2022-12-16
Yesterday, I discovered the defrost cycle on my new-ish portable heat pump. I thought that it might be broken, but it turns out I just had to trust it. Documenting the symptoms here in case anybody in the future has the same questions and finds this post (fat chance, I know…).
I was running the unit in heater mode. It had been working great for many days.
The outdoor temperature was around 45ºF with a relative humidity around 85%; the indoor temperature was around 63ºF. The past few nights have been a bit colder than usual (overnight lows in the high 30s F).
The unit started to blow air that was not as warm as usual. It had a hard time heating the room. The speed of the cold air coming out of the exhaust tube seemed lower than normal. After a few hours of this…
The compressor shut off, while the fan continued running, blowing cold air.
After several seconds, the fan stopped.
The compressor made its usual startup sounds, but then made a slow low whirring sound that increased in speed over 20-30 seconds and then leveled out.
The first couple of times this happened, I let it whir for a few minutes and then turned off the unit. Doing so immediately caused a wheezing sound, like pressure had been released somewhere in the system. If I then turned the unit back on, it would seem to work normally for some time (usually less than an hour) before repeating the strange behavior.
I just had to be patient and let it keep whirring. After maybe 10-15 minutes, it restarted again and then started working perfectly.
It turns out that heat pumps have something called a defrost cycle. When it’s particularly cold, frost can form on the surface of the outdoor-side coil, interfering with its heat-exchanging abilities. The defrost cycle runs the heat pump in reverse for several minutes, sending hot refrigerant to the the outdoor-side coil to melt the frost.
Unfortunately, this behavior doesn’t seem to be documented anywhere in the owner’s manual or the troubleshooting guide on the Whynter website. But the heat pump knows what it’s doing.