Heat Pump Thermostat Guide: Aux Heat vs Emergency Heat
Understanding aux heat, emergency heat, and optimal thermostat settings for your heat pump. The right settings can be the difference between an efficient system and an expensive one.
The Short Answer
Aux heat is normal and automatic — your system turns it on when it needs help. Emergency heat is manual and expensive — only use it if your heat pump is broken.
If you see “EM HEAT” on your thermostat and you didn’t turn it on, call your contractor.
What Is Aux Heat?
Auxiliary heat is your backup heating system — usually electric resistance strips inside your air handler. Your thermostat activates it automatically when:
- Outdoor temperature drops below ~40°F and the heat pump can’t keep up alone
- You raise the thermostat more than 2–3°F at once
- The heat pump enters a defrost cycle
- The heat pump is struggling to reach the set temperature
Seeing “AUX” on your thermostat in cold weather is normal. It means your system is working correctly.
Source: Trane, Lennox
When to Worry About Aux Heat
Runs for more than 30 minutes continuously
Aux heat should cycle on and off. Continuous operation suggests the heat pump is not contributing.
Runs when outdoor temp is above 40°F
The heat pump should handle the load alone in mild weather. Aux heat at 45°F+ indicates a problem.
Electricity bill spikes unexpectedly
A sudden increase in your electric bill often means aux heat is running more than it should.
Aux heat is on every day even in mild weather
Daily aux heat activation during moderate temperatures points to a system issue.
Any of these could indicate a refrigerant leak, dirty coils, failing compressor, or thermostat wiring issue. Call your contractor.
What Is Emergency Heat?
Emergency heat (EM HEAT) is a manual thermostat setting that bypasses the heat pump entirely and runs only the backup heating system.
Emergency heat shuts off your heat pump compressor. You lose all the efficiency advantage of the heat pump. Your backup strips run at COP 1.0 — which can cost 2–5 times more than normal heat pump operation.
When to Use Emergency Heat
- Your heat pump compressor has failed
- The outdoor unit is damaged (storm, ice)
- You are waiting for a repair technician
- NEVER as a regular heating mode
If you switch to emergency heat, call your contractor immediately. It’s a temporary measure, not a heating strategy.
Thermostat Settings That Save Money
Set it and forget it
Heat pumps work best at a constant temperature. Unlike a furnace that blasts hot air quickly, a heat pump delivers gentle, continuous warmth. Set your thermostat to 68°F and leave it.
Avoid large setbacks
A 2°F night setback (68 to 66) is fine. A 5°F setback (68 to 63) triggers aux heat on recovery in the morning — costing more than you saved overnight.
Don’t crank it up
Raising the thermostat from 66 to 72 when you’re cold triggers aux heat because the heat pump can’t close that gap fast enough. Raise it 1–2°F at a time.
Source: Trane, Jackson EMC
Smart Thermostats and Heat Pumps
Compatibility Requirements
Must support heat pump wiring (O/B terminal for reversing valve)
Should support aux heat staging (W1/W2)
Should have heat pump-specific algorithms that avoid unnecessary aux heat
Must support outdoor temperature sensor or weather data for intelligent lockout
Compatible Options
Ecobee Smart Thermostat
Built-in heat pump mode with aux heat management and remote sensor support
Honeywell T10 Pro
Heat pump compatible with multi-stage support and contractor-configurable settings
Google Nest Learning Thermostat
Heat pump compatible — verify wiring compatibility before purchase
Ask your contractor which thermostat they recommend for your specific heat pump. Not all combinations work well together. Using the manufacturer’s recommended thermostat is the safest choice.
Defrost Cycles — What’s Normal
In cold, humid weather, frost builds on the outdoor unit’s coils. Your heat pump periodically reverses to defrost — you’ll hear the compressor change sound, see steam rising from the outdoor unit, and feel cooler air from vents for a few minutes. This is normal.
Normal
- + Defrost runs for 5–10 minutes at a time
- + Happens a few times per day in cold weather
- + Steam rises from the outdoor unit during defrost
- + You hear the compressor change sound briefly
Not Normal
- – Ice stays on outdoor unit for 30+ minutes
- – Defrost cycle runs constantly or very frequently
- – Indoor temperature drops significantly during defrost
- – Outdoor unit is completely encased in ice
Source: D&B ClimateCare
Dual Fuel Settings
If you have a heat pump paired with a gas furnace, your thermostat needs a “balance point” or “changeover” setting — the outdoor temperature where it switches from heat pump to furnace.
Typical Changeover
25–35°FDepends on your heat pump’s efficiency curve and local gas vs electricity rates.
Below the changeover temperature, gas heat may be cheaper than running the heat pump. Above it, the heat pump is always more efficient. Your contractor should calculate the economic crossover point for your specific fuel costs.
This is different from the balance point discussed in our cold climate guide — that’s about capacity. This is about cost.
Frequently Asked Questions
Should I turn off aux heat on my thermostat?
No. Aux heat is a normal part of heat pump operation. Disabling it can leave your home cold during very low temperatures or after a defrost cycle. Let the thermostat manage it automatically — it activates only when the heat pump cannot meet the heating demand alone.
Why does my thermostat say "Aux Heat" when it’s not that cold outside?
This usually happens when you raise the thermostat several degrees at once. The heat pump cannot close a large temperature gap quickly, so the thermostat activates aux heat to assist. If aux heat runs frequently above 40°F without a large setpoint change, have your system inspected.
Is emergency heat more expensive than aux heat?
Emergency heat and aux heat typically use the same electric resistance strips. The difference is that emergency heat bypasses the heat pump entirely — so you lose all heat pump efficiency. Running on emergency heat full-time can cost 2–5 times more than normal heat pump operation.
What temperature should I set my heat pump thermostat to?
Most energy guides recommend 68°F (20°C) during waking hours. The key with heat pumps is consistency — avoid large setbacks. A 1–2°F night setback is fine, but dropping 5°F+ overnight triggers expensive aux heat during morning recovery.
Do I need a special thermostat for a heat pump?
Yes. A heat pump thermostat must support the O/B reversing valve terminal and ideally support aux heat staging. Standard furnace thermostats will not work correctly. Check that your thermostat is explicitly listed as heat pump compatible before purchasing.
Thermostat settings depend on your specific heat pump model, backup heat type, climate, and utility rates. This guide provides general best practices based on manufacturer guidance from Trane, Lennox, and industry resources. Consult your contractor or equipment manual for model-specific recommendations.
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