Emergency Heat vs Aux Heat — What's the Difference?
The difference between Aux heat and Emergency heat is one of the most misunderstood concepts in heat pump operation — and mistakenly leaving a system in Emergency heat mode can add hundreds of dollars to a monthly electric bill. Aux (auxiliary) heat means the heat pump compressor and the electric resistance heat strips are working together to meet the heating demand during extremely cold weather. Emergency heat means the compressor is completely locked out and only the electric strips are running — the most expensive possible heating mode. This guide explains exactly when each mode activates, how the balance point works, what the COP comparison means for your electric bill, and how the lockout relay wiring works on Carrier, Trane, Lennox, Goodman, Rheem, and Amana systems.
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Common Symptoms
- EM HEAT indicator light is on even though you did not select emergency heat
- Thermostat shows AUX in heating mode — wondering if this is normal
- Electric bill spiked dramatically after cold weather
- Heat pump runs but house only heats slowly or not at all
- Compressor is not running when thermostat is in heating mode
Most Likely Causes
- 1
Balance Point Exceeded — Aux Heat Activating Normally
Every heat pump has a balance point: the outdoor temperature at which the heat pump alone can no longer keep up with the home's heat loss rate, and the backup heat strips must assist. In most residential heat pumps this balance point falls between 25°F and 40°F outdoors depending on the system size and home insulation. When the outdoor temperature drops below the balance point, the thermostat's second-stage call (W2) energizes the heat strip sequencers — this is normal, expected Aux heat operation. The compressor continues to run alongside the strips.
- 2
Emergency Heat Selected Accidentally or Left On
Emergency heat mode can be activated by pressing EM HEAT on the thermostat or through app settings. In EM heat, the thermostat sends a lockout signal to a relay on the air handler control board that opens the Y circuit — preventing the compressor from running. Only the electric strips operate. The COP (coefficient of performance) of electric resistance heating is exactly 1.0 — every watt of electricity becomes exactly one watt of heat. A heat pump operating at 35°F outdoor temp typically achieves a COP of 2.0–3.0 — producing 2–3 watts of heat per watt of electricity consumed. Emergency heat therefore costs 2–3 times more to operate than normal heat pump operation at the same outdoor temperature.
- 3
Heat Pump Fault Triggering Automatic Lockout
Some heat pump control boards automatically activate the backup heat strips and lock out the compressor when a fault code is detected — such as a low pressure lockout, high pressure lockout, or defrost board fault. This mimics emergency heat mode without the thermostat EM HEAT switch being activated. The EM HEAT indicator may or may not light on the thermostat depending on whether the lockout signal came from the board or the thermostat.
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Quick DIY Checks
Do not run emergency heat mode as a substitute for a properly repaired heat pump. Electric heat strips are limited to the rated kW capacity — typically 5–20 kW on residential systems. Running strips continuously in very cold weather can cause the air handler to exceed temperature limits and trip the high-limit switch.
Electric heat strips operate at 240VAC and draw 20–80 amps depending on system size. Never bypass a sequencer or contactor to force heat strips on manually. The inrush current without staging causes voltage drops that can trip breakers or damage the compressor motor start circuit.
- 1Identify whether you are in Aux heat or Emergency heat. On the thermostat: if you see AUX on the display during heating, the heat pump compressor is running alongside the heat strips — this is normal Aux heat. If you see EM HEAT or the emergency heat indicator light is on, the compressor has been locked out. On Nest thermostats: look for the green leaf icon — it shows when Aux heat is engaged. On Ecobee: the home screen shows 'Aux Heat' in the heating status. Emergency heat will show 'Em Heat' or an icon.
- 2Check outdoor temperature against your system's balance point. Aux heat is expected below 35–40°F for most heat pumps. If outdoor temp is 15°F and you see AUX, that is completely normal. If you see AUX at 55°F outdoor temp with no good reason, investigate the heat pump for a fault — the compressor may not be running even though the system is calling for first-stage heat.
- 3Verify the outdoor unit compressor is running when in Aux mode. Go outside and listen — the compressor makes a sustained hum and the outdoor fan should be spinning. If the thermostat shows AUX and the outdoor unit is completely silent and off, the compressor has been locked out by a fault — check the air handler control board for a fault code or flashing LED sequence.
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Try Pro — $7.99/mo- 4Understand the COP economics. At 35°F outdoor temperature, a well-maintained heat pump delivers roughly a COP of 2.5 — meaning 2.5 units of heat per unit of electricity. Electric resistance strips deliver COP 1.0. At 12 cents per kWh, heating a home with the heat pump uses about 4 cents of electricity per 10,000 BTU of heat; emergency heat (strips only) uses about 10 cents per 10,000 BTU. Monthly cost difference on a 1,500 square foot home in a cold climate: often $150–$300+ per month more in EM heat.
- 5Check the emergency heat lockout relay wiring on the air handler control board. On Carrier and Trane air handlers: the emergency heat lockout relay opens the Y1 compressor call circuit when the thermostat's E terminal sends 24VAC. Measure voltage at the E terminal on the air handler board — 24V present means the thermostat has activated lockout. On Goodman and Amana boards: look for a terminal labeled 'EH' or 'EM' that receives the lockout signal. If you find 24V at this terminal when you didn't select EM heat, the thermostat is sending an incorrect signal — check thermostat configuration or replace the thermostat.
- 6Disable emergency heat correctly if it was accidentally activated. On most thermostats, EM HEAT is a physical button or switch — press it again to return to normal heat pump mode. On Nest: Settings → Equipment → verify 'Heat' is selected, not 'Emergency Heat'. On Ecobee: change the thermostat mode from 'Em Heat' back to 'Heat'. On Honeywell T6/T10: the EM HEAT mode switch is a dedicated setting — return to the standard HEAT mode. Always confirm the outdoor unit compressor restarts within 3–5 minutes of disabling EM heat.
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Repair vs Replace
If the system is locked in EM heat due to a compressor fault, diagnose the underlying heat pump issue — low refrigerant, capacitor, or contactor — rather than accepting ongoing high-cost strip heat. Parts for sequencers, contactors, and capacitors are low-cost. A heat pump over 15 years old with multiple concurrent failures may warrant replacement.
Est. Repair Cost
$0 (mode correction) to $100–$300 (sequencer or relay replacement)
Est. Replacement Cost
$4,000–$8,000 for a new heat pump system
Recommended Tools & Parts
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Heat Strip Sequencer Relay
Thermal sequencer that staggers electric heat strip activation 15–30 seconds apart to prevent voltage spikes. Replacement for failed sequencers that leave strips either always-on or always-off.
$15–$40
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Dual Run Capacitor for Heat Pump (45+5 MFD)
Replacement capacitor for heat pump compressor and outdoor fan motor. A failed capacitor prevents compressor start, causing aux heat to run without the compressor.
$15–$40
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Ecobee SmartThermostat Premium
Correctly handles Aux and Emergency heat modes with configurable outdoor temperature lockout settings. Prevents unnecessary emergency heat activation.
$189–$249
Links are Amazon affiliate links (tag: fixitfastai-20). Prices are estimates.
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Frequently Asked Questions
- Is it okay to leave my heat pump in emergency heat mode all winter?
- No — emergency heat mode is intended for short-term use when the heat pump has failed and needs repair. Running EM heat all winter typically costs 2–3 times more than normal heat pump operation because electric resistance strips deliver a COP of 1.0 while a heat pump at typical winter temperatures delivers COP 2.0–3.0. On a 2,000 sq ft home in a cold climate, this difference can be $200–$400 per month. If your heat pump is not working, have it repaired promptly rather than relying on emergency heat.
- At what temperature does aux heat turn on?
- Aux heat activates when the heat pump alone cannot maintain the set temperature — typically when outdoor temps fall below 35–40°F for most residential systems. The exact balance point depends on the heat pump's rated COP at low temperatures, your home's heat loss rate, and how much the thermostat is asking for (how many degrees below set point the house is). Smart thermostats like Ecobee and Nest also have configurable 'Aux Heat Maximum Outdoor Temperature' settings — this can prevent aux heat from running when outdoor temps are above a threshold you set.
- What is the difference between W1 and W2 (or Aux and W) on a heat pump thermostat?
- W1 is the first-stage heating call — on a heat pump system this is rarely used as a direct heat strip trigger; instead, W1 may not exist or may serve as a dual-fuel furnace trigger. W2 (also labeled Aux on some thermostats) is the second-stage heating call that triggers the electric heat strips. When the heat pump cannot maintain temperature and the thermostat drops into second stage, it sends 24V to W2/Aux, which energizes the sequencers in the air handler that power the heat strips in sequence.