Every spring the question arises: When is it warm enough to run my air conditioner?

To answer the question I looked at the Installer’s Guide and the Owner’s Guide for my split system central air conditioner.  These documents say absolutely nothing about this subject.

So I turned to other sources for this information, including some good HVAC contractors that I know and trust.  The consensus is that it’s never a good idea to run your air conditioner when it’s 60 °F or colder outside, and it’s probably not a good idea to run it even when it’s 65 °F or colder.  At these cold temperatures the lubricating oil can be too viscous and not flow well enough to provide good lubrication.  Also the refrigerant itself can be sluggish and that puts more stress on the compressor.

Some AC’s are installed with a crankcase heater.  Its job is to use some electric resistance heat to keep everything warm enough to run safely.  But according to the contractors I know only about 5% of residential AC systems in the Chicagoland area have crankcase heaters installed, and only about 35% of commercial systems.  And even if you do have a crankcase heater it has to be on and functioning for a while before the can safely run the AC.  How long should the crankcase heater be on?  Somewhere between 1 and 24 hours.

If an air conditioner has a crankcase heater then I’m going to assume that it’s necessary and not just a trendy add-on.  And if the breaker to the air conditioner is shut off then I’m going to assume that it’s shut off in order to shut off power to the crankcase heater.  So I won’t run an air conditioner that has the breaker shut off, because the crankcase heater would need to be turned on for 24 hours to allow for safe operation of the AC.

During an inspection I don’t run the air conditioner if the overnight temperature dropped below 60 °F or if the current temperature is below 65 °F.  It’s just not worth the risk, especially since we go more than half the year without being able to test the AC anyway.  So real estate agents and especially real estate lawyers know how to handle the situation to protect buyers.

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