Why Does My Heat Pump Thermostat Say AUX?

When your heat pump thermostat displays “AUX” or “AUX heat,” it indicates that your heating system has switched to its auxiliary or backup heat source. This typically occurs when the outdoor temperature is too cold for the heat pump to efficiently extract enough heat, or if the heat pump has iced over and needs to defrost. The auxiliary heat source can be either an electric resistance coil heater or a gas furnace.

Understanding the Auxiliary Heat Source

Electric Resistance Coil Heaters

Electric resistance coil heaters are installed directly into the home’s ductwork and are a cost-effective auxiliary heat source. However, they are less energy-efficient compared to other options, as they consume more electricity to generate the same amount of heat.

Gas Furnaces

Gas furnaces, on the other hand, are more expensive to install but can help reduce energy costs over time due to their higher energy efficiency. These furnaces are typically more powerful and can provide more consistent heating compared to electric resistance coil heaters.

Reasons for the AUX Heat Mode

There are several common reasons why your heat pump thermostat may switch to the AUX heat mode:

  1. Outdoor Temperature is Too Cold: Heat pumps are designed to extract heat from the outdoor air, but their efficiency decreases as the outdoor temperature drops. When the temperature falls below a certain threshold, usually around 40°F (4.4°C), the heat pump may not be able to extract enough heat to keep your home warm, and the auxiliary heat source is activated.

  2. Thermostat is Calling for a High-Temperature Increase: If your thermostat is set to a significantly higher temperature than the current indoor temperature, the system may switch to the auxiliary heat source to quickly reach the desired temperature.

  3. Heat Pump is in Defrost Mode: Heat pumps can accumulate ice on their outdoor coils, especially during periods of high humidity and low outdoor temperatures. When the system detects this ice buildup, it will enter a defrost cycle to melt the ice, during which the auxiliary heat source is activated to maintain indoor comfort.

Diagnosing Heat Pump Issues

If your heat pump is frequently switching to the AUX heat mode, it’s essential to diagnose the underlying issue. Look for the following signs:

  1. AUX Heat is On When It’s Warm Outside: If the AUX heat is activated when the outdoor temperature is relatively warm, it may indicate a problem with the heat pump’s ability to extract heat effectively.

  2. Higher Energy Bills Than Normal: Frequent use of the auxiliary heat source can significantly increase your energy costs, as it is less efficient than the heat pump.

  3. Home Feeling Too Warm: If your home feels too warm, even with the thermostat set to a lower temperature, it could be a sign that the auxiliary heat is being used excessively.

  4. Defrost Light is On When It Shouldn’t Be: If the defrost light on your heat pump is illuminated when the outdoor temperature is not cold enough to cause icing, it may indicate a problem with the defrost cycle.

Addressing Heat Pump Issues

If you suspect an issue with your heat pump’s AUX heat, it’s crucial to address it promptly to avoid increased energy costs and potential damage to your heating system. Here are some steps you can take:

  1. Check the Air Filters: Dirty air filters can restrict airflow and cause the heat pump to work harder, leading to more frequent use of the auxiliary heat source. Replace the air filters regularly.

  2. Inspect the Outdoor Unit: Ensure that the outdoor unit is free from debris, leaves, or snow, as these can impede airflow and cause the heat pump to ice over more quickly.

  3. Schedule a Professional Inspection: If the issue persists, it’s best to have a qualified HVAC technician inspect your heat pump system. They can diagnose any underlying problems and recommend the appropriate repairs or maintenance.

  4. Consider Upgrading to a More Efficient Heat Pump: If your heat pump is older and less efficient, it may be worth considering upgrading to a newer, more energy-efficient model that can better handle colder outdoor temperatures.

By understanding the reasons behind your heat pump’s AUX heat mode and taking the necessary steps to address any issues, you can improve the efficiency and performance of your heating system, ultimately saving you money on your energy bills and ensuring your home stays comfortable throughout the winter months.