Everything You Need to Know About Auxiliary Heat on Thermostat

Auxiliary heaters are meant to help your home’s heating system, but they can also be a good indicator that something is wrong with your heat pump system. There are many reasons why this happens.

Almost all homeowners dread seeing these two words on their thermostat: AUX heat. And it is even worse when the auxiliary heater stays on for long without automatically turning off. When this happens, contact your local HVAC contractor to check your home’s heat pump. 

Typically, auxiliary heat on thermostat should not stay on for longer than 30 min to an hour. So, if it stays on unusually long, you have a problem that will raise your electric bill.

What Is Auxiliary Heat Anyway, & Why Is It Important for My Home?

So, what does aux mean when you see it on your thermostat? The electrical heating system  in the air handler or gas furnace generates the heat to supplement heating in your home. The aux mode works in tangent with the emergency heat to raise the indoor temperature. 

You can manually turn on the emergency heat. Auxiliary heat and emergency heat help your heating system provide the best temperatures for your home. However, when they run into a situation where you have auxiliary heat on, but no heat is coming through, you should try turning on the emergency heat.

What Does AUX Mean on Thermostat and Indoor Heating System?

It’s not uncommon for people to see their thermostat has turned on auxiliary heat; meaning their heating system needs help to heat the house. The auxiliary heater’s main job is to help the heat pump meet the desired home temperature.

Therefore, you should not panic and wonder why the auxiliary heat is coming on your thermostat.

When Should Auxiliary Heat Come On? Importance of Auxiliary Heater

So, you don’t have to worry and ask “Why does my thermostat say aux heat?” The auxiliary heater is an important component of your indoor heating system; it will turn on aux mode when the temperature outside is too cold for the heat pump system to extract enough hot air to heat your home. When this happens, an electrical heating system setting is triggered to generate heat, so the temperature requirement is met. Once your home reaches the desired temperature, auxiliary heat turns off immediately to save the electric bill. 


Why Is the Auxiliary Heat Coming On? Other Reasons Why It Turns On

  • Cold Weather Causes Auxiliary Heat to Turn On

When the weather is extremely cold, you should expect the thermostat to turn on aux heat, meaning the electrical heater will kick in. 

What temperature should auxiliary heat come on?

The auxiliary heater will kick in when outside temperatures are below 40-35 degrees and your home is three degrees colder than the thermostat setting. Programmable thermostats are designed to turn on the auxiliary heater when it detects the mentioned temperature levels.

  • Raising The Thermostat Causes Auxiliary Heat to Turn On

Raising the thermostat temperature may cause the auxiliary heater to supplement the heat pump with enough heat for some time to meet the new temperature requirement.

  • The Heat Pump Is in Defrost Mode

The heat pump is fitted with a defrost control board that prevents ice formation on the outdoor condensing unit. Meanwhile, the HVAC auxiliary heat warms the house as defrosting takes place.

Why Does Auxiliary Heat Stay On? Possible Issues and Reasons

  • Faulty Condenser Fan Motor

A faulty fan limits the heat pump’s ability to absorb heat due to low circulating air. A broken, dirty, or frozen fan could be why you want to know how to turn off auxiliary heat.

  • Low Refrigerant

The refrigerant in your HVAC unit alternates between gas and liquid states to transfer heat between the indoor and outdoor units. Therefore, low refrigerant levels turn on aux heat.

  • Bad Compressor

The compressor performs two crucial functions: (1) turns refrigerant from gas to pressurized liquid and (2) pushes refrigerant through the system. When the compressor fails, auxiliary heat comes on.

  • Faulty Defrost Control Board

This problem prevents the condenser from defrosting, causing the auxiliary heater to stay on. While defrost mode will make you wonder why does the auxiliary heat come, a faulty defrost control board will make you wonder why it stays on.

  • Faulty Reversing Valve Problem

The reversing valve allows the heat pump system to perform heating and cooling functions by changing the refrigerant flow. When it gets stuck in cooling mode, the auxiliary heat will turn on because the hot refrigerant is heading to the outdoor unit instead of the indoor unit.

How To Stop Auxiliary Heat from Coming On? DIY Tips You Can Try

  • Keep Thermostat Set Point Low

Sometimes, learning how to fix aux heat on the thermostat requires you to reduce the home temperature.

  • Reduce Heat Pump Malfunctioning with Maintenance

Contact a maintenance professional to perform maintenance services like cleaning the heat pump components and repairing the auxiliary heat element.

Hire a Professional Auxiliary Heat and Heat Pump System Contractor

Blue Valley Heating and Cooling has over 30 years of experience providing quality HVAC repair and maintenance services. With our help, you won’t have to ask, “what is auxiliary heat on my thermostat, and why does it stay on?”

Contact us today for more information.

Similar Posts