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I am going to install some NEST thermostats. The directions say that I should turn the system off by turning off the power to it. However, I'm worried that an abrupt power cut will be damaging. Do I just shut off power to it, or do I need to do some sort of "soft" shutdown procedure to bring it down and back up?

What labels should I look for in the circuit breaker panel to turn the unit off? I see water heater and the like, but didn't see a label for "central heat".

  • Do you "shut it down gracefully" when there's a power outage? If your stuff breaks due to power loss there's something wrong with it. Find the breaker, switch it off, and be done with it. – apocalysque Feb 2 at 8:24
  • There are lots of things that can survive an abrupt power cut but do much better if properly shut down. It's a legitimate question. – Reid Feb 10 at 19:27
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Very few HVAC systems aren't able to handle abrupt power loss. I've flipped the switches on dozens of systems hundreds of times to change filters and perform other service. The gas valve will close and the control boards have algorithms to recover.

It may feel nicer to shut them down gently, but there's no good reason to worry about it. After all, power does fail at times, and when have you heard of an HVAC disaster as a result?

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    Although I kind of agree I don't like killing power on high efficiency furnaces because this leaves the heat in the exchanger but I have not seen a failure from this happening + – Ed Beal Feb 1 at 23:23
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    Ideally you wait until it finishes a cycle. But whatever yo, +1 – Mazura Feb 2 at 3:21
  • Thanks for the feedback, Ed. What's the concern with heat in the exchanger? Simple efficiency or metal fatigue issues? – isherwood Feb 3 at 3:26
  • @isherwood there's about a 30 second fan off delay (varies by manufacturer) to distribute the remaining heat after the burner de-energizes. Some units, usually fire tube boilers, have a post purge on the ventor motor to clear the combustion chamber of combustion products. I still wouldn't give it a second thought and shut her down. IMHO the door switch is a very convenient service disconnect for troubleshooting and restarting cycles. – Joe Fala Mar 5 at 7:10
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I would start by turning the thermostat off, including the fan setting to "off" and not "auto". The HVAC breaker should be a 240v double pole breaker. Most homes have four of those. One for the water heater, range, dryer and the HVAC. Turn the breaker to the HVAC off. If you have a heat pump, there will be a shut off near it outside (probably mounted to the wall). You can shut that down as well.

The label for central heat might say: HVAC, Heat Pump, or Pump. If you can identify the other three breakers I mentioned, you can determine the correct one by process of elimination. You can turn only the fan on at the thermostat and check it when you turn the breaker off to make sure it is the correct one.

  • In my area the furnace is typically on a single 15A breaker and the A/C compressor is on a double 15 or 20. – isherwood Feb 3 at 15:02

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