This question is related to a previous question of mine about what seemed to be a voltage drop after a night of heavy snowfall.

A couple of weeks ago, heavy snowfall resulted in what appeared to be a voltage drop in my house. I am no expert (so excuse my probable lack of correct use of terms), but I believe one out of three electrical phases was dead / missing (that's what my electrician tells me). Looking at my fuse box I could see that it reported ~115v and not ~230v as it should. This caused a variety of "interesting" results on our electrical equipment.

One piece of equipment which just acted in a not normal fashion was our heat pump. When I woke up the morning, the heat pump did not run (but it's lights were on indicating power).

After the power came back, and ever since, the heat pump has acted somewhat strange. It reports errors and simply just works irregularly (runs for some time, suddenly stops, goes through defrost cycle even though there are 5C - 10C degrees outside). I contacted a service company which told me that one or two of the heat pump PCB's most likely had been damaged and most likely needed to be replaced.

Is it plausible that the recent incident have caused damage to our heat pump, but only to some extent? Or is this simply a coincidence which is a result of "wear and tear"?

How can a reduced amount of power able to result in damage to a heat pump which results in irregular behavior? We are after all talking about reduced voltage, and not increased voltage.

  • lack of design specs can cause oversights like this. resulting in lack of fault detection/protection/verification. ask tech support if it will fail next time , of a power failure on 1 phase, either fault,fused open.or ground short Nov 21, 2016 at 11:12

2 Answers 2


If you do have 3 phase power loss of 1 phase cause what is called "single phasing". Sometimes a motor can run with the loss of 1 leg but it may start in the wrong direction and another time start in the correct direction. The motor will draw more than normal current on the working legs and usually overheats. With a heat pump single phasing can damage the compressor by starting in the wrong direction.


Sometimes we experience momentary power outages (fraction of a second)--lights blink, battery back-up to computer clicks on and then off. If the a/c (scroll) compressor was running at the time, I found it changed to running backwards under power. This overheats it leading to overtemp shutdown. The timer in the thermostat was supposed to prevent this 'short cycling', but didn't. I had a 'delay on break' timer installed in the condensing unit, but that didn't stop it.

Possibly this was caused by only one 120 V leg of the 240 V going out and the timers were on the other leg so I have switched the breaker to that circuit to the other leg. Waiting to see if that fixes it.

  • 1
    So far the backwards runs of the scroll compressor have not ruined it. It has been in service for 25 years. I checked the voltage on the incoming lines inside the panel 123 V to neutral on one side and 122 V to neutral on the other. However, there was a light load at the time. I wonder if under balanced heavy load a greater difference would develop. A possibility is damage to one leg in my service drop, i.e., between the pole and my house. Ten or 15 years ago I was trimming a tree over the line and cut the insulation--flash and bang teeth burned off saw, but power co said conductor OK. Nov 22, 2016 at 12:02
  • I have not seen a single phase motor run backwards with the loss of 1 Leg I seriously doubt the motor would have the electromotive force to run at all because single phase motors connect the field winding on only the hot legs if you remove one of the legs this is the same an a light bulb with only 1 wire going to it (it wont light).
    – Ed Beal
    Nov 22, 2016 at 13:51
  • I have been told by a/c techs that scroll compressors can run backwards if they are powered up against a high pressure. This usually comes from short cycling when the vanes of the scroll are being turned backwards by a pressure difference. I know that mine makes an awful clatter when in this state and it doesn't cool the air. A technician told me that piston compressors can also run backwards but that they still pump freon when they do and they do not overheat. My thought is that scroll compressors turn backwards for a short time after shutoff and so when powered they can run backwards. Nov 22, 2016 at 15:53
  • Discussion of scroll compressors running backwards after a momentary power outage: hvac-talk.com/vbb/… Nov 22, 2016 at 15:56
  • A power bump is much different than a lost leg or phase.
    – Ed Beal
    Nov 22, 2016 at 16:02

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