In the Seattle area, we’re usually spared the worst of winter temperatures, as opposed to…say…our neighbors in Canada to the north. That’s not to suggest it doesn’t get cold at times. Anyone who has spent a winter month in or near the Emerald City dealing with a seemingly never-ending drizzle of frigid rain can attest that it’s the type of cold that seems to permeate our bones. When it gets cold, you may notice on your thermostat that your heat pump HVAC system has switched to auxiliary heat, usually indicated on your thermostat as “Aux Heat”.
The good news is, there’s nothing wrong with your thermostat or system. Aux heat is simply your HVAC system assisting your heat pump in delivering warm air via the unit’s electric resistance heating strips found within your air handler or backup furnace (whichever option your system has). So why does that happen?
Heat pumps work by extracting heat from the outside air and sending it inside your home. The temperature outside might be (as one example) 55 degrees and you want the interior of your house to be 70 degrees. Even though the air outside is colder than 70 degrees, it still contains heat which your heat pump can separate from the air and transfer into your home. Think of a heat pump as a device for extracting and transferring heat rather than generating heat.
But when temperatures drop, there’s less heat available in the outside air to extract and the heat pump alone may not be enough to warm your house. It’s at those times when your system will switch to auxiliary heat out of necessity (with a modern system, your thermostat will do this automatically). Aux heat is a combination of the heat pump transferring heat from outside and being assisted by your HVAC unit generating its own heat.
In longer periods (several hours or days) of extremely cold temperatures you may want to manually set your thermostat to “EM Heat” or emergency heat, since most thermostats will not do this automatically. This bypasses the heat pump entirely and only uses your unit’s internal heat generating system (again, resistance heating strips or a furnace). For most heat pump HVAC systems, temperatures below 20 degrees are too cold for the heat pump to extract any significant heat from the outside air, rendering it ineffective. EM heat effectively tells your system “It’s too cold for the heat pump” so it will only utilize its internal heat-generating system.
