
Two years out of Northeastern University with a degree in HVAC Systems Design, I was working a project where a unit was very low on refrigerant (from the factory). So I got out all kinds of tools and test equipment to charge the unit by “Super-heat”. An old HVAC Service Tech came over and asked what I was doing and what did I need all that stuff for? He said “Let me charge it my way and you can check me when I’m done”. He grabbed a jug of refrigerant and just a refrigerant hose and charged the unit (his way). Well, he was right on…

Lesson learned; You can have all the education and tools you want, but you can still learn something new and out of the box (No matter how radical it may seem).

I got into HI because REA’s were tired of buying HVAC units for clients who purchased homes in the heating season and HI’s would not even start them. For Twenty-five years I inspected every HVAC unit on every Inspection. I never had to ever advertise, REA’s did it for me. I’m retired two years now and I just now got off the phone with a past client. Agents still call about every week. They take pictures, I tell them what the problem is.

I have HI’s from here that call me all the time and I teach them for free what I know. They just need to ask. If you want to talk “Latent” to me, it will only be how Latent Heat is screwing things up. If you want to argue about someones Latent problem when they probably don’t even know what Latent is, and never used the word, well this conversation ends here…


For a moron, you’re pretty high on yourself. How do you get your fat head in the door?

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Guys, you both have something to contribute, chill out :slight_smile: