Last fall we got into some weather that was about 20 degrees while using the heat pumps. Got up at 3:00am to the smell of electrical smoke but everything was still running. Did not expect it to run at that temp. Upon investigating in the morning found the transfer switch had fried. Guessing the heat pump was pulling a bunch of current and the contacts may have been loose. Loose contacts is a known problem and should be checked every year. I went out and bought an inch pound screwdriver just for that reason.