During thunderstorm our home had an instant power loss. Then one of the hvac's outside machines started roaring sound that was audible from inside of house, which I have rarely heard even when hvac is set to work hard. After some time and also some operations on the hvac's in-room panels, the sound stopped.

Question: What could cause such a roar, esp. when the hvac was NOT set to "work hard" (e.g. extreme target temperature was not set)?

Order of events:

  1. (Added on 20230831) The a/c in question was actively cooling the room (I verified just now looking at the smart system's log).
  2. In the midst of thunderstorm, whole house instantly lost a power, it came back on within less than a second. I immediately started hearing roaring sound from one of the hvac's outside machine (I later identified only one of them was making the noise, by turning off hvacs one by one).
    • (Added on 20230831) During the roar, the a/c was NOT cooling the room (at least that's what my smart system (Home Assistant) recognized) despite the target temperature was lower than the measurement right before this moment so it should've continued to cool..
  3. Maybe 10 minutes or so passed, I turned off the power of hvac systems (via smartphone app, one by one as mentioned earlier). Roaring sound stopped when turning off one of them.
  4. I turned hvac systems back on (no more roar heard).
  5. I went to one of the panels of thermostat, pressed some buttons manually. Commands didn't seem to get activated by that, so I wiggled the panel a bit too (this panel is not firmly attached to the wall. I don't know if this is related at all).
  • 1
    I would think the rapid change in the power. The unit did not have time to come to a stop before it was back getting full power, or a power surge.
    – crip659
    Commented Aug 29, 2023 at 21:49

2 Answers 2


Momentary power outages can cause some scroll compressors to run backwards while powered. What happens is that in normal operation when the compressor is running the high pressure side has a much higher pressure than the low pressure side. When the tstat signals to the contactor to open and so shuts off the power to the compressor, the scroll compressor briefly runs backwards and then stops.

But if the power to the compressor is turned on again when the compressor is spinning backwards, the compressor will keep running in that direction under power making a continuous awful racket.

When a scroll compressor runs backwards under power it makes a continuous racket and does not pump refrigerant. As a result of not pumping refrigerant the temperature inside the conditioned space does not get low enough to cause the thermostat to shut the compressor off.

The thermostat keeps sending the signal to the contactors to stay closed and the compressor runs backwards (making a continuous irritating nosise) until it overheats (the compressor gets so hot that you cannot leave your hand on it). Finally the temperature inside the compressor gets so high that the internal temp sensor shuts off the compressor. It can take two hours or more to cool to the point it will run properly.

To cool the compressor quickly, run a stream of water over it from a hose until it is cooled down. Then it should start and run correctly.

  • Sounds more plausible than the "what if" failures I surmised. +1 Commented Sep 2, 2023 at 0:45
  • Our new system (3 yo) is much more resistant to this faulty behavior than our older system. Years ago we were plagued by momentary outages. I would work at a computer protected by a UPS and it would click at these momentary outages and then I would find the house heating up. When I got near the window near the condensing unot I realized it was running but making a frightful continuous racket.. Our new 4 ton unit doesn't do this so the mfgrs have figured out how to prevent this. If your unit is doing this shut it off before it overheats, let it sit for 3 min and then it will start correctly. Commented Sep 2, 2023 at 0:58

The outside part of "HVACs" have a compressor. When the compressor is running normally, it builds up a lot of pressure on the output side. When power is turned off to the compressor, this pressure imbalance must dissipate or the compressor motor won't be able to push through the leftover high pressure and restart. There is a pressure balance switch that allows the compressor motor to restart when the pressure has dissipated. There is also a start system, possibly with a start switch, that engages a start winding on the motor to give the motor extra power at startup and to turn the correct direction.

A theory: The very small power outage did not give the motor enough time to fully turn off and when power was restored, it wanted to keep spinning. Or the pressure had not dissipated enough yet the pressure switch had switched to start mode. However it had slowed down enough to require the starting winding to kick in to build back up to speed, but the winding needs stopped rotor to reset the start winding.

However it happened, I surmise that the motor then went to a locked rotor state, still powered but not able to spin and not able to fight the high pressure.

Locked-rotor motors can make a lot of noise. You may want a technician to check the system out for why it didn't handle the very temporary loss of power smoothly, and see if damage was done.

  • "compressor motor won't be able to push through the extra pressure " Where does this pressure come from, the compressor has been slowed or stopped?
    – Gil
    Commented Aug 30, 2023 at 18:58
  • I'm no HVAC guru, @Gil, but as best I understand from videos and schematics, the pressure comes from the compressor, and remains for a short time after the compressor turns off. Eventually, the pressure will push on through the rest of the HVAC lines. Once the pressure equalizes enough, the pressure difference switch clicks and the compressor could start again. Commented Aug 31, 2023 at 11:38
  • A good compressor acts as a check valve and will not allow back leakage from high side to low side. Generally the Cap tube or Expansion device allows the high side to decompress into the low side until the pressures are equal when the compressor is off. The compressor can start before equilibrium is reached, that difference is not fixed.. Normally a shut off time is allocated for equalization. There are protections devices that will turn the compressor off after a short period of time if it does not start. The extra pressure is what I was questioning.
    – Gil
    Commented Sep 1, 2023 at 23:32
  • That all makes sense, @Gil. Probably less than knowledgeable wording on my part. I was thinking of the high pressure level generated normally by the compressor, which has to dissipate somewhat before the compressor can start again. Not "extra" I guess then just "normally high". Commented Sep 2, 2023 at 0:43

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