When frost builds up on your heat pump's outdoor unit, the system reverses refrigerant flow to melt it, which creates a loud whoosh sound. Heat strips keep your house warm while the reversed refrigerant pulls heat from inside to defrost the coil.
When it goes into defrost, it has to go on that cycle. It actually reverses the flow of refrigerant. So instead of being in heat mode, it switches to AC mode and it turns on those heat strips so you don't get cold in the house. So the reason it makes that loud noise is all that gas and refrigerant is saying back the other way. So that is the noise you're hearing. A lot of folks don't know that and probably don't need to know that, but it's actually going into AC mode because it's burning off the frost on the outdoor unit. So it's actually taking the heat that's in the house and blowing it back outside like it was. Like it was summertime. So that's how you melt that frost.
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