Heat pumps are an extremely energy-efficient and cost-effective option for both cooling and heating your home. During the summer, a heat pump functions exactly like a central air conditioner by using a refrigerant to capture and remove heat from the home to lower the indoor temperature. In the colder months, the heat pump reverses the direction that the refrigerant flows so that it absorbs heat from outside and transfers it inside to raise the building’s temperature. In colder northern climates like New Jersey, you may sometimes see that your thermostat shows your heat pump is running on auxiliary heating. To understand why this happens, here is everything you need to know about auxiliary mode and what it means for your heat pump.
Heat pumps are much more energy efficient than other heating options like gas furnaces or electric heaters. However, the fact that the units heat by absorbing heat energy from the air outside the building means that they don’t always work that well when the outdoor air temperature drops too low. The unit will still produce plenty of heat when the temperatures are above 40 degrees, but the amount of heat it can produce and its energy efficiency decrease as the temperature drops.
When this happens, the heat pump often can’t produce enough heat to maintain a consistent indoor temperature. For this reason, most HVAC systems with a heat pump also have a secondary heat source that can be used whenever the heat pump isn’t providing sufficient heating. This secondary source is usually either an electric resistance heater or a gas furnace.
If your thermostat is reading “AUX” or “AUX Heat” when your heat pump is running in heating mode, it means that the system has switched over to the secondary heat source for some reason. This is nothing to worry about, but it does mean that your heating costs will be higher whenever the system is relying on the secondary source instead of the heat pump.
There are typically three different reasons that your thermostat will switch over from the heat pump to auxiliary heating. The first reason is that the outdoor air temperature has dropped below 35 degrees. When this happens, the heat pump will produce very little to no heat. As a result, the thermostat will signal the heat pump to shut down and switch over to auxiliary heating to ensure the building stays warm.
Most heat pumps are also programmed so that the system will automatically switch to auxiliary heat whenever the indoor temperature is too far below what the thermostat is set to. The secondary heat source will almost always raise the temperature more quickly than the heat pump, and the system switches to auxiliary mode so that the building warms up to the set temperature more quickly.
Most systems are programmed with a 3-degree threshold or trigger point. If the indoor temperature ever drops to 3 or more degrees below what the thermostat is set to, it will automatically trigger the system to switch to auxiliary heating. Once the temperature goes back up, the system will then switch back to running off the heat pump as long as the temperature difference remains under 3 degrees.
The other instance where the thermostat will show it is in auxiliary heating is when the heat pump is currently defrosting. When the outdoor temperature is colder, the heat pump will occasionally need to run in defrost mode to prevent it from freezing up. When defrosting, the heat pump will switch over to run in cooling mode for a short time. This allows hot refrigerant to circulate through the outdoor unit to warm it up and keep it from freezing.
If the thermostat didn’t switch to auxiliary mode when the heat pump is defrosting, your HVAC system would start to blow out cold air since the heat pump is running in cooling mode. Switching over to auxiliary ensures that the system continues to produce heat while the unit is defrosting. As soon as the defrost cycle finishes, the system will then automatically switch back to normal heating.
If your thermostat is showing AUX heat in any situation other than these three specific instances, it typically means there is some issue that is preventing the heat pump from working correctly. In this case, you will want to have the unit inspected by a certified boiler repair technician as soon as possible. If not, your heating bills will increase by quite a bit since the system will only ever rely on the secondary heat source.