What is the minimum temperature that you inspect AC unit?

Hi, my name is (Jamie) Hyoenghoon and this is the first time to write the topic on a forum. I’m still studying the courses and having a question in regards to the minimum temperature to inspect the AC unit.

I learned that the outside temp of 65F (18.3degree C) is the comfortable level to run the AC due to the possible damage to the compressor in the condenser unit. (If this is incorrect, please revise me).

Here in Calgary, Canada, the typical temperature is pretty low except for the summer.
If I inspect the house from the early morning which is almost always below 18.3C and if the temperature rises to that level at around 1~3 pm, should I wait, turn on and inspect the AC unit? I don’t want to break the AC unit as well as I don’t want to be regarded as a negligent inspector.

I just want to have consistency before I start inspecting any AC units.
Thank you for reviewing my question.

I will run them at around 60F. Whatever you do, spell it out in your agreement and be consistent about it.

1 Like

Thank you for your quick reply. Just one more further question for you. Would you run the AC if the outside temp is around 40F in the morning and 60F at around 1-2 pm? I’m kinda confusing at this point with the significant temperature variation during the inspection day. Or, it would be really appreciated if you could let me know how you reflect this on your agreement.

My comment
“If the outside temperature has not been at least 60 degrees for the previous 24 hours, an air conditioning system in the cooling mode cannot be checked without the possibility of damage to the compressor. In this situation, it is suggested that the present owner of the property warrant the operational status of the unit on a one time start up and cool down basis when warmer weather permits.”

Yes, I would test it in this circumstance.

Your agreement should link to your standards of practice. Put a statement in your standards of practice as follows…We do not test outdoor AC units when the temperature is below 60F due to potential damage to the equipment.

Keep in mind, you should still “inspect” the AC.


Hi Roy, Thanks for your comments.
Here in Calgary, the annual temperature is pretty low. So, if we wait the day which is at least 60 F for the previous 24 hours, we may only have a chance to run the AC unit for about 10~20 days per year. Even on hot summer days, the morning temperature is only about 50 F.
But, thanks for the idea of asking the present owner for the status of the unit. It should be much better than not saying anything due to the weather limitations.

So, just to see how much disagreement I can generate… When I posed this question to an HVAC guy, his response was: It really doesn’t matter. If you think about it, heat pump outside units run during cold weather, and the only difference is the system is running in reverse, but it’s still running the same regardless of outside temperature.


Very true for Heat-Pumps. If the system is only AC, then just to CYA it is best to stick with the 60 or 65 and be consistent. Because if it does not work as expected and you check it or “if it fails in Anyway” when tested when temps were less, it won’t matter why it is not working now, it will be You that will be blamed (by the seller) for it no longer functioning now.

*I have been told by more than one HVAC guy that modern (less than 25 years old) units are stupid proof (have accumulators) and will not be damaged by operating in cold weather, but I still don’t want the lability.


Enough said and I don’t take chances. Never had a client question it.


Although a hvac company might run an A/C unit even in the low 40’s … Your SoP’s will tell you NOT to do it below 60’s for the preceeding 24 hrs or something similar. Follow your SoP

1 Like

For me…65°F. Regardless of whether it’s a Heat Pump or Split-system.
My report plainly states that the cooling system has not been operated during the inspection due to the outdoor temperature.
As of this date (19 years) - I have not had to pay to repair or replace a homeowner’s HVAC system.


There are a lot of variables in this conversation: if you need to ask this question, you should probably go with the 60° rule of thumb.

Heat pumps have an accumulator, air conditioners do not. The accumulator stores refrigerant that would otherwise flood back on the compressor under low ambient conditions. It also collects the oil that was flushed out of the compressor on a cold startup, so it can be returned at a slower rate through a small orifice at the bottom of the accumulator.

It depends on the type of compressor you have. Scroll compressor can handle testing at any condition as long as you don’t run it the entire time you’re on your inspection in the wintertime.

As for the question of wide variations of ambient temperature before testing: if the duration of cold weather has been a long time since it was last run, or if the compressor doesn’t have a heater, you should leave it alone. The heater can be a belly band or an immersion heater in the crankcase of the compressor, which keeps the refrigerant from being absorbed into the compressor oil. If it was just cold last night, you’re fine running it. Recommend that you turn on the heater and raise the temperature in the house and test that first. Then you have a significant load for a short period of time that will help prevent any liquid slugging of the refrigerant on the compressor.

As for air conditioners with accumulators: I know of no reason to have an accumulator on a unit less than 25 years old because most of the compressors younger than 25 have scroll compressors which can handle anything without the need of an accumulator. Maybe something in your regional area.

Mark; a split system is a two-part system, not just an air conditioner. Both heat pumps and air-conditioners can be split systems, otherwise they are a package unit.

Note: when you say “run the unit”, just how long is that?
Hopefully nobody tries to lower the air temperature in the building during the wintertime! All you have to do is bump the unit and see if the fans and compressor run, you don’t need to take any dam delta-t at the registers with your IR thermometer! If you are doing this, even at 60° with no load in the house, you can damage the compressor on an air conditioner. All it takes is a dirty filter or dirty evaporator coil, and you will slug the compressor. The key here is indoor ambient temperatures. That is the load that causes the refrigerant to change state. No load, no change of state, compressor slugging will occur. If The heat is not on in a vacant house and it was really cold the previous night, it is most likely that the indoor air temperature is below thermostat set point and therefore no load.

There are air-conditioners, primarily in commercial settings that have low ambient controls on them which turns off the outdoor fan and prevents the drop of refrigeration pressures, thus temperature in the system.These systems can be run anytime. They have been designed to do so. Depending on your geographical location, you may or may not see them in a residential setting. Also variable speed units (units that have fan and compressor speed control) can also be run.

Thank you for your explanation.

Thank you all for your inputs! It accomplished what I was hoping for… informational dialog which could help those out there who were/are unsure. Knowing the why is as important as knowing the what.

I will add one last reason we don’t run the systems below 60 degrees: Some newer systems have sensors which will not allow the system to run when it’s too cold to safely operate. So if you did try to test a system when its “too cold” and it doesn’t run, is the system not functioning correctly(malfunctioning) or is it working properly(temperature sensor preventing system from running)? You don’t know.

So… with all the great information above, follow the SoP.

1 Like