It is winter and your HVAC system which is equipped with a heat pump is keeping the temperature in your home comfortable and on the preset temperature. But you’ve noticed now for a while that the thermostat’s auxiliary heating icon is on. You now wonder what it exactly means if the auxiliary heat light is on. And if it means that the auxiliary heat is active, you want to know whether it is a bad thing or not. And will your energy bill be affected?

When the auxiliary icon is lit, it means extra heat is provided to help the HVAC heat pump to keep your home comfortable. This is the function of auxiliary heat, and thus not bad when it is providing extra heat. However, if extra heat is provided during milder weather conditions it might be an unnecessary waste of energy with a bad effect on your energy bill.

In this article, I discuss what auxiliary heat is and how it assists your heat pump to maintain the required temperature in your home. And I also provide tips on how to keep the use of auxiliary heat to a minimum.

Is it bad if the auxiliary heat comes on?

Is it bad if the auxiliary heat comes on? The short answer is no. Auxiliary heat is an important mechanism to keep your heat pump in proper working condition and to keep your home’s temperature comfortable. Your thermostat activates this function automatically whenever it is needed to keep your home’s temperature comfortable and at its required level.

But, just as many other HVAC devices and procedures can function incorrectly from time to time, the auxiliary heating system can become faulty. It might, for instance, stay activated longer than necessary. If this happens, it will affect your energy bill negatively.

In other words, it is not bad if the auxiliary heat is coming on during very cold weather, but it might be bad if it stays activated too long.

Understand heat pumps to understand auxiliary heat better

You will find it difficult to understand where auxiliary heat is coming from and its role in your HVAC system if you don’t understand the principle of how heat pumps work.  

The role/purpose of your heat pump

 A heat pump moves heat from one area to another for heating and cooling purposes. During the colder months, it draws heat energy from the outside air and transfers it to the inside of your house to heat your home. During the summer months, the heat pump has the opposite role. It then takes heat energy from inside your house and releases it outside. 

What is a heat pump?

The heat pump has two units. One is a condensing unit that is located outside your house, and the other one is an indoor air handler unit. The pump operates as a split system.  

The outdoor condensing unit circulates refrigeration for heat transfer. It is equipped with a reversing valve and can therefore change the direction of the refrigerant flow. This allows the heat pump to have a cooling and heating function.

In heating mode, the refrigerant in the outdoor condenser absorbs the heat energy from the outdoor air, pressurizes and heats it in the compressor to become a gas, and sends it to the air handler unit in your home. The vaporized refrigerant is then sent to the indoor unit coil where the cold air from your home is blown over the coil and absorbs the heat and heats your home.

Why does the auxiliary heat stay on?

Let’s look at the five main possible reasons why auxiliary heat sometimes stays on when it is not needed anymore to assist the heat pump.

1. Faulty defrost control board

The heat pump’s defrost control board and its sensors enable your HVAC system to detect when snow or ice is building up around the condenser coils. When the snow or ice built-up is detected, the unit automatically goes into defrost mode. When in defrost mode, the heat pump “fetches” hot air from inside the home and takes it to the coil to melt the snow or ice.

During the defrosting cycle, the auxiliary heat kicks in to keep your home warm. The whole defrosting procedure should normally not be longer than a few minutes, and when the procedure has been concluded the auxiliary heat provision should stop.

If the defrost control board is not functioning correctly anymore, it might be unable to cut off the auxiliary heat when it is not needed anymore. Thus, if the auxiliary heat stays on after defrosting mode has been terminated, it is recommended that you contact your HVAC technician to come and test and, if necessary repair or replace the board.

2. Low Refrigerant

The refrigerant in the system converts from liquid to gas and back to liquid again. This converting procedure allows your HVAC system to heat or cool your home.

Because the purpose of the refrigerant is to absorb heat, a low level of refrigerant can cause the moisture on the evaporator coil to freeze. This triggers auxiliary heat to be activated.

Thus, a low level of refrigerant might be the reason why the auxiliary heat stays on to keep your home’s temperature comfortable.   

Your HVAC technician will be able to fill up the refrigerant and also check for possible leaks causing the low level of the refrigerant.  

3. Faulty reversing valve problem

In a heat pump system, the reversing valve allows the system to change the refrigerant flow to perform heating and cooling functions. When the valve becomes faulty and stays in cooling mode, the auxiliary heat turns on automatically because the hot refrigerant is flowing to the outdoor unit instead of the indoor unit.

When the valve is repaired, the auxiliary heat will come on and shut down correctly again. Your HVAC technician will be able to repair or replace the faulty valve.

4. Bad Compressor

The heat pump compressor is designed to turn off the compress when the outside temperature goes below 35 degrees. If the compressor is faulty and is not turning off when it reaches 35 degrees, the system will keep on providing auxiliary heat to keep the home comfortable. If you are living in an extremely cold area and the auxiliary heat stays on, the chances are great that a bad compressor is a cause of why the auxiliary heat stays on. It is recommended that you contact your HVAC technician to check and repair the compressor.

5. Faulty Condenser Fan Motor

 The condenser fan motor forms part of an outdoor HVAC system’s condensing unit and it influences the comfort level in your home. A faulty condenser fan motor can cause the auxiliary heat to stay on when it is not needed. Your HVAC technician will be able to determine whether the motor is faulty and if so, repair or replace it.

Can I stop the auxiliary heat from coming on?

Because auxiliary heat is generated by an electric element, it is affecting your energy bill, and sometimes you may feel that you would rather be without the “extra” heat and keep your energy bill low.  Although you can’t actually “turn off” the auxiliary heat, you can take other steps to create an environment not needing the extra heat. The following are some of the things you can do:

Set a lower heat temperature

The temperature set in your home is very often too high. A few degrees lower will not make your home uncomfortable, but you will save money if the auxiliary heat is not activated. Generally speaking, a setting between 62-68 degrees will keep you comfortable and keep the auxiliary heat inactive.

Let the sun come into your home during the day

You can keep your home warmer by opening the curtains and window shades of the rooms you normally use during the day. The sun will warm the rooms where possible. The temperature will then less frequently be so low that auxiliary heat is necessary.

Keep unused rooms and spaces closed

Close the doors to rooms you are not using frequently and keep their curtains and window shades closed. By closing off seldom-used rooms the warm air can be redirected to the rest of your home.


Q) When is auxiliary heat needed?

Auxiliary heat is “extra” heat to help the heat pump to provide enough heat to keep your home at a comfortable temperature. In very cold weather, it often happens that the heat pump cannot provide enough heat to keep your home comfortable and supplemental heat is needed. This extra heat is provided by an electric heating element inside the air handler.

Usually, the thermostat activates auxiliary heat when the temperature inside your home, and thus on your thermostat, reach 2-3 degrees colder than your preset required indoor climate temperature.

Q) Is emergency heat the same as auxiliary heat?

Although emergency heat and auxiliary heat are both created by this electric heating element, there is a “technical” difference between the two. You have to manually switch on emergency heat, while auxiliary heat turns on automatically when there is a sudden drop in temperature. Emergency heat should only be switched on in temperatures below 30 degrees.

Auxiliary heating is automatically activated by your thermostat when the desired temperature cannot be provided by the heat pump alone. It will shut off once the thermostat has reached the required temperature.

Q) How will I know when auxiliary heat is active and why is it good to be on?

When auxiliary heat is on, the auxiliary icon or the “AUX” will be lit on the thermostat. They will be shown until the auxiliary heat is turned off again by your thermostat.

Auxiliary heat is “good” when it, apart from other things, assists the heat pump when

  • temperatures are extreme,
  • you’ve raised your thermostat’s temperature setting, or
  • your heat pump is going into defrost mode.

Q: When is auxiliary heat on too long?

A: The auxiliary heat should only run for as long as it is necessary to get the temperature in your home at the preset temperature. If it runs every time you turn your system on, or for longer than 30 minutes at a time, it is recommended that you call an HVAC expert.

Q: Why does the auxiliary heat stay on too long?

A: In most instances, when your auxiliary heat is active for an unusually long time, the problem is either a malfunctioning heat pump or a broken or dirty condenser fan motor.


In principle, it is not bad if auxiliary heat is coming on – it has to become active in certain circumstances to keep your home’s temperature comfortable and your heat pump in working condition. But because auxiliary heat is created with elements using electricity, it affects your energy bill negatively. Therefore it is good practice to utilize aspects such as the closing of rooms not frequently used to help to keep your home’s internal temperature from going so low that auxiliary heat is necessary.


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