Aux Heat

What Does Aux Heat Mean on My Thermostats? Everything You Need to Know to Keep Your Home Warm This Winter

When it gets cold outside, you may notice your thermostat set point has an aux heat setting. But what does aux heat mean on a thermostat, and how do you know when to use it?

Auxiliary heat is a backup heat source for your home. You only use it when the temperature outside is below freezing and your central heating systems cannot keep up.

If you utilize aux heat frequently, it may be time to upgrade your central heating system.

What Does Aux Heat Mean on My Thermostat?

If the aux heat indicator is on, your heat pump feature may not be operating at its optimum level. Auxiliary heat, also known as emergency heat, is supplied by the same source as emergency heat. Still, it works in conjunction with your heat pump, making it less expensive than emergency heat. Your heat pump will activate the auxiliary heating strip inside your secondary heating source if it struggles to maintain its desired temperature. If your heat pump is not working correctly, your aux heat will turn the light and help it out.

For example, the thermostat turns on auxiliary heat when the temperature drops below the goal temperature. The aux heat indicator on the thermostat lets you know when this is happening.

What Is Auxiliary Heat?

Auxiliary heat is a term used to describe a heat source that kicks in when the primary heat source isn’t providing enough heat. The auxiliary heat source is usually a backup used when the temperature outside is freezing and the primary heat source can’t keep up. There are different auxiliary heat sources, but an electric resistance heater or a gas furnace is the most common.

In winter, the primary heat source is a furnace, but it’s not always effective in cold weather, so heat pumps are used as an auxiliary heat source.

Why Does Auxiliary Heat Stay On?

In the winter, your furnace works hard to keep your home warm. But sometimes, even when the furnace is running, your home can still feel chilly. The furnace isn’t the only heat source in your home; auxiliary heat is.

Auxiliary heat is a backup system that kicks in when the furnace can’t keep up with the demand for heat. It usually comes in electric resistance coils or a heat pump. And while it’s great to have a backup system, it can be frustrating when auxiliary heat stays on all the time.

There are a few reasons why this might happen:

Faulty Condenser Fan Motor

The condenser fan motor is responsible for indoor air quality exchange through the condenser coils. Reduced airflow can cause the coils to freeze or overheat if they fail. It’s essential to repair or have an AC replacement for a defective condenser fan motor as soon as possible.

Low Refrigerant

A heat pump uses a refrigerant to transfer heat between the outdoor coil and the indoor coil. Low refrigerant means that the heat pump will have difficulty absorbing heat, making it necessary for the system to rely on external heat sources.

Bad Compressor

The compressor is responsible for circulating the refrigerant throughout the heat pump system. If the compressor isn’t working correctly, the heat pump won’t be able to effectively move heat, meaning your home won’t be as warm as it should be. Some heat pumps use auxiliary heat or a supplemental heat source to heat your home. It’s essential to check the compressor to ensure it’s working correctly.

Faulty Defrost Control Board

Your heating system relies on the backup heat to keep your home at a comfortable temperature during defrost mode. If the defrost control board goes haywire, the aux heat will be on for a long time. If the condenser cannot defrost the ice accumulation, it will impede critical heat transfer.

Faulty Reversing Valve Problem

The reversing valve is responsible for changing the refrigerant flow in the air conditioner. If the valve is not working correctly, the air conditioning will not be able to cool the home properly. There are a few things that you can do to troubleshoot the issue. The reversing valve is the main component in a refrigeration system that allows the refrigerant to flow in the opposite direction. You must check it for correct wiring, any obstructions, and leaks.

How Do I Know If Auxiliary Heat Is On?

If your HVAC system has auxiliary heat, it will turn on automatically when the temperature inside your home drops below the set temperature. The types of auxiliary heat vary depending on the make and model of your HVAC system, but it is typically electric resistance heat or a fossil fuel furnace.

Resistance heating is a type of heating that uses electrical resistance to convert electrical energy into heat. The heat is then transferred to the object to be heated.

It is essential to know when your auxiliary heat is on if you have a heat pump. Here are some things to look for: 

  • Auxiliary heat will make your heat pump run longer and make more noise than usual.
  • Your home’s auxiliary heat could be on if it is taking longer to get warm or if it is cooler than usual.
  • Using auxiliary heat will increase your energy bill because it uses more energy than your heat pump alone.

What Causes Auxiliary Heat To Come On?

Your heat pump might come on when the temperature outside drops and the heat pump cannot meet the demand. It uses the auxiliary heat to supplement the heat it is pulling from the outside.

Heat pumps run at a lower temperature when working, using auxiliary heat to keep your home warm. A poorly sized heat pump will require more work to heat your home and require auxiliary heating more often. Or, if it is not maintained correctly, it can start to have problems that will cause it to need to use the auxiliary heat more often.

Why Does Auxiliary Heat Stay On?

In a perfect world, your home’s heating system would keep up with the need for heat 100 percent of the time. However, several factors can contribute to your heating system not being able to keep up, such as:

  • The heat pump may be malfunctioning
  • When the thermostat is unable to reach and maintain a specific set of temperature
  • The outdoor temperature is freezing
  • Your home is vast and has a lot of windows

 Auxiliary heat is more powerful than your home’s primary system.

How to Stop Auxiliary Heat From Coming On?

The heat pump system is efficient when temperatures are above 40 degrees, but auxiliary heat is needed when temperatures are below 40 degrees. Here are some ways to stop auxiliary heat:

Lower The Set Heat Temperature

Turning down the thermostat and dressing warmly during the winter is a better solution than trying to turn up the heat. Using the aux heat feature on a heat pump is another option for keeping warm without strain on the system.

Make Your Home More Comfortable

You can make your home more comfortable by insulating and using a programmable thermostat. Use the auxiliary heat feature to provide additional warmth during the winter months, or use it to keep the air inside your home cool and comfortable during the summer months.

Shut Off Unused Rooms

Turning off unused rooms in your house can save money on heating bills. Make sure your home is well-insulated and turn the heat down to around 55 degrees in a guest room and 60 degrees in a formal dining room while maintaining comfort for your family in your home.

When Do You Not Want Auxiliary Heat On?

It would be best if you didn’t use auxiliary heating on your heat pump because it might be more expensive to operate than your regular heat pump. In addition to putting extra strain on your heat pump, leading to more frequent repairs, auxiliary heating can also be more expensive. Finally, if you reside in a cold environment where your heat pump cannot sufficiently satisfy demand, you may be forced to rely on auxiliary heat more frequently than you prefer.

How to Use Your Heat Pump Thermostat Setting in Cold Temperatures?

If you have a heat pump, you know that it is a great way to save money on your energy bill. But did you know that you can save even more money by using your heat pump thermostat settings in cold temperatures?

Here are some tips on how to use your heat pump thermostat settings in cold temperatures:

  • Emergency heat is the most expensive choice for heating your home. It relies exclusively on the secondary heating source, which is more costly than aux heat.
  • Don’t use auxiliary heat unless you increase the thermostat setting by only two degrees.
  • You can save money on your energy bill by turning off your thermostat during the winter and turning it back on when temperatures rise above 40 degrees.

What Is Emergency Heat?

Emergency heat is a setting on your thermostat that controls your home’s backup heating system. If you have an EM heat setting, you likely have a heat pump, and it will activate the secondary heating source when outside temperatures are low.

When using the EM heat setting, only use it if you need to. It is less efficient than the primary heating system, so we only use it in real emergencies.

When Should I Turn My Emergency Heat On?

The emergency heat setting on your thermostat is there for a reason to keep you warm in a power outage. But what many people don’t realize is that you can also use emergency heat to supplement your regular heating system. Here are a few instances when you should turn on your emergency heat:

You can use emergency heat to keep your home warm if the temperature outside is below freezing. Monitor the temperature closely to prevent overheating.

Turning on emergency heat when your heating system is on the fritz can buy you some time until a repair technician can fix the problem.

Depending on your emergency heat when you’re away from home, it is good to prevent pipes from freezing and bursting.


Auxiliary heat is essential even when you don’t have radiant heating in your home. It’s one more method of keeping your house warm from November through March, keeping your heating as low as possible while remaining comfortable. While you may be familiar with auxiliary heat from some of the commercial buildings you’ve visited, it might not be a term you’re all that familiar with. Fortunately for us, it’s easy to understand and might make the next winter easier to handle tentatively.

Always make an appointment and seek a professional team to avoid any problems. AC repair commercial service is a great way to keep your business or office running smoothly and efficiently. However, it is essential to remember that not all commercial AC repair service providers are created equal. When you are looking for a reputable and reliable commercial AC repair services provider, there are a few things you will want to keep in mind.


Is auxiliary heat more expensive?

Auxiliary heat is more expensive to operate than the heat pump alone. Heat strips use more electricity than the heat pump, and the amount of heat they produce is indicated by a heat strip on the thermostat.

Why is my auxiliary heat blowing cold air?

Your auxiliary heat could be blowing cold air if the air exchange is dirty or the heat pump is not working correctly. If your heat pump is not working correctly, it could be because the outdoor unit is not working correctly, the indoor team is not working correctly, or there is a problem with the ductwork.

Should I leave my heat pump on all the time?

There are several factors to consider when deciding whether to leave your heat pump on all the time. Maintenance is one and the type of heat pump and the climate. Ultimately, it’s up to the homeowners to decide what’s best for their heat pump repair and home.