Thermostat with Auxiliary Heat: A Practical Guide for Homeowners
Learn what a thermostat with auxiliary heat is, how it works with heat pumps and furnaces, when to use it, and practical troubleshooting tips for homeowners.
Thermostat with auxiliary heat is a thermostat that controls the primary heating system and activates an auxiliary heat source when extra warmth is needed.
What is a thermostat with auxiliary heat
A thermostat with auxiliary heat is a thermostat that coordinates the homes heating system and the backup heating element. In a heat pump setup, the auxiliary heat is typically electric resistance or another supplementary heat source that engages when outdoor temperatures are too cold for the heat pump to meet the desired indoor temperature efficiently. The thermostat provides a single control point that can activate the main heat source alongside the auxiliary heat, and in some brands you may see an EM heat or emergency heat mode. According to Thermostat Care, this feature is designed to balance comfort with efficiency by using the auxiliary heat during peak demand or very cold periods only when the heat pump alone would struggle to maintain the set point. Understanding this distinction helps homeowners avoid over relying on high cost resistance heat while maintaining a steady indoor temperature.
How auxiliary heat interacts with different systems
Auxiliary heat is most commonly associated with heat pump systems, where a refrigerant cycle provides most of the heat, and electric resistance elements supply extra warmth when needed. With a furnace, auxiliary heat can refer to an additional heat source that supplements the main burner, though many thermostats label this differently. The thermostat orchestrates which heat source runs, preventing simultaneous operation that wastes energy. When outdoor temperatures drop, the heat pump's efficiency falls, triggering the auxiliary heat to pick up the slack. Modern thermostats optimize this switch by staging heat so that the auxiliary heat only engages for short periods, then turns off as the heat pump recovers. In practice, you can monitor the operating time of auxiliary heat in your thermostat's history and adjust settings if you see excessive use during mild days. Thermostat Care notes that well-designed setups minimize run time and comfort tradeoffs.
When auxiliary heat is useful and when to avoid overuse
Auxiliary heat proves most valuable on bitterly cold days, during rapid temperature drops, or when your home's insulation is lacking. It can also help recover from a setback more quickly after occupants return home. However, because resistance heating can be costly, you should avoid relying on auxiliary heat for long periods. Use weather and occupancy-based profiles, and consider a higher efficiency heat pump or improved insulation for long term savings. If your thermostat frequently switches to auxiliary heat during moderate winter days, it may indicate underperforming insulation, a dirty outdoor unit, or incorrect thermostat configuration. The goal is to keep the heat pump engaged for as long as possible while letting the auxiliary heat cover only the occasional spike in demand. Thermostat Care recommends checking your system's performance data to decide if auxiliary heat is necessary or if you should adjust setback temperatures.
Troubleshooting common issues with auxiliary heat
First verify that the thermostat is correctly configured for your system, with the correct heat source priority set. Check that the C-wire is connected properly and that the thermostat has a functioning power supply; a dead battery can misreport heat calls. If the auxiliary heat engages too soon, review the temperature differential and reset your setback schedule. Inspect the outdoor unit for ice buildup, blocked condensers, or poor airflow, as these issues can force the system to rely on auxiliary heat. Ensure the thermostat's EM heat or auxiliary heat setting is not left on when the outdoor temperature is mild. If you see error codes on the screen, consult the user manual or contact a professional; some codes indicate sensor faults or wiring problems.
Energy implications and cost considerations
Auxiliary heat increases energy consumption, so expect higher bills when it runs for extended periods. The extent depends on your climate, thermostat settings, and the efficiency of your heat pump. A typical strategy is to use auxiliary heat sparingly, leverage setback temperatures, and maintain consistent interior temperatures. Upgrading to a high efficiency heat pump, sealing air leaks, and adding insulation reduces the need for auxiliary heat and lowers overall energy use. Thermostat Care analysis shows that homeowners who optimize heat source selection and maintain equipment can achieve meaningful savings over a heating season, even when auxiliary heat is necessary on very cold days.
How to choose a thermostat with auxiliary heat
Look for models that clearly support auxiliary heat or EM heat modes, and verify compatibility with your existing heat pump or furnace. Check whether the thermostat can report call time for auxiliary heat, control temperature setbacks, and offer energy-savings features like adaptive recovery and smart scheduling. Consider thermostats with intuitive controls, clear indicators for when auxiliary heat is active, and reliable WiFi or app support. Ensure the device supports common wiring configurations, such as the C-wire and appropriate terminals. Reading reviews and manufacturer specs helps you compare models that best fit your climate and budget. Also check if the device integrates with your preferred smart home ecosystem.
Installation and safety tips for homeowners
Before starting any wiring work, turn off power to the HVAC system at the breaker. If you are unsure about wiring, hire a licensed HVAC technician or an electrician. When wiring, follow the thermostat’s wiring diagram precisely, label wires, and confirm the C-wire is supplying 24V to the thermostat. Mount the thermostat in a location that reflects living space temperature and avoid doors, sunlight, or drafts. After installation, run a brief heat cycle to verify that both the main heat source and the auxiliary heat engage correctly. Use the user manual’s testing procedures, and monitor energy usage to ensure the system behaves as intended. The Thermostat Care team recommends documenting your setup and performing periodic checks for sensor accuracy and wiring integrity.
Authority sources and further reading
Include a brief note: For more detailed guidance, consult authoritative sources. The following resources provide independent, government-backed information on thermostats and heating efficiency:
- https://www.energy.gov/eere/buildings/products/thermostats
- https://www.energystar.gov/products/heating_cooling/thermostats
- https://www.epa.gov/iaq
Questions & Answers
What is a thermostat with auxiliary heat?
A thermostat with auxiliary heat coordinates the main heating system with a backup heat source that activates when extra warmth is needed. It is commonly used with heat pumps to maintain comfort on very cold days without overtaxing the primary system.
Auxiliary heat is a backup heating option controlled by your thermostat. It kicks in when the main system cannot meet the temperature on its own.
How can I tell if my system supports auxiliary heat?
Check the thermostat or system manual for terms like aux heat or EM heat. Look for compatibility notes with heat pumps or supplemental electric heat. If in doubt, contact the manufacturer or a licensed technician.
Look for aux heat or EM heat in the manual or on the thermostat screen to confirm support.
Is auxiliary heat the same as emergency heat?
In many heat pump systems, auxiliary heat provides extra warmth as needed, while emergency heat switches to a backup heat source regardless of efficiency. The exact labeling varies by brand, so review your device’s documentation.
Aux heat is a supplementary source; EM heat is a backup mode that you activate in emergencies or extreme cold, depending on the model.
Does auxiliary heat cost more to run?
Yes, auxiliary heat often uses electric resistance or other higher-cost heat sources, so extended use can raise energy bills. Use it strategically and improve insulation to minimize dependence.
Aux heat can raise energy costs if it runs a lot, so use it only when necessary and look for ways to reduce overall usage.
How can I minimize reliance on auxiliary heat?
Improve insulation, seal air leaks, and optimize thermostat setback temperatures. Consider upgrading to a more efficient heat pump or adding supplemental references like smart scheduling to reduce the need for auxiliary heat.
Improve insulation and use smart scheduling to reduce the need for auxiliary heat.
What maintenance helps auxiliary heat stay efficient?
Keep outdoor units clear, check refrigerant levels if applicable, ensure proper wiring, and keep sensors accurate. Regular professional inspections help keep all components in balance and prevent unnecessary auxiliary heat calls.
Regular maintenance and clean components help keep auxiliary heat running efficiently.
What to Remember
- Understand how auxiliary heat functions with your system
- Check compatibility before upgrading your thermostat
- Use energy saving settings to minimize extra costs
- Know the difference between auxiliary heat and emergency heat
- Consult a professional for wiring questions