Heat pump emergency heat and auxiliary heat are two different things that share a heating element, and the distinction matters on your bill. Auxiliary heat is the control system adding electric resistance capacity while the compressor keeps working. Emergency heat is a manual mode that switches the compressor off completely and heats the house with resistance alone. One is normal winter behavior. The other is a temporary measure for a broken outdoor unit.
How a heat pump splits the heating job
A heat pump moves heat from outdoor air into the house rather than creating it, which is why it can deliver more heating energy than the electrical energy it consumes. That advantage shrinks as outdoor temperature falls, because there is less heat available to move and the compressor works harder for a smaller result. Below some outdoor temperature the compressor alone cannot keep up with the building load. Manufacturers therefore pair most air-source heat pumps with a backup: electric resistance elements in the air handler, or in some systems a gas or hydronic furnace. The control system decides when to bring that backup in, based on indoor temperature, outdoor temperature and how long the compressor has been running.
What the auxiliary indicator is really telling you
When the thermostat displays auxiliary heat, the compressor is still running and the resistance elements are supplementing it. That is a designed condition, not a fault. It commonly appears on the coldest mornings, when someone raises the setpoint several degrees at once, and during and after a defrost cycle, when the outdoor coil is being cleared and the indoor air needs tempering. Frequent auxiliary operation in mild weather is worth investigating, because it can signal low refrigerant charge, a dirty outdoor coil, restricted airflow, a thermostat with an aggressive staging setting, or a heat pump that was undersized for the house. A technician can watch the staging behavior and settle which it is.
Why emergency heat costs so much more
Emergency heat locks the compressor out of the circuit and carries the entire heating load on resistance elements. Resistance heating converts electrical energy to heat at essentially a one-to-one ratio, while the heat pump moves several units of heat per unit of electricity across most of its operating range. Running in emergency mode therefore multiplies the cost of every hour of heating, and people who flip the switch in November and forget about it often discover the difference in a January bill that is several times what they expected. The mode exists so a household is not left cold when the outdoor unit fails, not as a seasonal comfort preference or a way to heat faster.
When emergency heat is the right call
Switch to emergency heat when the outdoor unit is clearly not working: it is silent during a heating call, it is buried in ice that does not clear, it is tripping its breaker, or a technician has told you to keep it off until a repair. In those cases the mode keeps the house habitable while you arrange service. It is also worth using briefly if the outdoor unit is making a sound that suggests mechanical distress, because continuing to run a struggling compressor can turn a repair into a replacement. Once the unit is fixed, put the thermostat back into normal heat so the compressor resumes carrying the load it was bought to carry.
What the wiring behind the modes looks like
On a typical air-source system the thermostat controls the reversing valve with an O or B conductor, the compressor with Y, the blower with G, and the backup with W or W2. Selecting emergency heat simply tells the thermostat to stop energizing the compressor and reversing valve outputs and to call the backup directly. Resistance elements draw substantial current and are fed by their own circuits with their own overcurrent protection, sequenced by relays so they do not all energize at once. That is why the outdoor unit and the air handler usually have separate breakers. If your emergency heat produces no heat at all, the fault is often in an element circuit or a sequencer rather than in the thermostat.
What to check before you act
- Confirm whether the display says auxiliary, which is automatic, or emergency, which is manual.
- Look at the outdoor unit during a heating call to see whether it is running.
- Check the setpoint history if you have been making large temperature jumps.
- Return the thermostat to normal heat as soon as a repair is completed.
Common questions
Does auxiliary heat mean something is broken?
Not by itself. It is normal in very cold weather, after a large setpoint change and around defrost, but frequent use in mild weather deserves a service check.
Will emergency heat warm the house faster?
Usually not meaningfully, and it removes the compressor’s contribution, so you generally get similar comfort at a much higher cost per hour.
The practical takeaway
Auxiliary heat is the system helping itself and belongs on the thermostat automatically. Emergency heat is a manual fallback for a failed outdoor unit and should be temporary. If auxiliary keeps appearing in mild conditions, treat that as a diagnostic signal about charge, airflow or sizing rather than as something to override with a switch.
References and further reading
- Department of Energy: estimating how much energy home equipment consumes
- ENERGY STAR: efficiency information for heating and cooling equipment
- NFPA: electrical safety guidance for household equipment circuits
Numerical scenarios are illustrative unless identified otherwise. Follow the exact product instructions; component ratings and local installation requirements can differ.
