Have you ever wondered about the auxiliary heat feature on your thermostat? Auxiliary heat offers additional heating power for your system, but only when it needs it. This shows up as “aux mode” or “aux heat” on most thermostats.
Many people aren’t sure what to do when this appears on their thermostat. Most assume there’s something wrong with their heat pump, but that’s not the case. To clear the air about this misconception, your system taps into auxiliary heat when the heat pump needs a temporary boost. You don’t have to do anything when the auxiliary heat is activated on your system.
So why does auxiliary heat turn on? What is its purpose? Let’s dive in and take a closer look at this mysterious feature.
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On this page
- What is auxiliary heat?
- When does it kick in?
- Auxiliary heat vs. emergency heat
- How to prevent auxiliary heating
- Is auxiliary heat essential?
What is auxiliary heat and what does it mean?
Auxiliary heat is a supplemental heating source in your heating system. This setting utilizes secondary heating, which differs from your outdoor heat pump system. The auxiliary heat source is located inside your home and can be gas, electric, or heat strips.
The specific type of indoor unit depends on the heating system you have at home. During auxiliary mode, the backup heating source is activated to increase the heating capabilities of your heat pump. This allows your home to reach the desired temperature faster.
Auxiliary heat is designed to kick in when you need to heat your home or space quickly and your main heat pump is not generating enough heat to meet the required thermostat setting. For example, a sudden drop in temperature by 2-5 degrees inside your home can trigger auxiliary heat.
Auxiliary heat shouldn't stay on for a long period of time. If it does, there could be an issue with your heater. Your heating system should automatically revert to its primary heat pump and deactivate auxiliary heat after 15 to 30 minutes.
When does auxiliary heating mode kick in?
Extremely cold temperatures
During the winter, temperatures tend to dip quickly after sunset – especially in areas prone to experiencing cold climates. Your heat pump will typically work harder when the outside temperature drops in order to maintain a warm, cozy environment inside your home. If your heat pump is having a difficult time maintaining the required indoor temperature setting, it will leverage “help” from your auxiliary heat source. Auxiliary mode is automatically deactivated and your system goes back to relying solely on the heat pump upon reaching the desired set temperature.
Sudden temperature increase on a thermostat setting
Rapidly increasing the temperature by a few degrees on your thermostat can cause your auxiliary heating system to kick in. This is because there is a major imbalance between the new temperature setting on your thermostat and the actual temperature reading of your home. Your heater wants to make up for the temperature difference as quickly as possible, so it activates the heat pump auxiliary for extra juice. When the temperature setting on your thermostat rises two degrees, auxiliary heat is turned off.
Heating defrost mode
Auxiliary heat is used when your heat pump undergoes a defrost cycle. This happens when outdoor temperatures dip below 32 degrees and frigid, cold air causes moisture to freeze on the heat exchanger. Icy buildup is not good for your heating system as it can reduce operating efficiency or damage the heater. Your heater automatically detects these issues when outside air causes such problems. As a solution, it will go into defrost mode for about 5 to 15 minutes.
When your heating system is in defrost mode and is able to transfer heat more efficiently, auxiliary heat is activated to keep your home warm. Auxiliary heat is deactivated and reverts back to the heat pump after the system finishes defrosting.
What is the difference between auxiliary heat and emergency heat?
| Auxiliary heat | Emergency heat |
| Automatic activation and deactivation | Manual thermostat setting |
| Works with heat pump | Works without heat pump |
| Does not point to potential heater damage under normal circumstances | May point to potential heat pump repair |
| Supplementary feature | Critical feature |
| Typically runs for 15-30 minutes | Runs as needed |
Auxiliary heat is an automatic, supplementary feature that kicks in when your heat pump is having a difficult time warming up your home or reaching a desired Fahrenheit temperature setting on your thermostat. Both the outdoor unit and secondary heat source are in use when auxiliary mode is activated. You cannot manually turn auxiliary heat on or off.
On the other hand, the emergency heat setting is reserved for unforeseen emergencies, such as when your main heat pump is malfunctioning. You can manually turn emergency heat on or off, as needed. Consider that in emergency mode, your heat pump is turned off and heating only comes from the secondary heat source. This heat setting should only be turned on when temperatures dip below 30 degrees.
How to prevent auxiliary heating
Slowly adjust your thermostat setting
As previously mentioned, rapidly increasing the temperature setting on your analog or digital thermostat can cause your heating system to activate auxiliary mode. You can easily prevent this by gradually adjusting your thermostat’s setting by 1 to 2 degrees every 5 to 15 minutes until you reach the desired temperature.
Keep a regular maintenance schedule for your heater
If your system persistently activates auxiliary mode, there could be something wrong with your primary heating. This is a sign that your heat pump is struggling to keep your home warm or at specific temperature settings. To prevent this issue, homeowners should regularly maintain their heater.
Be sure to do the following when maintaining your heater:
- Replace your filter every 30 to 90 days
- Clean the fan, chambers, and blowers
- Check for signs of rust or small holes
- Test the pilot light
- Check your vents and ducts for any blockage
In addition to preventing auxiliary mode, performing regular maintenance can keep your energy bills consistent and more predictable as your heater would be working smoothly. Like your air conditioner or HVAC system, your heater tends to consume more energy than normal when it has to work harder than it should.
Auxiliary heat is essential for a comfortable home
Auxiliary heat is nothing to worry about. In fact, it is a very important feature that allows your heater to operate effectively by giving it a boost during sudden temperature fluctuations. Without auxiliary heat, your system would have a difficult time keeping your home warm, as it would solely rely on the heat pump. Auxiliary heat mode is automatic and does not require any manual settings – unlike the emergency heat function.
While there are ways to prevent your heater from turning on auxiliary heat, there’s really no need to avoid the feature. This is because auxiliary mode does not cause damage to your system. If you’re concerned about damaging your heater and want to take preventive measures, you should focus on keeping a regular maintenance schedule. This ensures your system is working optimally at all times.