That “whoosh” sound isn’t necessarily a sign that anything is wrong with your heat pump if it’s happening during the winter.
As it gets colder the relative humidity in the air goes up, resulting in condensation forming on the heat exchanger in your heat pump unit outside. If it’s cold enough that moisture will freeze, which will reduce the heat pump’s efficiency.
Thankfully, your heat pump is good at creating heat, so it can defrost itself. To do this it has to temporarily reverse a valve and send the heated air back towards the outside part of your HVAC system.
The whooshing sound is that valve reversing to defrost the heat exchanger, which essentially means entering cooling mode temporarily. The refrigerant will run backwards in this mode, so the heat pump will essentially be pulling the hot air out of your home.
Some heat pumps have a supplemental electric heating element that will take over heating during the defrost. If it doesn’t have a secondary heating element the system may just turn off the blower temporarily.
You can check to make sure this is what’s happening by putting your hand next to one of your supply registers (air vents) the next time you hear it. You may feel a blast of cool air before the supplemental heat kicks on.
For most units the system will run defrost until the outdoor coils reach 57 degrees.
If you are alarmed by the whoosh and go outside to check on your heat pump, you may notice steam coming out of the unit or water draining off it. This is normal since it’s steam from the frost being rapidly melted off the heat exchanger.
Try not to panic when you hear the whoosh and don’t turn off your thermostat. If you do turn it off it won’t be able to finish defrosting and the heat pump will take longer to heat back up when you turn it on again.
Every heat pump should have a “frost thermostat” attached to the bottom of the coil in the outdoor unit. When that thermostat reaches freezing the heat pump should enter defrost mode. How long or frequently the defrost cycle runs depends on the unit. Some units will engage the defrost cycle ever 30 or 60 minutes when the conditions are met.
How long defrosting takes depends on the performance of the unit and the temperature outside. Some may take as little as 30 seconds while others may take three or four minutes. If your system is in defrost mode for longer than that you may want to schedule a HVAC maintenance check.
The defrost function can potentially malfunction.
Some older heat pumps are known to enter defrost cycles when there’s no actual frost present. You may be able to diagnose this yourself by visually inspecting the outdoor unit if your heat pump seems to be entering defrost mode frequently when it’s not cold enough to justify it. If you don’t see any frost on the unit and no water runoff or steam from the defrosting it may be indicative of a unit that’s entering defrost mode when it’s unnecessary.
The other somewhat common defrost malfunction is that it doesn’t kick on or won’t run long enough. You can also check this yourself by examining your outdoor unit. If it resembles a big metal ice cube and your heat pump is no longer operating, it’s a sign the defrost cycle isn’t functioning properly.
Some heat pump problems that can lead to ice buildup include:
Home and business owners in Maryland, Virginia and the Washington, DC area enjoy relatively mild winters – most of the time. There may be a few months each year when heat pump owners will hear the sometimes disconcerting and startling whooshing noise from a defrost cycle. If you’re hearing the whooshing when it’s nowhere near freezing outside you may want to schedule HVAC service.
If you haven’t had your HVAC serviced in a while or you’d like to learn more about getting regularly scheduled maintenance and discounted repairs through our Vito VIP Club give us a call at (800) 438-8486.
If you have a heating emergency, please don’t hesitate to call our 24/7 emergency number at (866) 211 8536.