All homes that use a heat pump for heating also need a backup or auxiliary heat source. This is because there are occasions when the heat pump won’t be able to work effectively or can’t heat quickly enough. The auxiliary heat source also works to maintain the temperature in the home so that it doesn’t get too cold whenever the heat pump needs to defrost. Most new heat pump systems have electric heating elements that serve as the auxiliary heat source, but you can also choose to use a furnace as the backup source.

If you ever see that the screen on your thermostat reads “Aux” or “Aux Heat,” it simply means that the auxiliary heat source is currently running and heating your home. This typically isn’t something you need to worry about, but there are times when it can be a sign that your heat pump isn’t working properly or won’t turn on for some reason. Discover a few reasons that your thermostat will switch to auxiliary heating so that you can know when it’s normal or when it indicates a problem.

Outdoor Temperature Is Too Cold for Heat Pump to Work Effectively

Many modern cold-climate heat pumps can keep working in temperatures as low as -10 to even -22 degrees Fahrenheit, but some units may stop working effectively when the temperature is around zero degrees. Heat pumps don’t have a temperature sensor to know whether it is too cold for the unit to heat effectively. Instead, the thermostat continually monitors the performance of the heat pump by measuring how quickly the temperature increases.

Many heat pump systems are set up so that the heat pump always turns on first when the temperature drops below a set level and the thermostat calls for heat. In cold conditions, a heat pump may run almost constantly to ensure the home stays warm and the temperature stays consistent. However, if the heat pump runs for more than 15 to 30 minutes and the temperature doesn’t increase, the thermostat will typically signal the heat pump to shut off and switch over to auxiliary heat so that the home stays sufficiently warm. Once the auxiliary heat source has fully warmed the home, the thermostat will typically switch the heat pump back on and then only switch back to auxiliary heat if the temperature in the home starts to drop.

Some smart thermostats do have the ability to monitor the outdoor air temperature. In this case, the thermostat may always switch to auxiliary mode whenever the temperature outside is below a set level where the heat pump won’t work effectively or won’t provide any heating.

Additional Heating Is Required

Some systems are instead set up so that the thermostat will always switch to auxiliary heat when the temperature in the home is more than 3 to 5 degrees colder than it should be. This typically happens in situations when the thermostat is programmed for different temperatures at different times of the day, such as if you set it so the heat turns down at night and back up to normal in the morning.

Although heat pumps are extremely efficient, they tend to heat more slowly than furnaces and other electric heating options. A heat pump can usually maintain a consistent temperature in cold weather if the home is already warm, but it will sometimes take up to a few hours for it to raise the temperature by more than a few degrees. Auxiliary heating is less efficient and will use more energy, but it will also bring the temperature back up to where it should be in much less time. Once the house is up to temperature, the thermostat will exit auxiliary mode as long as the heat pump continues working effectively.

Heat Pump Is in Defrost Mode

Heat pumps work by pulling heat energy from the air outside and sending it into the building, and this process can only work if the refrigerant flowing through the heat pump is colder than the outdoor air temperature. Since the refrigerant has to be so cold for this heat transfer process to work, frost and ice always begin forming on the coil in the heat pump as it runs. If too much ice forms on the coil, the process will no longer work since the ice will insulate the coil from the surrounding air.

The way that heat pumps overcome this issue is by using sensors to monitor the coil temperature and switching to defrost mode whenever the sensor detects the coil is frozen. In temperatures around 40 degrees, the unit may only need to defrost every few hours. In colder weather, the defrost cycle could run every 30 to 45 minutes.

When the defrost cycle runs, the reversing valve activates so that the heat pump runs in cooling mode. This means that, instead of transferring heat inside from outside, the refrigerant pulls heat from inside so that it can send hot refrigerant flowing through the coil outside. The temperature inside would quickly start dropping if the thermostat didn’t switch to auxiliary mode during the defrost cycle. In most situations, the coil will fully thaw within 15 minutes at the most. Once the sensor detects that the coil has heated up to a set temperature, it will signal the defrost cycle to end and the thermostat will exit auxiliary mode.

How to Know When Auxiliary Mode Is a Sign of a Problem

Unless the temperature is extremely cold, your thermostat should typically never stay in auxiliary heat mode for all that long. If the outdoor temperature isn’t that cold and the thermostat shows it’s in auxiliary mode for more than an hour or two, it generally indicates that your heat pump has some issue and isn’t working effectively or won’t run at all.

In this situation, the first thing you should do is go outside and check if the heat pump is running. You also want to make sure that there isn’t lots of snow piled up around the unit since this will prevent it from working effectively. If the thermostat shows auxiliary mode and your heat pump is running, it indicates that the unit is currently in defrost mode. When the unit is defrosting, you’ll usually see lots of ice or frost inside and possibly even outside the unit.

If it keeps running and is still frozen after a few hours, you’ll need to have it inspected as this indicates the defrost cycle isn’t working as it should. The most common reason that a heat pump won’t defrost is due to a broken, stuck, or malfunctioning reversing valve. If your heat pump won’t turn on at all, it could be that it has an electrical issue or that one of the motors has burnt out. If your thermostat frequently switches to auxiliary mode, it could also be that the heat pump keeps shutting off because it overheats or has some other issue.

Healthy Home Heating & Cooling LLC is the top choice if you need any heat pump services in the De Pere area. We offer professional heating and air conditioning repairs and maintenance, and our team can also handle your HVAC installation and repair needs. For more information or to schedule a heat pump inspection, give us a call today.

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