A/C units

Why do you not operate central a/c units when the temperature is below 65 degrees outside? Also will they even come on when it is 32 degrees outside?


Theoretically, one could test air conditioner at any temperature below 65, BUT, one would have to have sufficient test equipment and knowledge of thermodynamics (Bernoulli’s Equation etc) to properly test due to the temperature being below the heat exchanger design limits and things like that; I really don’t think we want to get into that though. Other than that, it’s just a guideline I guess.

Some will not come on, my 2001 model Bryant/Carrier heat pump units have outdoor sensors that disable the cooling mode when 55 or less.

I ran three units on a foreclosed house yesterday in cold weather for a few minutes just to make sure the refrigerant levels were not zero and that the contactor, fan and compressor would at least come on. If all of the refrigerant has leaked out a sensor will keep the compressor off. My client said he would rather know at least something about the three A/C units than zilch.

I had a client once that is a professional HVAC inspector, guess what he did in really cold weather? Yep, he turned on the A/C system for a minute just to see what it would do. I just wish he had not done it while I was standing over the unit checking the inside of the disconnect box…

Its normally 60* not 65*

and you been a home inspector since when? hows your liability insurance?


Be NICE, I know that it has been a LONG old winter!

I believe that the 60 degree limitation has a couple of reasons. First and foremost, some manufacturers state specifically that their units should not be operated when outdoor temperatures are below 60 because “the unit will cycle excessively and damage the fan cycling switch.” I have also read that the cooling coil can frost up, which might not be a problem as long as the defrost goes the right way.

But, I have also heard that the compressor could be damaged if its heater is not working properly, and possibly even if it is. In a cooled state, the Freon could mix with the oil in the compressor. The heater helps keep the oil warm to keep the Freon as a gas. If it is in liquid form, it can slug and damage the compressor.

So, I guess the bottom line is you might be able to run a compressor with outdoor temps below 60 for a VERY short time. However, I would not recommend it if the unit is less than 5 years old so that you don’t screw up someone’s warranty. I would especially not do it for a buyer client without the homeowner’s permission because you know who is going to pay for the repairs if damage is done. And finally, you might not even be able to run the unit if it has protective controls installed. If the unit doesn’t work because of the controls, some inspectors might say that the unit needs repair when it doesn’t.

I just tell the client that the condensing unit oil cannot circulate properly and will pool in cooler temperatures and if it is operated will damage the unit at temps below 60 F. Also offer to come back at no charge if they want me to inspect it when it is cooler.