What Does Auxiliary Heat Mean? (And Why It Turns On)

What does ‘auxiliary heat’ mean on your thermostat? You might have noticed this little setting the last time you changed your heat pump’s temperature and wondered what it meant. In this quick guide, we’ll explain what AUX heat is, why it turns on, and whether it means you need an HVAC professional to inspect your system.

Key Takeaways

• Noticing the word ‘AUX’ on your thermostat means your heat pump’s auxiliary heater is running.
• Heat pumps may use auxiliary heating during extreme cold spells, big temperature changes, or if there’s a problem with the system.
• It’s normal for auxiliary heat to run occasionally, but if it runs too often, it can significantly increase your energy costs.

What Does “AUX” Mean on a Thermostat?

If you have a heat pump, you may occasionally notice that your thermostat says, “AUX heat on.” Auxiliary heat refers to a secondary electric heater that kicks in when your heat pump is unable to heat your home for whatever reason. Think of it as an invaluable backup, ensuring your home is always being heated, even if there’s a problem with the heat pump.

Why Does Auxiliary Heat Come On?

Why does auxiliary heat turn on? There are a few typical scenarios, including:

Cold outdoor temperatures

Considering their cost, heat pump installations in Indiana can be a great investment. However, even the best systems only function effectively down to -13ºF. If it’s colder than that outside, your heat pump won’t be able to pump warm air into your home. 

In these cases, the auxiliary heat kicks on to ensure you don’t freeze in the meantime. This is why some Indiana homeowners opt to keep their gas furnace over a heat pump.

Sudden temperature changes

If you rapidly change the temperature on your thermostat, there’s a chance the heat pump will turn on auxiliary heating to help the space warm up more quickly. This helps reduce strain on the heat pump, making it less likely to require heat pump repairs.

Heat pump not working efficiently

Heat pumps can face winter efficiency challenges, especially if you haven’t kept up with regular maintenance over the past year. If your system is struggling to operate optimally, it may turn on auxiliary heating to bring your home up to temperature.

Poor insulation or air leaks

Another potential cause of auxiliary heating turning on is issues with your home’s air sealing or insulation. These help keep warm air inside the home, preventing it from escaping through gaps in the structure or uninsulated building materials. If warm air is escaping too quickly, your heat pump may activate auxiliary heat to maintain the temperature.

Does Auxiliary Heat Increase My Energy Bills?

Now that we know what ‘AUX’ means on a thermostat, let’s get to the real question: Does it cost you more money to run this extra system? 

Heat pumps don’t generate heat themselves. Instead, they extract heat from the outdoor air and simply pump it into your home. As a result, they offer superior heating efficiency benefits compared to furnaces and other power-based heating systems.

Auxiliary heating, on the other hand, functions more like a furnace or other heating unit. This means it can significantly increase your energy bills over time, reducing the savings you’ll see from the heat pump.

How Long Is It Normal for AUX Heat to Run?

If the auxiliary heating only engages during a temperature change or a heat pump defrost cycle, you can expect it to run for 15 to 30 minutes at a time. This is normal and shouldn’t give homeowners too much to worry about. 

The issue comes if your AUX heating runs for hours at a time. Because of the much lower efficiency of this setting, you’ll pay a lot more in utility bills. Plus, you’ll likely notice your home just isn’t as comfortable as it is when the heat pump is up and running.

How Do I Stop Auxiliary Heat from Coming On?

There are a few simple ways to prevent auxiliary heat from kicking in, potentially saving you hundreds of dollars each winter. Here are a few tips:

• Look for ‘balance point’ or ‘AUX settings’ on your thermostat, adjusting them to a lower outdoor temperature
• Only adjust your thermostat 1 to 2 degrees at a time
• Keep up with heat pump maintenance, including changing air filters and cleaning coils
• Have your home inspected for air leaks and insulation issues

When Should You Call an HVAC Professional?

If the steps above don’t seem to make a difference, it might be time to have an HVAC professional inspect the system. Dugan Air regularly provides heat pump services in Indianapolis, Columbus, Greenwood, and beyond, helping homeowners resolve troublesome heat pump issues.

An HVAC technician will inspect the entire heat pump system, including the indoor air handler, outdoor compressor unit, and thermostat. They’ll look for anything that might be causing the auxiliary heater to switch on too often and can make repairs, tune-ups, and minor changes that help your heat pump run more smoothly and efficiently.

Enjoy Year-Round Home Comfort with Dugan Air, Heating & Cooling

Heat pumps provide a comfortable home at a significantly lower energy cost. However, unexpected auxiliary heat operation can pose a problem, which is where we come in to help. Our team has the skills, tools, and experience to address any heat pump issue you may be facing, helping your system run reliably and efficiently again. For more information or to schedule a heat pump service appointment, don’t hesitate to get in touch.

Dan Dugan

President
Dan Dugan is the President and co-owner of Dugan Air Heating & Cooling. A licensed HVAC professional with over 25 years of industry experience, Dan brings extensive technical expertise and leadership to the company he and Sue founded in 2009. He remains deeply involved in training, innovation, and maintaining the company’s reputation for integrity-driven service across Central Indiana.