What is Auxiliary Heat? What to Do When it Comes On

What Does Auxiliary Heat on My Thermostat Mean?

If you have the auxiliary heat on in your thermostat, you have a heat pump and not the conventional air conditional. A heat pump heats your house by extracting heat from the outside air while a standard air conditional is paired up with a furnace.

Auxiliary heat turns on when the heat pump cannot extract enough heat from the surrounding, usually when the temperature dips below 35 degrees. The heat pump activates the secondary heat source (electric resistance heating) to heat the house until the temperature rises when the AUX heat is then turned off.


Your thermostat has what is called goal temperature which is basically your set temperature. When the indoor temperature drops 2-3 degrees below that temperature, the heat pump powers up the secondary/complementary heat source to heat the house temporarily until the temperature outside increases.

Heat pumps can cool and heat the house, which is their main difference from conventional air conditioners. However, heat pumps will extract heat from the outdoor air as long as temperatures don’t fall below 35 or sometimes 40 degrees.

Because temperature will fall below those temperatures in winter, heat pumps always have a secondary source of heat. That is usually in the form of electric resistance heating strips.

When working normally, heat pumps are very effective. That however changes when auxiliary heat is activated. Your utility bill will definitely be high if the aux heat is on for long periods of time.

Auxiliary heat is more expensive than the primary source of heat (the heat pump). When aux heat is on, your house is being heated by electricity while a heat pump simply extracts heat from the outside air which is very efficient.

Your heat pump activates the auxiliary heat automatically. You don’t need to manually turn it on as is the case with emergency heat. Whenever indoor temperatures fall about 3 degrees lower that the thermostat setting, aux heat will kick in at once.

Often, most homeowners confuse between auxiliary heat and emergency heat on their thermostat. I will explain the difference between the 2 later.

Auxiliary heat comes on when the indoor temperature is set too high or when the outdoor temperature falls below 35 degrees hence heat pump cannot produce enough heat. It will also turn on when the heat pump is in defrost mode.

To stop auxiliary heat from coming on, lower the indoor thermostat setting and close rooms not being utilized. Unless you have a defective heat pump sometimes you should let the auxiliary heat on otherwise you are going to freeze.

How a Heat Pump Works

To properly understand the function of aux heat on your thermostat, you first need to understand how a heat pump works.

When used to cool your house in the months of summer, a heat pump works very much like an air conditioner. As I wrote in this post, it is actually not easy to tell the 2 apart unless you know where to look.

Just like a central air conditioner, a heat pump has 3 main components:

  • Compressor
  • Condenser coil
  • Evaporator coil

The compressor and condenser coil forms the outdoor unit also known as the condenser unit while the evaporator coil is the indoor unit. The 3 components are joined together using copper tubes.

To help in heat transfer, a refrigerant is used. The refrigerant (used to be Freon but now usually R-410A) moves between the 3 components where it changes state from gas to liquid and back to gas again.

During the cooling process, the refrigerant moves from the compressor to the condenser coil and then to the evaporator coil before going back to the compressor. The cycles goes on and on and on.

When the heat pump is used to heat the house, the cycle is reversed. The refrigerant moves from the compressor to the evaporator coil and then to the condenser coil. That is made possible by a reversing valve.

The compressor is probably the most important component in HVAC. When low-pressure refrigerant gas enters the compressor, its pressure increases which also results in an increase in temperature. That is what creates a temperature differential.

In thermodynamics, heat is transferred from a point of high concentration to a point of low concentration.

After cooling the house, the refrigerant needs to remove the heat to the surrounding but remember the outside temperature is high as well since that is usually during summer.

By compressing the refrigerant, its temperature increases way more than the outdoor temperature allowing it to dissipate heat to the surrounding.

The opposite of that happens during heating. Instead of the compressed and superheated refrigerant gas flowing to the condenser coil, it is reversed to the evaporator coil inside the house.

The cold air inside the house absorbs the heat from the refrigerant and supplies it throughout the house. From the evaporator the refrigerant is sent out to the condenser coil to extract more heat from the surrounding.

In a short, when used for heating, a heat pump’s evaporator works like a condenser while the condenser coil works like an evaporator coil.

This process works seamlessly until outdoor temperature dips below 35 degrees Fahrenheit. At that temperature, the heat pump cannot extract enough heat from the outside temperature.

And that is where auxiliary heating comes in. To prevent you from freezing, the heat pump is designed with a secondary source of heating to back it up when it cannot cope up with your heating demands.

When your indoor temperature falls a few degrees below the set thermostat temperature, a signal is sent to the heat pump to activate supplementary heating. At that point, the aux heat will turn on in your thermostat.

Auxiliary Heat vs Emergency Heat

I am not surprised that most homeowners cannot tell the difference between auxiliary heat and emergency heat. So, what is the difference?

As we have seen, auxiliary heat is activated automatically by the heat pump when it cannot cope up with the heat demands inside your house. However, emergency heat must be activated manually when the heat pump fails.

When you look at your thermostat, you will see a button labelled EM, EMER or EMERGENCY HEAT. When your heat pump fails or when it is damaged either by snow or a tree crushing into it, that is when you activate emergency heat.

Emergency heat as its name implies is only for emergencies and you should call in an HVAC technician to fix your heat pump as soon as you turn it on.

What you need to remember however is that both the Aux heat and EM heat draw energy from the heat pump’s secondary source of heat. While most heat pumps have an electric secondary heat source, some have an oil, natural gas or even hot-water source.

Unlike the aux heat which works together with your heat pump, when the emergency heat is turned on, the heat pump is completely bypassed, meaning 100% of the heat will be drawn from the secondary heat source.

The cost of running your heat pump on EM or aux heat depends on the price of electricity or natural gas in your area. If your heat pump uses natural gas as its back-up heat source and the cost of natural gas is at an all-time low, running it on EM or aux heat will not be too bad for your pocket.

What Causes Auxiliary Heat to Come On?

One thing to note is that it is hardly anything to be concerned about when your auxiliary heat comes on. It is part of how your HVAC system works.

There are 4 reasons why you auxiliary heat is on. They are:

1. The Outdoor Temperature is too Low

As I have mentioned above, a heat pump will only heat your house properly if the outdoor temperature is above 35 degrees. When the outdoor temperature falls below that, the secondary heat source will have to complement it.

If therefore your aux heat comes on and the outdoor temperature is below 35 degrees Fahrenheit, you have no reasons to be concerned. Your system is working properly.

2. The Heat Pump is on “Defrost Mode”

It is not unusual for ice to form on your heat pump’s outside unit, especially around the coil. That happens when temperatures fall below freezing point.

Heat pumps are designed to deal with that problem. As I explained earlier, instead of the hot refrigerant flowing from the compressor to the indoor unit, it will be reversed to flow to the condenser coil so that it can melt the ice.

During that time, the auxiliary heat will be on so that your house will remain warm as the ice buildup on the unit is being melted.

3. The Indoor Temperature is Set Too High

Needless to say, when the indoor temperature is set too high, the auxiliary heat will have to come on to supplement the heat pump.

The ideal thermostat setting is between 68 and 72 degrees. If you however prefer a higher setting, it means that your aux heating will come on sooner than someone whose temperature setting is way lower.

4. Your System is Faulty

If the outdoor temperature is way above 35 degrees and your thermostat isn’t set too high but still your auxiliary heat is on, you most likely have a problem with your heating system.

In that case what you should do is have a technician come over and have a look.

Why Does Auxiliary Heating Stay On?

Your auxiliary heat should not stay on for long. Once, the temperature outside has increased, the aux heat should automatically turn off and let the primary heat source to heat the house 100%.

If however the auxiliary heat stays on even after the temperature outside has improved, you definitely have a problem with your heat pump. The following are some of the causes when your thermostat’s auxiliary heat stays on all the time:

1. Low Level of Refrigerant

The refrigerant is responsible for the actual heat transfer between the inside and outside of your home. If there isn’t enough refrigerant in your system, it goes without saying that your heat pump will not extract enough heat from the surrounding, prompting your auxiliary heating to kick in.

Although the heat pump’s system is close-looped, a leak can develop in one of the coils or tubes, and that could be the reason you have low levels of refrigerant in the system.

Old heat pumps and air conditioners used a refrigerant known as Freon. Freon has however been phased out due to its effects (global warming) and instead R-410A is now used.

Take your time to know what type of refrigerant your heat pump uses and then call in a technician to fix it.

2. Problems with the Compressor

The compressor is responsible for compressing the refrigerant and in the process increase its pressure. A sit happens with gases, when you increase their pressure, their temperature increases as well.

As a result, the refrigerant leaves the compressor as a superheated high-pressure gas which is what actually heats your house.

Due to electrical faults or wear and tear, the compressor can fail to work as designed and at that time the auxiliary heat will come on.

When the compressor fails completely, even the aux heat will not come on and you will need to manually activate emergency heating.

3. Bad Condenser Fan Motor

The condenser fan is responsible for accelerating the rate of heat exchange between the refrigerant inside the condenser coil and the surrounding. It does so by rapidly circulating air over the coil.

The fan is powered by an electric motor. If the motor fails, the fan will not spin and that means poor heat exchange. As such, auxiliary heat will come on to compensate for the lack of heating from the primary heat source.

You can actually check if that is the problem by peering from the top of the condenser unit. If the fan is stationery then that is the source of your problem

4. Faulty Reversing Valve

As I mentioned, it is the reversing valve that makes a heat pump different from a central air conditioner. It reverses the direction of flow of the refrigerant so that heat can go to the house and not outside.

Sometimes the reversing valve can malfunction and instead of heating the house, the heat pump will be operating in cooling mode. What is even worse with this problem is that the auxiliary heating will come on and try to heat the house while on the other side the heat pump will be cooling it.

If your aux heat is on but cool air is being discharged by the ducts, this could be the problem you’re having. What you can do is to turn on EM heat (which will bypass the heat pump) and call in a technician to fix the problem.

5. Faulty Defrost Control Board

As I mentioned, when ice accumulates around the condenser unit’s coils, the reversing valve will reverse the direction of flow of the refrigerant to melt the ice, which is called the defrost mode. That is made possible by the heat pump’s defrost control board.

At that time, auxiliary heating will come on to keep the house sufficiently heated.

If the defrost control board malfunctions, the ice on the coils will not be melted which will inhibit heat transfer. For that reason, the auxiliary heating will stay on until the problem is fixed.

Wrap Up

And basically that is everything about auxiliary heating. I hope you now know why the aux heat is on in your thermostat as well as how different it is from emergency heating.