What Is Auxiliary Heat on a Heat Pump

Heat pump systems often include an auxiliary heat setting that activates when the primary heat source cannot maintain indoor temperature. This article explains what auxiliary heat is, how it differs from emergency heat, when it operates, cost and efficiency implications, and how homeowners can manage and troubleshoot auxiliary heat. Understanding auxiliary heat helps prevent surprises on utility bills and improves home comfort during cold weather.

Topic	Quick Fact
Also Called	Aux Heat, Backup Heat, Second-Stage Heat
Main Purpose	Supplement heat pump when outdoor temperature is low or when defrost cycles occur
Typical Source	Electric resistance heating or gas furnace
Cost Impact	Usually More Expensive Than Heat Pump Mode

What Auxiliary Heat Means

Auxiliary heat is a secondary heating source integrated with many heat pump systems to provide additional warmth when the heat pump alone cannot meet the thermostat’s setpoint. It is commonly referred to as aux heat or backup heat.

The auxiliary system typically uses electric resistance coils or a conventional furnace fueled by gas, propane, or oil. When engaged, auxiliary heat delivers immediate, reliable heat but often at a higher operating cost per unit of heat than the heat pump’s standard mode.

Why Heat Pumps Need Auxiliary Heat

Heat pumps move heat rather than generate it, extracting energy from outdoor air or the ground. As outdoor air temperature falls, heat pumps become less efficient and may struggle to deliver sufficient heat.

Auxiliary heat activates when the heat pump cannot sustain the thermostat setpoint, during rapid temperature drops, extended cold periods, or when the system enters a defrost cycle. Auxiliary heat ensures consistent indoor comfort when the primary heat source is insufficient.

Types Of Auxiliary Heat

Electric Resistance Auxiliary Heat

Electric resistance is the most common auxiliary heat type in many modern heat pump installations. It uses heating elements that convert electricity directly to heat. These elements heat quickly and are easy to control, but they are typically the most expensive option per heating unit.

Fossil Fuel Backup Furnaces

Some systems use a gas, propane, or oil furnace as the auxiliary source. In these setups, the heat pump works as the primary source and the furnace provides supplemental heat when needed. Furnaces can be more economical in areas with low electricity costs or high gas availability.

Dual-Fuel Systems

Dual-fuel systems combine a heat pump with a fossil-fuel furnace and switch between the two based on outdoor temperature or system efficiency. A control board or thermostat typically governs the swap to optimize efficiency and cost.

Auxiliary Heat Versus Emergency Heat

Two thermostat settings often confuse homeowners: auxiliary heat and emergency heat. While both involve the secondary heating source, they serve different roles.

Auxiliary Heat is an automatic supplement that the heat pump control activates when needed. It operates together with the heat pump during hard pulls or long recovery periods.

Emergency Heat is a manual thermostat setting that disables the heat pump, forcing the system to rely solely on the auxiliary heating source. Emergency heat is used when the heat pump is broken or when it cannot operate properly.

How The Thermostat Controls Auxiliary Heat

Modern thermostats manage auxiliary heat using algorithms that monitor temperature difference, runtime, and outdoor conditions. When the thermostat determines the heat pump cannot reach the setpoint within a reasonable time, it commands the auxiliary stage.

Some thermostats include settings for outdoor temperature locks or set thresholds to control when the furnace or electric resistance engages, enabling users to tune the balance between comfort and cost.

When Auxiliary Heat Activates

Auxiliary heat commonly activates in several scenarios: during very cold outdoor temperatures, when the heat pump’s capacity is overwhelmed by a sudden temperature change, or during defrost cycles when the outdoor coil temporarily reverses and the heat pump output drops.

Thermostats may also trigger auxiliary heat if indoor temperature falls several degrees below the setpoint or if compressor runtime suggests the heat pump is struggling. Activation logic varies by system and thermostat model.

Energy Use And Cost Considerations

Electric resistance auxiliary heat is generally more expensive to run than heat pump operation because it converts electricity directly to heat at a 1:1 input-to-output ratio, while heat pumps move heat and can deliver multiple units of heat per unit of electricity when operating efficiently.

Using auxiliary heat frequently can significantly raise energy bills. Homeowners should monitor runtime and consider thermostat settings, property insulation, and heat pump capacity to minimize auxiliary use.

How To Reduce Auxiliary Heat Usage

• Improve insulation and air sealing to reduce heat loss and lower demand on the heat pump.
• Install a programmable or smart thermostat to better manage setpoints and avoid frequent calls for heat.
• Ensure correct heat pump sizing and regular maintenance to maintain optimal performance.
• Consider a dual-fuel strategy or outdoor temperature lockout on auxiliary heat to prevent excessive use at mild temperatures.
• Use adaptive staging thermostats that delay auxiliary heat activation and prefer longer run cycles for the heat pump when feasible.

Signs Auxiliary Heat Is Running Too Often

Frequent or prolonged auxiliary heat activation may indicate underlying issues. Common signs include a sudden spike in electric bills, the thermostat indicating second-stage heat is active often, and heat pump short-cycling or long recovery times after setback periods.

Homeowners noticing these signs should check air filters, outdoor unit condition, refrigerant charge, and ductwork for leaks. A professional HVAC technician can diagnose whether the heat pump needs repair, replacement, or resizing.

Troubleshooting And Maintenance

Regular maintenance reduces unnecessary auxiliary heat usage. Key tasks include cleaning or replacing filters, clearing debris from the outdoor unit, and scheduling annual professional inspections for refrigerant levels, electrical connections, and mechanical wear.

If auxiliary heat engages unexpectedly, homeowners should inspect thermostat wiring and settings, verify that the compressor runs during calls for heat, and confirm that outdoor temperatures haven’t crossed any thermostat-defined lockout points.

Smart Thermostats And Auxiliary Heat Optimization

Smart thermostats provide tools to optimize auxiliary heat use. Features like learning algorithms, geofencing, and weather compensation can reduce unnecessary auxiliary activation by adapting schedules and anticipating temperature changes.

Some smart thermostats allow setting an outdoor temperature threshold for auxiliary heat or support staged heat strategies to prioritize the heat pump before switching to the auxiliary stage.

Cost-Benefit Of Upgrading Systems

Upgrading to a high-efficiency heat pump, improving insulation, or installing a smart thermostat can reduce reliance on auxiliary heat. While upfront costs exist, energy savings and improved comfort often justify investments over several years.

For homes in very cold climates, a dual-fuel approach with an efficient furnace may offer better economics than electric resistance backup. A professional energy assessment helps determine the most cost-effective path.

Common Misconceptions

One widespread misconception is that auxiliary heat is always bad. In reality, auxiliary heat is essential for maintaining comfort when the heat pump cannot keep up. The concern lies in unexpected or excessive use, not its existence.

Another misconception is confusing emergency heat with auxiliary heat operation. Emergency heat should be used sparingly and only when the heat pump is inoperative.

Safety And Code Considerations

Auxiliary heating elements and backup furnaces must meet local building codes and safety standards. Electric auxiliary elements require correct wiring, overcurrent protection, and proper sequencing to avoid short-cycling or overloads.

Gas or oil backup systems must be vented and inspected regularly. Any changes to auxiliary heat systems should be performed by licensed HVAC professionals to ensure compliance and safety.

Questions To Ask An HVAC Professional

• Is the current auxiliary heat source electric resistance or a fossil-fuel furnace?
• Is the heat pump correctly sized for the home and climate?
• Can thermostat settings be adjusted to reduce unnecessary auxiliary activation?
• Would a dual-fuel system or heat pump upgrade improve efficiency and lower bills?
• What maintenance schedule will keep the heat pump operating optimally?

Key Takeaways For Homeowners

Auxiliary heat is a necessary supplement that keeps the home warm when the heat pump cannot meet demand. It provides reliable heating but usually at higher operating costs.

By combining proper maintenance, insulation upgrades, and intelligent thermostat strategies, homeowners can minimize auxiliary heat use while maintaining comfort and controlling energy bills.