What is Auxiliary Heat?

A homeowner adjusting the temperature on the thermostat.

If you’ve ever seen the words ‘auxiliary heat’ or ‘aux’ on your thermostat and panicked, you’re not alone. It’s alarming to see anything out of the ordinary on our thermostats, but in this case, it’s not a reason for concern.

If your furnace has an auxiliary heat setting, it means you have ‘backup heat.’ Typically, this setting turns on when the temperature plummets and your heat pump can’t keep up with the temperature you set on your thermostat.

It’s a great way to provide peace of mind, knowing your house will be comfortable, but there are advantages and disadvantages to using aux heat.

Here’s everything you should know about auxiliary heat and how it works.

Definition and Function of Auxiliary Heat

At this point, you’re probably wondering what is auxiliary heat and how do I use it?

The good news is that you don’t have to do anything to make your aux heat work. If your furnace has one, it automatically kicks on if the temperature outside plummets.

When this happens, your outdoor heat pump may be unable to keep up with your home’s heating needs, so the aux heat automatically turns on to boost your heat pump. This simple process helps keep your home at a comfortable temperature without any effort on your part.

Your aux heat should only be on during freezing temperatures. If it runs too frequently, it could be a sign that your heat pump is malfunctioning. Running aux heat excessively can be more expensive and harder on your heating system.

Also, if your heat pump and aux heat aren’t working, your home’s temperatures can plummet fast. This can happen when the heat pump stops working. In this case, you can switch to your emergency heat setting.

Unlike aux heat, which is a supplemental heat source to help your primary heat pump, emergency heat is a secondary heat source that should only be used in emergencies. Most systems turn ’em heat’ on themselves, but you can also turn it on manually. If you do, it’s a sign that you need HVAC repair quickly, as you shouldn’t run the emergency heat for long.

Auxiliary Heat vs. Emergency Heat

Aux heat and emergency heat settings seem like they should be the same. After all, they both provide heat when the furnace can’t operate independently. However, there are some vast differences.

Aux heat kicks on when the furnace can’t keep up, usually when temperatures dip below 32 degrees Fahrenheit or if you increase the thermostat temperatures by two or three degrees and the furnace cannot keep up. It runs temporarily, usually no more than two hours, to get the home up to the desired temperature, but then should revert to the heat pump.

Unlike aux heat, emergency heat must be turned on manually. It doesn’t kick on when the heat pump cannot keep up. 

Emergency heat should only be used when temperatures dip below 30 degrees or your heat pump isn’t working. When you switch to emergency heat, you must call for HVAC repairs immediately. That isn’t the case with auxiliary heat.

Understanding When Auxiliary Heat is Needed

Aux heat isn’t a feature you’ll use often and shouldn’t run frequently. Here are some reasons it may turn on:

  • Increased thermostat temperature: If you increase the temperature set on your thermostat, your heat pump may be unable to keep up. When this happens, the aux heat may kick on to get your house to the temp you set. This typically happens when you set the thermostat two to three degrees higher.
  • The heat pump is defrosting: If ice builds up on the condenser coils, the HVAC system automatically goes into defrost mode. This means it pulls the hot air from inside the house to the coil to defrost it. Most furnaces switch to aux heat while this occurs.
  • Extreme drop in temperatures: If the temperatures drop drastically outside, your furnace may not be able to keep up. When this happens, it needs support, which the auxiliary heat provides.
  • System issue: Aux heat may turn on when it shouldn’t. If you notice it’s on, but there aren’t any indications it should be, consider calling for HVAC repair to determine the reason.

Preventing Auxiliary Heat From Running Unnecessarily

Auxiliary heat is there to help keep your home at a comfortable temperature, but it shouldn’t be used regularly. Here are a few ways to keep it from turning on unnecessarily:

  • Keep temperatures low: Raising the temperatures in your house when it gets below freezing can be tempting, but this only makes your heating system work harder. Keep your thermostat at 62 to 68 degrees Fahrenheit to prevent the system from working too hard. If the house feels cold, find other ways to warm it (or yourself) up.
  • Open shades during the day: On cold days, if the sun is out during the day, open the shades to heat your house. This increases the temperature inside and decreases the need for the furnace. By using the sun’s natural light, you increase the heat in your house and don’t have to put too much pressure on the furnace.
  • Close off unused rooms: If you have spare rooms in your house that you don’t use, keep their doors and vents closed. This allows the heat to go where needed and doesn’t make your furnace work as hard.
  • Get regular HVAC maintenance: Keeping your heat pump in good condition ensures it can work efficiently and will rely less on your auxiliary heat. This may help you save money on energy bills and emergency HVAC repairs. Have your system inspected annually and call for repairs immediately when you notice something is wrong.
  • Use layers: Layering up is not as convenient, but it helps you stay comfortable and reduces your heating costs. Use blankets and layer clothing to keep your house at the same temperature even when it turns freezing outside.

How to Know When Aux Heat is On

Most thermostats with aux heat will show it on the thermometer. It may appear as a big red ‘AUX’ or in small print near the temperature. Pay attention when it is on because it shouldn’t run long or too often.

Efficiency and Cost Considerations

Auxiliary heat is a great ‘backup’ and provides peace of mind when the temperatures drop. However, there are downsides, including energy efficiency and cost considerations.

Typically, auxiliary heat is less energy efficient because it uses two heat sources: the main heat pump and the aux source. This can increase your energy bills by as much as 50% or more. Running too long can cause premature failure of your furnace and require emergency HVAC repairs.

Maintenance and Troubleshooting

Aux heat is there to keep your house comfortable. However, common issues with your HVAC system can cause it to run excessively or not at all. Here’s what to consider.

  • Faulty fan motor: If the fan motor stops working, the heat pump doesn’t get enough heat to push into the house. This may cause the aux heat to run excessively.
  • Faulty compressor: The compressor is responsible for turning the refrigerant from a gas to a liquid, pushing it from the outdoor condenser to the indoor air handler. This prohibits the heat pump from transferring heat and causes the aux heat to run.
  • Low refrigerant: Without refrigerant, there is no heat absorption, which means your furnace cannot run properly. Low refrigerant is usually due to a leak, making your furnace run inefficiently.


Understanding auxiliary heat and when it should and shouldn’t run is important. Since it’s not something you must run manually, it’s easy to let your furnace work how it should. The key is recognizing when it’s running too much or when you need HVAC repair.

Aux heat differs from emergency heat, but both provide a backup source when your furnace isn’t running properly. The key to ensuring you don’t lose heat when you need it most is to complete proper HVAC maintenance and repair on time.

An HVAC professional can detect problems early and reduce the need to use auxiliary heat. Check out this list of HVAC professionals near you to keep your system running optimally.


Is it bad if the auxiliary heat comes on?

Auxiliary heat isn’t bad, but it shouldn’t run too long. Consider it a supplemental source of heat that should only run in certain circumstances. If you notice your aux heat running frequently or not shut off, it’s time for an HVAC inspection and potential repair.

Does auxiliary heat cost more?

Auxiliary heat can cost twice as much as regular heat, so it’s important to run it only when necessary. When you run aux heat, you run two heat sources, so you are using double the energy and risking the integrity of your HVAC system if it runs too long.

How do I stop my auxiliary heat from coming on?

To prevent your aux heat from turning on, you can do a few things. First, keep your home as warm as possible naturally without relying on the furnace. Close vents in unused rooms, open window treatments when the sun is out, and wear layers around the house.

Next, avoid increasing the house temperature when the outside temperature drops drastically. While it’s tempting to want much higher temperatures in the house, doing so can damage your HVAC system and increase your utility costs.

When should you use auxiliary heat?

Aux heat should turn on automatically when it’s needed. Unlike emergency heat, you shouldn’t turn it on. However, if you notice your aux heat runs frequently or when it’s not below freezing outside, it could be a sign that something is wrong with your system that requires HVAC repairs.

At what temperature should I turn on auxiliary heat?

Aux heat should turn on only when the temperatures outside are below freezing. However, it may also turn on if you increase the set temperature in your house by two or three degrees. The aux heat may supplement the heat pump to quickly warm the house. If it runs any other time, you should consider calling for professional HVAC repair.

What happens if auxiliary heat runs too long?

If your auxiliary heat runs too long, it’s a sign that something is wrong with your heat pump. Letting it run too long can get costly and damage other parts of your HVAC system. If you notice it running more frequently or for more than 30  minutes to a couple of hours at a time occasionally, call for professional repair.

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