I have a portable air conditioner in my bedroom set to 68 degrees. On cooler days it brings the room down to 68 and the compressor shuts off and it runs in fan only mode. Once the temp in the room goes back up, the compressor comes back on and gets the room back to 68 and shuts off and goes to fan only mode again.

When it is 100 degrees out it is unable to cool to 68. The compressor runs and runs and doesn't seem to cycle on and off. It is able to keep the room at 72 which is fine. I assume this is okay as it has been working great. But I thought the compressor had to take a break and cycle on and off to let coils evaporate or something? Is this normal for a portable AC unit compressor to never cycle on hot days?

4 Answers 4


There is no problem with the unit running continuously without a break. The most wear occurs when the compressor starts up from being off. A unit that is sized to run continuously is actually more efficient than one that cycles on and off.

We have a slightly undersized Carrier central a/c unit (12 SEER) which is 26 years old, but still works as well as could be expected for one that age. The blower fan motor is original and the compressor is original. The original fan in the condensing unit failed and was replaced 2 years ago. On very hot summer days (>100 F) it turns on at say 11:00 am and runs continuously until 5 pm.


It will cycle off when the room temperature reaches your set temperature. On hot days, though as you've noticed, it can't keep up, so it keeps running. That is normal, if it only happens on very hot days. Failure to keep up on hot (maybe 90 degrees) could indicate a problem with the unit like a refrigerant leak.

You may want to consider getting a larger unit if it has trouble keeping up for many days throughout the summer. Alternatively, set the desired temperature a bit higher, if you can tolerate it, so it can cycle off. Running continuously is not great for the unit.

  • Thanks. Limited budget keeps me from upgrading but I was always under the impression that the system (portable A/C in my case - like a window unit accept the whole unit is in the bedroom and vents the warm air thru the window) had to rest so the coils don't freeze?
    – user71259
    Jun 28, 2017 at 3:37
  • So long as it's not running the condenser too cold, the coils don't freeze. In many cases, the condenser freezing is an early sign of failure (low refrigerant - with a portable A/C normally not serviceable, with a whole-house unit you need to have a leak found & fixed and recharge.) As an air conditioner (not freezer) the coils should be running somewhat above freezing, so water merely condenses and drips off, rather than freezing up and blocking the fins.
    – Ecnerwal
    Jun 28, 2017 at 13:21

What about a sensor failure? I couldn't find any leaks. Top just freezes. I'm wondering if the room temperature sensor or the pipe temp sensors fail?


These units are not designed to be running 100% of the time. It will lead to premature failure and probably will not be covered by warranty. So

  • Either get a larger unit, your installer should have spec'd it properly in the first place.
  • Set the thermostat to 73 or 74, so that the compressor does get a break.

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