During the heating season, my heat pump makes a “whooshing” sound and I feel cool air coming from the supply registers. Is that normal?
In one word, yes. In fact, it’s generally a sign that your heat pump is working exactly as it should.
Heat pumps warm your house by extracting heat from the outside air and then sending that heat into your home. When the outside air cools, the relative humidity increases and moisture in the air condenses on the heat exchanger. If the outside temperature is close to or below freezing, that moisture turns into ice. A heat exchanger that’s coated with ice can no longer work efficiently, so at that point your heat pump has to temporarily switch into defrost mode in order to get rid of the ice.
The “whooshing” sound you hear is made by a valve as it reverses your unit from the heating mode to the defrost mode. Essentially, it’s like your unit is briefly switching over from heating to air conditioning. This way the heat gets directed towards the outdoors, melting the ice on the heat exchangers, instead of into your house. As a result, cold air comes in through your supply registers rather than warm air.
Once the ice is gone, the heat pump will switch automatically back into heating mode. You shouldn’t turn your heat pump off from the thermostat while defrost mode is happening because it will only delay the removal of the ice. Not only does the ice build-up make your heat pump less efficient, if it’s not defrosted promptly, it can cause damage to the unit.
In addition to the whooshing noise and the cold air, when your heat pump is defrosting you may see steam rising from it or water running off. Again, these are both normal parts of the defrost cycle. The temperature in your home may drop a few degrees as well. But don’t worry—your heat pump is doing what it’s supposed to do and you’ll be warm and cozy again soon!