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We have hit record low temps. High temps only reaching 17 for several days. I have recently installed a nest thermostat, about a month prior to the cold snap, and everything is working fine. I noticed the other day the outside compressor making a very loud noise, like a huge air compressor/ jackhammer. I turned off the heat pump as quickly as I could. I sent the video to the hvac guy and he said to turn it to emergency heat. There is no setting for emergency heat only aux heat.Could it be the temp outside is just too cold? Could the nest thermostat cause the sound? Here is a picture of how the previous thermostat was wired. enter image description here

here is a pic of how the nest is wired. enter image description here

  • Aux heat and Emergency heat are sometimes used interchangeably - is that what you're asking? – JPhi1618 Jan 12 '18 at 16:53
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When it gets extremely cold outside air to air heat pumps don't work very well.

That is why they are usually set up with auxiliary resistive heating coils. Referring to it as "emergency" heat is a little over the top since you will probably have to use it often in the deep winter.

Just turn the auxiliay heeting on and see if your compressor quiets down.

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    "Emergency" probably refers to the effect on your electric bill. – Harper Jan 12 '18 at 17:50
  • Heat pumps are not effective at freezing temperatures. Normally they have resistance heaters that come on then. – blacksmith37 Jan 12 '18 at 19:15
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Emergency heat and auxiliary heat both come from the same source, usually electric strip heat. Emergency heat is activated at the thermostat, a switch or button. Auxiliary heat should be activated by an outdoor temperature sensor when the temperature drops below a pre determined temperature usually around 45 or so. This is to prevent using the expensive strip heat when the temperature is mild. I have not seen too many wired properly in my area though. According to your picture you have auxiliary heat. I am not sure if your thermostat allows for emergency heat or not. It should say in the manual. As for the noise, hard to tell by your description. It may be something is caught in the fan blade. Try spinning it by hand. A compressor won’t typically make noise like you described.

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