What is AUXILIARY HEAT on your thermostat? Easy answers to 10 common questions

Being a homeowner can be a bit overwhelming, especially when you notice your thermostat flashing ‘Auxiliary Heat’ or ‘EmHeat’. It’s natural to feel concerned and wonder about the best way to heat your home.

Auxiliary heat serves as a supplement to your primary heat source. It doesn’t replace your regular heat, but kicks in automatically when the primary system struggles to keep up with extra demand.

Whether you’ve owned your home for a short or extended period of time, questions about heating and cooling are bound to arise.

Learning about different heating types and deciding on the best way to heat your home might seem daunting. In this article, we’ll demystify auxiliary heating to empower you to make informed decisions.

What is the Difference between Auxiliary Heat and Conventional Heat? 

There’s a significant difference between conventional heat and auxiliary heat. Auxiliary heat acts as a supplementary source, and is triggered by environmental factors, adjusting the thermostat, or by issues with your heating system.

Honeywell thermostat showing auxiliary heat

Overall, auxiliary heat is much more powerful than your conventional heating source, but it comes at the cost of less efficiency and greater expense.

Another difference between your conventional heat (heat pump) and your auxiliary heat is that your regular heat pump is located outside of your home, either in the ground or above, pulling in outside air. In contrast, your auxiliary heat is located inside your home, within your furnace or air handler.

Furnace diagram showing airflow

The heat pump extracts the “warmth” out of the outside air, and then pumps that warm air into your home. But there is only so much warmth that can be pulled out of the air, especially when the ambient outside temperature is below freezing.

That’s where the auxiliary heat comes into play. When the heat pump can no longer be effective with heat extraction, the auxiliary heat kicks on and uses electric resistance heaters to heat the air the rest of the way.

It is exactly because of this temperature change limitation of a conventional heat pump, that an auxiliary heat setup is necessary. You will not typically find it on any other systems, except in special cases.

Does Auxiliary Heat Turn on Automatically?

Once you set your programming for your regular heating system, it operates automatically. Depending on your system, this can be done in multiple ways: 

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  • Setting your controls manually with your thermostat 
  • Using a smart system with an app 
  • Using a voice-controlled device like Amazon Alexa or Google Nest

Your auxiliary heat is pre-programmed to also be automatic. That way, you don’t have to worry about it. It simply will notify you with a ‘EmHeat’ or ‘Aux Heat’ icon on the thermostat when it is running.

There are generally four different reasons it may turn on, and it usually does not signify that there is an issue with the system.

#1 It may kick on if the temperature outdoors dips below 35 degrees outside, and the air is so cold that there is not enough heat to draw out of it. 

#2 It may toggle on if the indoor air temperature is three degrees colder than what the thermostat is set to. 

For example, if your thermostat is set to 63 degrees and the temp in your house is 59 degrees, the auxiliary heat may kick on.  

The manufacturer typically programs the three-degree trigger point. 

Three degrees is standard, but the variance may differ depending on your unit and brand of equipment.

This can be changed, but it does need to be done by an HVAC professional.  

#3 Although your heat pump is otherwise functioning, condensation on the outside can lead to the pump freezing. 

Your auxiliary heat will kick on if your pump is frozen and not sending warm air into the air handler. 

#4 Lastly, if it is cold enough, the system may think it is frozen when it is not, and your pump can get stuck in defrost mode, triggering the auxiliary heat to kick in.  

Read: Why Is Auxiliary Heat On, But No Heat?

heat pump in snow

How Do I Turn Off Auxiliary Heat? 

If you do not want your system using the auxiliary heat source, it can be programmed to turn off completely, or the set point can be adjusted. 

Because HVAC systems are so technical, it is best to have an HVAC technician come out and program it not to toggle on.  

If you feel like this is a challenge that you can take on yourself, or if you don’t have the extra money to have a tech come out, you can do some additional research online based on your unit. 

You should be able to find at least an instruction manual for your unit’s make and model.

Some manufacturers make it easy to change, and some make it more challenging, to the point where you would need a tech to set it up for you. 

The simplest option is merely turning your heat down so that your system is not working too hard.  This option is less convenient, but saves the cost of hiring a tech.

Turn your heat down.  When it comes to the set temp, kick it up a few more degrees and continue the pattern until you reach your desired indoor temperature. 

The small incremental increase will keep your aux heat from getting toggled on.  

Can I Change Features When the Auxiliary Heat Turns On?

The great thing about technology is that it is adaptable, and we can change it to fit our needs. 

Although it may be tricky to do yourself, you can change certain features that toggle the auxiliary heat.   

Some systems may come with instructions, and often you can find instructional videos online based on your specific unit.

There are two common features that you can change or have a professional change.  The first is what outside temp the heat toggles on for. 

Generally, it is set between 32 to 35 degrees for the auxiliary heat to kick on. 

If you live in a place where it is common for the outside temps to be that low, you may want to see about getting the temp setting updated for a lower toggle point.

The second most common feature you can change is when the thermostat is three or more degrees higher than the indoor temp. 

If you want it to toggle sooner or later, you can get a tech, or yourself, if so inclined, to change the toggle point. 

If you are away from your home for long periods and leave the heat off, you may want a smaller toggle point to heat your home quicker and vice versa. 

Whatever your circumstances are, most machines are made to be adaptable to some degree. 

If you cannot figure it out on your own, a local tech is always just a call away.  Sometimes a small service fee is better than a large power bill or accidentally damaging your system.  

Is Auxiliary Heat More Expensive to Use?

Auxiliary heat and conventional heat use very different methods to heat the air pumped through your home.

Because auxiliary heat is straight resistance heat, and will heat up your home much quicker than a heat pump, it will be significantly more expensive. 

It will vary depending upon your energy provider, but typically auxiliary heat is well over twice as costly than conventional heat.  

Auxiliary heat is different from your average heat source because instead of using a heating pump, it uses electric heating strips. 

Auxiliary heat strips

The heating strips use a lot more power than a compressor pump.  The pump heats the air with its motor (by compression), using a minimal amount of energy, while the strips use extra electricity to heat the air quicker.  

So, if your auxiliary heat is kicking on often, you will be paying an inflated rate for your heat. 

Keep an eye on your thermostat, and if you find that your auxiliary heat is on fairly often, you may want to reach out to a local heating company for a heating system check-up.  

How Long Does Auxiliary Heat Stay On? 

Your auxiliary heat will stay on for as long as it needs to.  How long the auxiliary heat depends on how your system is set and what the environment is like. 

If it gets freezing at night and warms significantly during the day, then your auxiliary heat may only turn on for a little while in the wee hours of the morning at the coldest temperatures. 

If you live in an area where it stays cold during both the day and night, then your auxiliary heat may be on more to keep up with demands.  

Depending on your system’s features and what they are set to, your aux heat timeframe will vary. 

The best way to understand your system is to research your specific make and model in your general climate.  

Should you Worry if it Stays On?

Your auxiliary heat should only stay on when it is needed.  For the reasons mentioned earlier, when the indoor heat meets the household heating needs, your auxiliary heat should have kicked off automatically.  

If your auxiliary heat is staying on for extended periods, you may want to do a few tests to see if you have a larger problem.  Some of the things you could try include:

  • Turning your heat down, so there is not as big of a difference between what your thermostat is set to and what the indoor temp is
  • Checking your pump (if possible) to see if frozen
  • Watching the outdoor temp to see if it may affect your toggle point 

Although these may not solve your problems, these are easy ways to see if the auxiliary heat and your conventional heat are working correctly. 

If you have tried these things and the auxiliary heat is still on, it may be time to call a heating expert.

Because auxiliary heat is so much more expensive to heat with, it may be in your favor to pay a tech and get a repair rather than pay inflated heating costs over the cold winter months.  

Is it Better to Use Auxiliary Heat? 

Auxiliary heat is an excellent supplemental heat source, but it is not intended for long-term use, and the more you use it, the more expensive your heating bill will be overall.

side view of emergency heat strips

If you are having problems with your heating system keeping up with your demands, you may want to consider the following over using auxiliary heat:

  • Shutting doors and closing vents to rooms that are not utilized
  • Turning the heat down and add more layers to your wardrobe.  Opting for a chillier home, even just a few degrees, can monumentally affect your power bill and the use of your auxiliary heater
  • Utilizing a fireplace if one is available – although you need to purchase wood or pellets, this can put off a significant amount of heat and are very efficient
  • Making sure that you try to seal off any drafty areas in the house. Windows and doors that have a draft can be a huge issue when trying to heat or cool your home
  • Using a portable heating source to heat a small, enclosed area – if you will be spending most of your time in one area/room and you can enclose that room, it may be in your favor to use a personal heating source for that area rather than heat into your whole house if it is not needed
  • Depending on your climate and circumstances, solar or wind power may be a viable option

Because auxiliary heat is much more expensive, it should only be used when needed.  Suppose features need to be reset for your auxiliary heat.

In that case, overall, it may be cheaper to pay a professional to reset options to be more efficient than it would pay an excessively high heating bill during the winter months. 

Is Auxiliary Heat the Same as Emergency Heat?

Auxiliary heat and emergency heat do virtually the same thing, but are simply controlled differently. 

Auxiliary heat is automatically triggered by environmental reasons, as noted above. 

Emergency heat is turned on manually or automatically and typically should only be done when the outdoor temperature is below 30 degrees.

The main difference is that the auxiliary heat is a backup or supplement to your conventional heating or external heat pump. 

Both will be running simultaneously; the auxiliary kicks on to help the external heat pump out.

When emergency heat is toggled, it sometimes turns the external pump off and only uses the heater’s interior system. 

So rather than taking in the cold air to warm and pump through the house, using the emergency heat turns the external pump off and only uses the interior air that is already much warmer than the outside air. 

This should only be used in an emergency and when the outside temperature is below freezing.  This is done for two specific reasons:

  • To keep your exterior heat pump from being damaged from the freezing weather; and
  • To heat your home more efficiently when it is freezing outside

Your emergency heat should only be used when necessary to avoid damage to your external heat pump. 

Although your heat pump will be pumping hot air through its lines, your coils can freeze with the condensation build-up and refrigerant. 

When this happens, or before it happens, is when you would want to use your emergency heat setting, avoiding any damage to your exterior heat pump.  

How Does Emergency Heat Work?

Emergency heat is a lot like auxiliary heat, with a few key differences.  Once the outdoor temperature is below 40 degrees, your pump will need to defrost every few hours.  

If the outdoor temperature dips into the 20’s and 30’s your external pump will not be able to produce any heat as there would not be enough to extract.

When the temperature gets this low, it makes it extremely hard to compress, and it makes it even harder to pull the little heat that is available in the air.

When this happens, it can cause immense damage to your heat pump.  When the temperatures are this low, you would want to switch your emergency heat on. 

When you switch your emergency heat on, it ceases operation to the external pump and utilizes the interior pump, thus avoiding damage to the external pump.

When using your emergency heat, the most significant consideration is that it uses a less efficient method to heat and should only be used in an emergency – hence the name emergency heat. 

You may want to try alternative heating methods, if possible, before turning on your emergency heat.  Emergency heat should only be used temporarily.

How Do You Know if Your System Can Keep Up with the Heat Demand?

Much like an air conditioner, your heater or furnace can only pump out so much heat. 

The two most significant things that become heating system issues are 1) not having the correct system size and 2) not having an energy-efficient system.

If your system is not the right size or is old, your heater will be working overtime, your heating bill could go through the roof, and your auxiliary heat would be kicking on more often.

There are things you need to keep in mind as well that could be contributing to your unit not being efficient enough, like:

  • Drafty house
  • Skipping out on regular system maintenance
  • Leaving the unit on 24/7 – if you leave, turn it down; it needs a break now and then
  • Not changing out your filters
  • Blocking the air vents with pieces of furniture
  • Cranking up the dial when you’re cold – this will heat your home faster but will put added stress on the system
  • Not keeping your vents free of dust

Some of these things seem like no-brainers, but when we get busy or things get in our way, we tend to forget the little things. 

We can do small things like setting reminders to change our filters and schedule our annual maintenance ahead of time. 

Keeping your system in good shape is essential.  Imagine a cold night when the temps dip below freezing and your heater malfunctions!

What Can I Do to Keep My Heating System Proficient?  

To keep your auxiliary heat from kicking on as much as possible, it is essential to keep your heating system in top condition. 

There are a few things that you can do yourself or a professional may need to do from time to time.  These can range from:

  • Changing your filter regularly
  • Vacuuming your vents so that the dust on the exterior does not get sucked in
  • Getting an annual tune-up – this can include topping off fluids, cleaning vents, and servicing parts that are not functioning properly
  • Paying attention to any changes in functions – not pumping out the same amount of heat, clunking noises, etc. 
  • Keeping an eye on your bill – if you notice a spike in your statement and there were no significant exterior temperature changes, this may signify an issue with your system
  • Noticing that your system is acting differently – turning off and on rapidly, etc.

Ensuring that your system is in top condition will prevent your auxiliary heat from coming on unless necessary. 

Although it can be time-consuming, it is essential to make sure your system is functioning correctly.  Otherwise, it undoubtedly will mean high service repair bills down the road.


There is a difference between auxiliary heat and conventional heat.

Now knowing all the ways and reasons to use auxiliary heat, you will be able to make a more educated decision for your household and situation. 

You can determine whether you want to turn the additional heat function off since not all environments require it. 

This can save you money. Knowing how to make the best decisions for your household gives you the option to be more efficient.