Auxiliary heat is a secondary or supplemental heating source integrated into a heat pump system that activates when the primary heat pump alone cannot satisfy the thermostat setpoint. It most commonly consists of electric resistance heating elements housed within the air handler unit, and it engages automatically when outdoor temperatures drop to a point where the heat pump’s heating capacity falls short of the building’s demand. Auxiliary heat should not be confused with emergency heat, which bypasses the heat pump entirely and relies solely on the backup heating source.
Technical Details and Specifications
Electric resistance heating elements used for auxiliary heat are typically rated between 5 kW and 20 kW, selected based on the system’s calculated heating load and climate zone. Because electric resistance heat operates at a Coefficient of Performance (COP) of 1.0, it converts energy to heat on a one-to-one basis. This stands in contrast to a properly functioning heat pump, which can achieve a COP of 2.0 to 4.0 during moderate outdoor temperatures, making auxiliary heat significantly more expensive to operate per unit of warmth delivered.
Most thermostats allow configuration of the activation differential, which is the temperature gap between the current room temperature and the setpoint that triggers auxiliary heat. This differential is commonly set between 1 and 3 degrees Fahrenheit. Many systems also employ staged activation, bringing electric elements online in increments rather than all at once. This staging approach more precisely matches heating output to actual demand, reducing unnecessary energy consumption. Auxiliary heat also activates during the heat pump’s defrost cycle, compensating for the temporary loss of heating output while the outdoor unit reverses its refrigerant cycle to clear frost from the evaporator coil.
Applications and System Integration
Auxiliary heat is found in virtually all ducted air-source heat pump installations in climates where outdoor temperatures regularly fall below the system’s balance point. The balance point is the outdoor temperature at which the heat pump’s capacity equals the building’s heat loss. Below this temperature, supplemental heating becomes necessary. Common applications include:
- Residential single-family homes with split-system heat pumps
- Light commercial buildings using packaged heat pump units
- Multi-zone systems where individual air handlers contain dedicated strip heaters
In some installations, fossil fuel furnaces serve as the auxiliary heat source in dual-fuel or hybrid systems, offering a more cost-effective alternative to electric resistance heat in regions with low natural gas prices.
Relevant Standards and Codes
Installation of electric resistance auxiliary heat must comply with NEC (NFPA 70) requirements for circuit sizing, overcurrent protection, and conductor ratings. AHRI Standard 210/240 governs the performance rating of heat pump systems, including conditions under which supplemental heat is factored into seasonal efficiency calculations such as the Heating Seasonal Performance Factor (HSPF/HSPF2). Local building codes and energy codes, including IECC provisions, may limit the use of electric resistance heating or require controls that minimize its operation to meet efficiency targets.
Practical Significance
Frequent auxiliary heat activation is one of the most common causes of unexpectedly high utility bills in heat pump households. Technicians should verify proper thermostat differential settings, confirm adequate refrigerant charge, and ensure the outdoor unit is not restricted. Monitoring auxiliary heat runtime through modern smart thermostats can provide valuable diagnostic insight into overall system performance and help identify when a heat pump is underperforming relative to expected capacity.