Cielo
Key Takeaways
- Aux heat automatically kicks in when the heat pump can’t maintain the set temperature, usually in very cold weather.
- It uses more energy than the primary system, so limit its use to avoid high bills.
- Emergency heat is manually activated and should only be used when the heat pump is not working properly.
Heat pumps are highly efficient systems that can both heat and cool your home, making them an excellent choice for comfort and energy savings. They perform especially well in mild climates, but during extremely cold weather, their heating capacity can drop. That’s when auxiliary heat steps in to provide extra support and keep your home comfortably warm.
If you’ve ever noticed ‘Aux Heat’ on your thermostat and wondered what it means, you’re not alone. This guide will break down what auxiliary heat is, why it’s needed, and the different types of auxiliary heat sources. Additionally, it will also explain how aux heat differs from emergency heat, and how to use it effectively to maintain comfort without wasting energy.
What Is Auxiliary Heat?
Auxiliary heat is a supplemental or backup heating system that automatically activates when your heat pump struggles to maintain the set temperature during extremely cold weather. This often happens when outdoor temperatures drop to around 35–40°F or lower.
Think of auxiliary heat as a supportive sidekick that assists your main heating system when it needs an extra boost. Once your home reaches the desired temperature, auxiliary heat turns off automatically to prevent unnecessary energy use.
Budget-friendly smart thermostat for central HVAC systems. A thermostat that guarantees savings.
Award-winning smart thermostat for mini-splits. Technology that goes above & beyond.
Sleekest smart thermostat for mini-split, window & portable ACs. A design that magically blends.
Smart mini-split thermostat that is engineered for affordable comfort. Small in size, big in benefits.
Types of Auxiliary Heating Systems
- One common type is electric resistance heat strips integrated with the air handler. These use electric coils that heat up quickly to provide additional warmth. While effective, electric auxiliary heat is energy-intensive and typically the most expensive option to operate.
- If your home has access to natural gas or propane, a furnace or a boiler is an ideal choice for your secondary heat source. It takes over once the outdoor temperature drops below a certain threshold.
What Causes Auxiliary Heat to Switch On?
The aux setting on your thermostat is designed to switch on under specific conditions. Here are some scenarios that can cause the auxiliary heat setting to kick in:
1. The Outside Temperature Is Too Low
When outdoor temperatures fall to around 40°F or lower (depending on the system), auxiliary heat will be activated. The backup heating works alongside your primary heating source to meet the increased demand, allowing your home to reach the set temperature without the system struggling or running nonstop. This ensures steady comfort indoors, even when it is freezing outside.
2. Heat Pump Enters the Defrost Mode
When a heat pump goes into defrost mode, you will notice frost or ice on the outdoor unit. This is normal during cold weather. To prevent that, heat pumps are equipped with defrost mode.
During this process, the heat pump sends warm refrigerant to the outdoor unit to melt the ice. While this is happening, the system cannot heat your home as usual, so auxiliary heat turns on to maintain indoor temperatures. Once the ice is cleared, the heat pump returns to normal heating, and the auxiliary heat turns off automatically.
3. When You Turn Up the Thermostat by 3 Degrees or More
Raising your thermostat by 3°F or more at once often triggers auxiliary heat. The heat pump alone cannot meet the demand for rapid temperature increases, so auxiliary heat is engaged to speed up the process.
4. Heat Pump Malfunctioning
If you are experiencing heat pump issues, such as mechanical faults or refrigerant problems, the system may struggle to produce enough heat. Just like in cold weather or large temperature changes, the system senses that your home is not warming fast enough and activates auxiliary heat. The key difference is that, in this case, the heat pump may never catch up, which means auxiliary heat stays on much longer than usual.
If your heat pump has mechanical issues or refrigerant problems, it may struggle to produce enough heat. In this case, the thermostat detects a large difference between your indoor temperature and your setpoint, triggering auxiliary heat. Unlike normal operation, auxiliary heat may stay on longer because the heat pump cannot catch up.
Auxiliary Heat vs. Emergency Heat
Before diving into the differences, keep in mind that both are thermostat settings that use the same secondary heating system. Often, they are used interchangeably, as some heat pump systems have only an auxiliary mode, while others offer a separate emergency heat mode.
Auxiliary heat refers to when your second-stage heating, or, in other words, a supplemental heat source, switches on to aid your heat pump when it cannot produce enough heat to warm your home. The aux setting turns on automatically when needed and turns off as soon as your home reaches the desired temperature.
When your heat pump isn’t working, or you want to switch on your backup heating system intentionally, that’s called emergency heat. It’s like skipping the first stage and going straight to the second stage of heating. You would manually set the EM mode on your thermostat to warm your home while you wait for a technician to fix your heat pump.
When emergency heat is switched on, your heat pump shuts down completely. This makes it more expensive to operate and should be used only in emergencies.
Is Auxiliary Heat Expensive to Run?
Yes, auxiliary heat does cost more to run than a heat pump’s normal heating mode. The extra cost depends on two things: the type of auxiliary heat your system uses and how often it runs. In many homes, auxiliary heat can raise heating costs by up to 50 percent when it operates frequently or for long periods.
Auxiliary heating is temporary, though. It is only switched on for a limited time to give your heating a required boost. Once your set temperature is reached, it automatically switches off.
How to Limit Auxiliary Heat Usage
Reducing auxiliary heat use is mostly about helping your heat pump work under stable, efficient conditions. Small adjustments can significantly cut down how often backup heat is needed.
1. Avoid Large Thermostat Changes
Raising the thermostat by several degrees at once often triggers auxiliary heat.
- Set your thermostat to around 68°F in winter for maximum savings.
- Make small temperature adjustments (1-2°F at a time).
- Setback your thermostat from your usual settings when you are asleep or away from home to prevent unnecessary aux triggers.
2. Improve Insulation and Seal Air Leaks
Poor insulation and drafts cause heat to escape quickly, making the system struggle to keep up.
- To insulate your home, seal air leaks around windows, doors, and other openings.
- Add insulation to your attic, walls, and floors.
3. Maintain Your Heat Pump Regularly
Dirty filters, blocked airflow, or low refrigerant levels reduce heating performance.
- Clean air filters every 2 weeks & replace them after every 3 months.
- Keep outdoor units clear of debris, snow, and ice.
- Schedule professional maintenance once a year before heating season.
Conclusion
Auxiliary heat is not a sign that your heat pump is failing. It is a built-in backup designed to protect comfort during extreme cold, defrost cycles, or sudden temperature changes. Frequent use, however, can lead to higher energy costs and signal efficiency issues. Cielo smart thermostats offer an automation feature called Cielo Linked, which automatically switches between your residential HVAC systems. It helps ensure backup heating is used only when truly needed, improving comfort while avoiding unnecessary energy waste.
Equip your HVAC system with smart features and achieve the perfect balance between comfort & savings.
Learn more
Frequently Asked Questions
Does Auxiliary Heat Increase My Energy Bills?
Yes. Auxiliary heat, especially electric resistance heat, uses significantly more energy than a heat pump and can raise heating costs if it runs often or for long periods.
How Do I Stop My Auxiliary Heat From Coming On?
You can reduce auxiliary heat use by keeping thermostat temperature changes small, improving insulation, sealing air leaks, and ensuring your heat pump is properly maintained.
Is It Bad if Auxiliary Heat Comes On?
Auxiliary heat turning on is normal during very cold weather or defrost cycles. However, if it runs often, for long periods, or in mild temperatures, it may signal poor efficiency or a system problem and can increase energy bills.
