Massachusetts Heat Pump Auxiliary Heat Explained
Auxiliary heat is backup heat that may turn on automatically when a heat pump needs help meeting demand, during some defrost cycles, or during a large thermostat recovery. Emergency Heat is different: it is generally a manually selected backup mode.
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Auxiliary heat vs. emergency heat
Auxiliary heat
Usually controlled automatically. In many ducted systems this is electric resistance backup. It may assist during very cold weather, rapid recovery or defrost.
Emergency heat
Usually a user-selected thermostat mode intended for situations where the heat-pump portion cannot operate normally. It should not be treated as an everyday efficiency setting.
Why auxiliary heat can increase an electric bill
Electric-resistance backup converts electricity directly to heat and can use substantially more electricity than normal heat-pump operation. If auxiliary heat runs frequently, total electric use can rise even when nothing is technically broken.
Possible drivers include colder weather, thermostat setbacks, undersized equipment, poor insulation/air sealing, an outdoor-unit fault, improper controls, airflow problems or a heat pump that is not carrying the expected share of the load.
See why a Massachusetts heat-pump electric bill may be high.
When aux heat deserves investigation
- Auxiliary heat stays on for very long periods during ordinary winter weather.
- The house still loses temperature with backup heat operating.
- Aux heat begins appearing much more often than it did previously under similar weather.
- The outdoor unit is not operating, is heavily iced, or repeatedly faults.
- The thermostat is unexpectedly in Emergency Heat.
Massachusetts thermostat strategy
Heat pumps often work well with relatively steady setpoints. Large setbacks can create a high recovery demand and, depending on controls, trigger backup heat. That does not mean every home should use the exact same thermostat schedule; equipment type, utility rate, occupancy and building performance differ.
For broader winter behavior, defrost and icing, use the Massachusetts Heat Pump Winter Guide.
Need diagnosis?
If auxiliary heat use suddenly changes, the home cannot maintain temperature, or the outdoor unit is not operating normally, use the Massachusetts heat pump repair guide. For replacement/design questions, see heat pump installation in Massachusetts.
Auxiliary Heat, Emergency Heat and the Heat Pump Are Not the Same Thing
“Aux heat” usually means a secondary heat source that the thermostat or equipment controls can call automatically. “Emergency heat” is generally a manual operating mode that disables normal heat-pump heating and relies on backup heat. The exact behavior depends on the equipment and controls.
Electric resistance backup
Common in ducted air handlers. It can add substantial electrical demand because resistance heat converts electricity directly to heat rather than moving heat like the heat pump.
Furnace backup
Dual-fuel systems can pair a heat pump with a gas or oil furnace. Controls may switch heat sources based on outdoor temperature, capacity or configured economic/comfort logic.
Defrost support
Backup heat can operate during defrost to offset the temporary cooling effect indoors. Brief auxiliary-heat operation during winter weather can therefore be normal.
When Auxiliary Heat Use Deserves Attention
| What you see | Possible interpretation | What to do |
|---|---|---|
| Aux appears briefly during very cold/wet weather | May be normal defrost or supplemental operation | Watch whether the house maintains temperature |
| Aux runs after a large thermostat setback | Controls may be responding to a large recovery demand | Use smaller setbacks and observe behavior |
| Aux runs almost continuously in moderate weather | Control issue, heat-pump performance problem or sizing/distribution limitation may exist | Request professional evaluation |
| Emergency Heat is selected | Normal heat-pump operation may be disabled | Use only as intended by the system/contractor and schedule diagnosis if the heat pump failed |
| Electric bill jumps dramatically | Resistance backup may be running more than expected | Compare thermostat history, outdoor conditions and system performance |
Why Massachusetts Homeowners Should Care About the 2025–2026 Heat-Pump Rates
Eligible Eversource, National Grid and Unitil customers who heat with heat pumps can receive lower seasonal winter delivery-rate components from November 1 through April 30. That can reduce winter operating cost, but it does not make unnecessary resistance-backup operation irrelevant because the rate applies to household electricity usage rather than making backup heat free.
See the Massachusetts Heat Pump Incentive Tracker and High Heat-Pump Electric Bill Guide.
Questions to Ask at Installation or Service
- What type of backup heat does my system have?
- What conditions are intended to trigger it?
- Is there an outdoor-temperature lockout or balance point?
- How does the system behave during defrost?
- What thermostat setting should I use for normal winter operation?
- What pattern would indicate a fault rather than normal supplemental heat?
Official context: ENERGY STAR air-source heat pumps · Mass.gov seasonal heat-pump rates.
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