What Does 'Auxiliary Heat' Mean on a Thermostat?

By Michael Franco

October 31, 2024

If you've ever noticed the "AUX" setting light up on your thermostat, you may have wondered what it means and why it's activated.

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Understanding this function is essential, particularly if you're looking to optimize your home's heating system and avoid unnecessary energy costs.

What Does 'Aux' Mean on a Thermostat?

In simple terms, "Aux" stands for auxiliary heat. It is an additional heating source that kicks in when your primary heating system can't keep up with the demand. This setting is more common in systems that rely on heat pumps, which transfer heat from the outside air to warm your home during cooler temperatures. Heat pumps are highly energy efficient when outdoor temperatures are moderate, but as the temperature drops, they may struggle to maintain the desired indoor temperature. This is where auxiliary heat comes into play, serving as a backup heat source when the heat pump alone can't do the job.

What Is an Auxiliary Heat System?

Auxiliary heat refers to the backup heating system that supports your primary heat pump. The auxiliary heat is usually an electric resistance heating element, similar to what you would find in a space heater or electric stove. It’s also known as "strip heat" or "emergency heat," though "emergency heat" specifically refers to a mode that homeowners can manually engage when the heat pump is not functioning properly.

In most cases, auxiliary heat is automatic. You don’t have to turn it on manually—it kicks in when your thermostat detects that the heat pump is struggling to maintain the set temperature. For instance, if you have set your thermostat to 70 degrees Fahrenheit but the outdoor temperature is too low for the heat pump to efficiently pull in warm air, the system automatically turns on the auxiliary heat to help bridge the gap and maintain comfort indoors.

Why Does My Auxiliary Heat Come On?

Auxiliary heat usually comes on under a few specific conditions. The most common reason is that the outside temperature has dropped too low for your heat pump to operate efficiently. Heat pumps work by extracting heat from the outdoor air and moving it inside, but as the outdoor temperature drops below 40 degrees Fahrenheit, there is less heat available to transfer. At this point, your system switches to auxiliary heat to maintain the set temperature in your home.

Another reason auxiliary heat might engage is if you drastically increase the temperature setting on your thermostat. Let’s say you come home to a chilly house after the thermostat has been set low during the day, and you bump it up by 10 or 15 degrees. In this case, the system will struggle to make up the difference quickly, so it activates auxiliary heat to help warm the house more rapidly. This is common when you make significant temperature changes or during extreme cold spells.

Additionally, auxiliary heat might turn on if there’s an issue with your heat pump, such as a malfunction or if it has gone into defrost mode.

Also, heat pumps occasionally need to reverse their operation to defrost themselves when ice accumulates on the outdoor coils. During this defrost cycle, the heat pump can’t provide heat, so the auxiliary heat kicks in to keep the house warm.

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Does Auxiliary Heat Cost More?

Yes, auxiliary heat does cost more to run than your standard heat pump operation. This is because electric resistance heating, which is commonly used as auxiliary heat, is far less efficient. Consequently, relying on auxiliary heat for extended periods can cause your energy bills to spike.

If you notice that your auxiliary heat is coming on frequently, it’s worth investigating why. In some cases, it could be due to external weather conditions, and there’s little you can do about that except wait for the cold snap to pass. However, if you find that auxiliary heat is coming on in milder weather, it may indicate a problem with your heat pump. Issues like low refrigerant levels, a malfunctioning compressor, or a dirty air filter can all reduce your heat pump’s efficiency, leading to more frequent use of auxiliary heat.

How to Minimize the Use of Aux Heating

To reduce your reliance on auxiliary heat and keep energy costs under control, you can take a few proactive steps. First, make sure that your heat pump system is properly maintained. Check and replace air filters on a regular basis, and schedule yearly inspections to ensure that all aspects of the system are functioning correctly. Dirty or malfunctioning parts can decrease the efficiency of the heat pump, causing it to rely more on auxiliary heat.

Next, consider adjusting your thermostat settings gradually instead of making large jumps in temperature. For example, if your house is set to 60 degrees Fahrenheit while you’re at work, don’t immediately set the thermostat to 75 degrees Fahrenheit when you get home. Instead, increase the temperature gradually over time. A programmable or smart thermostat can also help manage temperature changes more efficiently, preventing auxiliary heat from kicking in unnecessarily.

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