What is Auxiliary Heating: Everything You Need to Know

Ever wondered what is auxiliary heating and what “aux heat” means on your thermostat? Aux heat is short for auxiliary heat and is when your heating system activates the secondary heat source to reach the set temperature. This usually happens when the heat pump cannot provide sufficient warmth due to freezing weather.

Does this mean that aux heat is the same as emergency heat? While they are both similar in nature, aux heat and emergency heat are slightly different. Moreover, knowing the differences can help you save hundreds of dollars every month.

Keep reading on if you want to find out more about what is auxiliary heating, its differences, and the best way to reduce electricity bills this winter!

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What is auxiliary heating and how does it work

As mentioned, auxiliary heating is when the heat pump employs the secondary heat source to reach the set temperature when the primary heat source (heat pump) cannot provide enough heat during cold weather. To fully understand how that works, we must first look at how the heat pump system functions.

A heat pump transfers heat from one location to another.

In a heat pump system, heat is transferred between your home and the outside (even during cold weather). To warm up your home, the heat “pumps” warm air from outside into your house. The inverse applies the same technique; heat is transferred outdoors from the inside of your home. So, a heat pump transfers heat from one location to another rather than generating heat as fireplaces do.

Fans usually assist this heat transfer in both the outdoor and indoor units. However, the primary source of heat for your home via heat transfer may not be enough. During icy weather or wintertime, there is simply not enough heat outside to transfer in. This is when auxiliary heating comes into play.

How does auxiliary heating (aux heat) work

When the heat pump is facing difficulty reaching the temperature you set on your thermostat, auxiliary heating activates to support the heat pump. You can know when auxiliary heating is activated when you see the aux heat indicator present on your thermostat.

Auxiliary heating employs a secondary heat source which is usually some heating coils or electric resistance heating. The secondary heat source runs concurrently with the heat pump to make up the difference in your home’s temperature. There is also a trigger point for aux heat to turn on, usually 3 degrees below your thermostat’s set temperature.

For example, if you set the thermostat to 86°F (30°C) but the temperature inside your home suddenly drops to 85°F. Then, your thermostat will automatically turn on auxiliary heating until it reaches 86°F. Once it has reached the goal temperature, your thermostat will also automatically deactivate auxiliary heating.

Aux heat usually comes on when the outside temperature is 35-40°F (1.6-4.4°C). If you find that aux heat frequently turns on when the outside temperature is warmer than 40°F, it may be time to contact your contractor to have a look at your heat pump.

How much does auxiliary heating cost?

Aux heat will significantly increase your electricity cost because it runs two heat sources simultaneously. Remember, auxiliary heating engages electric resistance heating aside from the primary heat pump. Another reason is that electric resistance heating is much less energy-efficient than the primary heat pump. Hence, auxiliary heating only comes on when necessary. Empirical results for aux heat’s electricity usage varies from manufacturer to manufacturer. You may contact your heating system’s manufacturer for a graph on the electricity usage between aux heat and heat pump. What you must know is that auxiliary heating generally costs 50% more than the conventional heat pump. Keep reading to find out know how to save cost with your heat pump and aux heat. But, before that, you need to know when aux heat should and should not come on. Knowing these will help you determine whether to contact a serviceperson and make any changes to your home heating system.

When auxiliary heating should come on

Aux heat usually comes on during cold weather to support the heat pump.

Auxiliary heating is necessary when you need to heat your home quickly. As mentioned, aux heat supports the primary heating source during cold times. Hence, the temperature you set, how often you adjust the thermostat, and where you live contribute to how frequently aux heat comes on.

Generally, there are four regular occasions where auxiliary heating is required.

When the temperature outside is below 35-40°F (1.6-4.4°C)

To recap, a heat pump system transfers hot air from the outside to the inside of your home. Naturally, when there is simply not enough hot air to draw in, auxiliary heating is required. Typical times when aux heat comes on are during the wee hours or in winter.

If you live in a state or country that is cool all year round, you may find that aux heat comes on frequently. This is entirely normal. But, if you are unhappy about aux heat coming on frequently, you should consider contacting the manufacturer or changing to a different heating system.

When you adjust the thermostat

Auxiliary heating usually activates when the indoor temperature is about 3 degrees colder than the thermostat’s setting. So, if you suddenly increase the thermostat setting by, say, 8 degrees, the heating system will automatically energize aux heat to meet the set temperature.

Once the indoor temperature has reached the thermostat setting, the aux heat indicator should turn off itself. If it doesn’t, check if the outdoor unit is defrosting and if it isn’t, contact a technician immediately.

When the outdoor unit is defrosting

Ice can build on your heat pump’s outdoor unit (the box with a fan). Most times, the heat pump will detect something wrong with the outdoor unit and initiate defrosting mode.

Defrosting mode is when the heat pump switches to the air-conditioning mode for a while. Instead of transferring heat outside-in, heat is transferred to the outdoor coils for a few minutes or until it reaches about 57°F.

Then, as the coils warm up and melt any frost or ice, defrosting mode will automatically shut off. However, this also means that your home is being cooled down during winter or an icy morning. To counteract this, the heat pump system activates the supplemental heat source to heat the home.

For most heat pump systems, this would be electric resistance heating or auxiliary heating. Of course, defrosting mode is uncomfortable during cold weather but is necessary to prevent costly damages to the outdoor unit.

Key things to know about defrost mode

Depending on the amount of ice on the outdoor unit, the defrosting cycle could last anywhere from 30 seconds to 10 minutes. Moreover, the heat pump should have a timer to start the defrost cycle every 30, 60, or 90 minutes.

Signs your heat pump is defrosting:

  • When the indoor fan stops moving
  • You hear a valve near the outdoor unit initiating a defrost cycle
  • The defrosting indicator light is blinking
  • The outdoor fan is running

When the emergency heat is engaged

Emergency heating comes on when the primary heat source is facing a problem and cannot start. The outdoor unit could be facing one of many issues, such as being completely frozen over or a component failure. If it continues to run, the outdoor unit will continue to damage and become unusable.

When that happens, the thermostat engages the secondary heat source or electric resistance heating. During emergency heating, the home heating system relies solely on that secondary heat source which can be costly when left running for long periods.

When the door is left open

Auxiliary heating can also come on when someone has left the door open. An open door could very well change the indoor temperature significantly. As the thermostat detects this temperature difference, it then engages aux heat to reach the temperature on the thermostat.

When aux heat should NOT come on

Aux heat should not come on when the heat pump can provide sufficient heat on its own.

When the outdoor temperature is above 35-40°F

Auxiliary heating should automatically turn off once the heat pump efficiently transfers heat from outside into your home. This would be when the outside temperatures have increased, such as during the day or warm seasons.

However, if you find that the aux heat still comes on even though the temperature outside is above 35-40°F (1.6-4.4°C), it could mean that your heat pump is malfunctioning.

When adjusting the thermostat in small increments

Your thermostat will automatically activate auxiliary heating when the difference between the temperature indoors and the set temperature is greater than 3°F. So, aux heat should not come on when you increase the thermostat’s temperature in small increments.

However, if you find that aux heat still comes on, check if the heat pump is in defrosting mode or if the weather outside is frigid. If there is nothing out of the ordinary, it’s time to call a professional to give your heating system a check.

Cost of auxiliary heating and emergency heating

Auxiliary heating and emergency heating consume much more electricity than regular heat transfer. This is because they utilize electricity resistance heating which is less efficient than heat pumps. Aside from efficient energy transfer, heat pumps also have zero emissions.

Another factor in saving electricity costs is through tax credits. For example, if your heat pump is energy-efficient enough, you could be obliged to Non-Business Energy Property Tax Credits. Only your heat pump’s manufacturer can provide the exact number of your aux heat’s electricity usage.

However, we estimate aux heat to be 50% more expensive than heat pumps. Emergency heating should cost even higher because it engages only the secondary heat source.

Remember, the secondary heat source is meant as a backup because it is less energy-efficient. Therefore, using it for prolonged periods will reflect on a more expensive electricity bill at the end of the month.

How to reduce cost and avoid aux heat issues

Schedule a regular maintenance check to ensure that your home heating system is issue-free.

There are two main ways to minimize the cost of auxiliary heating: reduce aux heat usage and maintain it properly. Aside from electricity bills, you should also consider the cost of repairing your heat pump if it is not maintained well.

In this section, you will learn five tips to reduce the cost of your heat pump system without sacrificing comfort.

Adjust the thermostat in small increments

You should only increase the temperature on your thermostat in small amounts. Since aux heat only engages when the temperature difference is more than 3°F, avoid increasing the temperature by more than 3°F every time.

Service the heat pump regularly

You should call a professional to regularly inspect your heat pump and ensure that it is in good shape. Prevention is always cheaper than repairs, so we recommend you schedule a maintenance check at least once a year. The serviceperson should inspect the ducts, motors, coils, electrical wiring, and any crucial component for optimum operation.

Aside from professional inspection, there is some maintenance work that you can DIY. For example, you can inspect and change the heat pump filter once a month. A dirty filter restricts airflow to the outdoor fan, stressing the motors and bringing dirty air indoors.

Clean the outdoor unit regularly

You should make sure that the surroundings of the outdoor unit are free of any debris. Leaves, grass, and other common lawn debris can get inside the outdoor unit and obstruct airflow.

You can also clean the outside coil by yourself once it gets dirty. However, make sure you follow the manufacturer’s instructions when cleaning the outside coil.

Use economy mode if available

Some thermostats, such as the Google Nest Learning Thermostat, have an Eco mode. The Eco mode will either heat or cool your home to keep it within the Eco Temperatures range. This Eco Temperatures range depends on the model of the thermostat that you own. For the Google Nest Learning Thermostat, the Eco Temperatures range from 40-70 °F (4-21 °C) for heating mode and 76-90 °F (24-32 °C) for cooling mode.

You can also contact a private energy company or government energy office to give you a free energy audit depending on where you live. The energy audit will assess how much energy your home uses and what measures you can take to make it more efficient.

Replace with another heating system

Under ideal conditions, a heat pump can transfer 300% more energy than it consumes. However, heat pumps can rack up electricity costs over extended periods of cold weather. So, more and more households are adopting dual-fuel systems — a gas furnace and heat pump in one heating system.

A dual fuel system activates the heat pump during a hot spell This is because heat pumps are much more energy-efficient than gas furnaces (300% efficient vs. 95%). Then, during colder weather, the gas furnace will take over the heat pump in providing warmth for your home.

While gas furnaces are less energy-efficient than a heat pump, it prevents the engagement of auxiliary heating. This allows for optimum comfort for the lowest cost possible. Regardless, a dual-fuel system only works at its best when there is regular inspection and maintenance.

If you are interested in learning more about the different types of home heating systems, read on! In a later section, we briefly laid out the various types of home heating systems and the average cost of each heating system.

Signs that your aux heat and heat pump is malfunctioning

As much as we want to save costs, we can only do so much since heat pumps and like all things, will inevitably begin to malfunction. Nonetheless, knowing the early warning signs of a malfunctioning heat pump system can help you do early repairs and avoid more significant issues.

Aux heat is running during hot weather

Generally, aux heat engages during winter or the wee hours of the morning. Aux heat will usually come on when the outside temperature is lower than 35-40°F (1.6-4.4°C), which is slightly above freezing. However, if auxiliary heating is still engaged when the temperature outside is above 40°F, then it’s time to contact a technician.

Unusually high electricity bill

As mentioned above, a malfunctioning aux heating system will cause it to come on when it’s not supposed to. So whether you notice the aux heat coming on or not, the malfunctioning secondary heating will reflect in an oddly high electricity bill.

A high utility bill at the end of the month is that auxiliary heating uses more energy than heat pumps. So, naturally, using more electricity leads to a more expensive electricity bill.

Defrost cycle happens too frequently

When the defrost indicator comes on one too many times, it means that there is an issue with your thermostat or heat pump. Defrosting should take only a few minutes at a time and only occurs when there’s ice in the outdoor unit. If you notice the heat pump going into defrost mode too often, call a technician immediately! Defrosting for extended periods will lead to hefty electricity bills.

Not maintaining the set temperature

If you have adjusted the thermostat, but the temperature in your home does not reflect that change, there’s something wrong with the heat pump. It may be tempting to give your thermostat a few whacks, but the best course of action is to call a professional.

Aux heat still running despite the house being warm enough

Similarly, the thermostat should detect that the house is too warm and automatically switch off the aux heat. However, if you find the aux heat is still running when you’re sweating beads, then it’s time to call for help.

Difference between aux heat and emergency heating

When reading the above sections, you may notice that we referred to aux heat and emergency heating as two different things — that’s because they are!

Auxiliary heating and emergency heating are similar in that they both activate the secondary heating source. This secondary heat source is usually electricity but can also be oil or gas. The core difference is that while aux heat runs concurrently with the heat pump (transferring heat outside in), emergency heating shuts down the heat pump and relies only on the secondary heat source.

Auxiliary heating is also meant to help the heat pump fill in the difference in temperature indoors. On the other hand, emergency heating only comes on when something is wrong with the heat pump.

Leaving the heat pump to run despite having issues will lead to irreparable damage. So, the emergency heating mode is meant to prevent further damage to the outdoor heat pump system by shutting it off entirely. Moreover, you must switch the emergency heating mode manually, but some heat pumps do activate it automatically.

Types of home heating systems

Fireplaces are beautiful home heating systems but are also less energy-efficient.

By now, you may be considering home heating systems that do not use auxiliary heating. Perhaps, you’ve installed a heating system that is not suitable for your home and wants to change to something else. Fortunately, you’ve come to the right place! Aside from heat pumps, these four heating systems are most commonly found in homes due to their cheap installation and maintenance costs.

Furnaces: Furnaces generate heat by burning natural gas, propane, oil, or electricity. Then, it distributes hot air through ducts or a blower. Modern furnaces can achieve energy efficiency ratings of up to 97% but still falls short of heat pumps that are regularly 100% energy-efficient.

Boilers: Boilers boil water and distribute it all over the house through pipes and eventually to radiator units. While common, boilers cannot function as an air-conditioner during warmer weather. Moreover, boilers are best suited for providing zone heating rather than warming the entire house.

Radiant heating system: Radiant heating provides heat through tubes installed under the floor or in concrete slabs. Since it uses electricity, it is more energy-efficient than boilers and is quiet compared to other heating systems. However, maintenance is complex because of the hidden piping system.

Dual-fuel heating system/Hybrid heating: Dual-fuel heating systems utilize heat pumps and a gas furnace for optimum comfort. The heat pump activates during warmer periods, whereas the gas furnace takes over during colder times such as winter.

Which home heating system has the lowest annual cost?

The truth is, optimizing your home heating system for the lowest electricity bills is more complicated than it seems. It involves many variables such as the climate, heat insulation, amount of windows, lighting, etc.

The best way to cut your utility costs is to consult a professional for an energy audit. Then, by upgrading to the right equipment, conducting regular maintenance, optimizing the thermostat settings, and improving your home’s insulation, you will find yourself saving up to 30% on your monthly electricity bill.

That being said, some groups have attempted to estimate the annual cost of different types of home heating systems. According to this home heating cost calculator by Efficiency Maine, the home heating systems with the lowest yearly costs are wood stoves and geothermal heat pumps.

Wood stoves and geothermal heat pumps have an average annual cost of $1,169 and $1,168, respectively. In contrast, an electric baseboard and propane furnace have average annual costs of more than $3,500.

Average heating costs graph chart

Which home heating system is the right one for you?

Average annual costs are important factors to consider when buying or changing your home heating system. However, it isn’t the only factor in play. Some aspects of a home heating system that you have to consider include:

  • Energy efficiency and tax credits
  • Fuel and maintenance cost
  • Life expectancy
  • Zoned heating vs. central heating
  • Size and installation cost

Auxiliary heating can be costly, but it may still be the best home heating system for some homes. We recommend you get an energy audit from your local government energy office. After assessing your home’s conditions and taking the right actions, you should be able to save hundreds of dollars every month on utility bills.


How do I check if my home heating system has aux heat?

You can identify if your home heating system has aux heat by looking at your thermostat. The auxiliary heating mode will appear as a setting on your thermostat. If your thermostat does not have aux heat, your home heating system also doesn’t have it.

How to change the trigger point of auxiliary heating?

Auxiliary heating automatically activates when the difference between the indoor temperature and the thermostat setting is larger than 3°F. Depending on the model of your thermostat, it may be possible to adjust the trigger point of aux heat.

However, we recommend you contact the manufacturer. Calling a professional is better than risking damaging any expensive components or circuitry of your home heating system.

Is aux heat better than emergency heating?

Both modes are not better than one another because they have different functions. However, engaging aux heat is less costly than emergency heating. This is because while aux heat runs the secondary heat source concurrently with the heat pump for a while, emergency heating completely shuts down the heat pump and solely runs on the secondary heat source.

What is the difference between heat pumps and air-conditioning?

Heat pumps and air-conditioning work on the same principle in both transfers heat from one location to another. The critical difference is that while a heat pump transfer heat in both directions (outdoors and indoors), an air-conditioner can only transfer heat from inside to the outside.

Key takeaways

When the aux heat indicator is displayed on your thermostat, it means that the secondary heat source is engaged. Aux heat is crucial because heat pumps cannot transfer enough heat indoors when the temperature outside is too low. So, aux heat will run concurrently with the heat pump to make up for the temperature difference.

Auxiliary heat also engages when the temperature indoors is lower than the thermostat setting by more than 3°F. While aux heat keeps you warm and cozy at home, you’ll want to avoid using it 24/7 because its inefficient energy usage can cause your electricity bills to skyrocket.

You shouldn’t confuse aux heat with emergency heat because while both activate the secondary heat source, emergency heat completely shuts down the heat pump. Emergency heat’s primary function is to avoid further damage to the heat pump when it has issues rather than to make up any temperature difference.

Thank you for reading this far, and we hope that you found this article useful! Also, if you found our tips helpful or know of suggestions to save electricity costs, let us know in the comments below!

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