If your thermostat is showing “AUX” and you’re not sure whether to worry, here’s the short answer: aux heat is the backup electric strips your heat pump calls automatically when it needs help. That’s normal. Emergency heat is a different mode that shuts the compressor off entirely and runs only the strips. Use it when the compressor is broken, not for routine cold snaps.
Mixing them up costs money. Running in Emergency heat all winter when your compressor is fine can triple your heating bill.
What is AUX heat?
AUX (auxiliary) heat is the electric resistance backup that supplements your heat pump when the outdoor temperature drops and the compressor can’t keep up. It fires automatically. You don’t flip a switch.
On the wiring side, the backup strips are usually connected to the W2 terminal. When AUX activates, the thermostat closes W2 alongside Y1 (compressor), so both the heat pump and the strips run at the same time. The heat pump still contributes; the strips pick up the slack.
Most ducted heat pump systems have electric strips. Some use a gas furnace as backup instead (dual-fuel system, different wiring). If you don’t know which you have, check your air handler for a kW rating on the elements: “10kW” or “15kW” stamped on the unit means electric strips.
What is emergency heat?
Emergency heat is a manual override that disables the compressor and routes heating entirely through the backup strips (or, on dual-fuel systems, the furnace). It’s wired to the E or Em terminal and operates separately from the AUX relay.
Your thermostat will show “Em,” “E-Heat,” or “Emergency Heat” when this mode is on. The compressor won’t run at all.
Use emergency heat only if the heat pump is broken. Signs the compressor has failed: the outdoor unit doesn’t spin, you hear grinding or clicking from the outdoor unit, or HVAC tech confirms a refrigerant leak or compressor failure. Running EM heat with a working compressor means you’re paying strip prices (COP ≈ 1.0) for heat your heat pump would have provided at COP 2.0–3.0.
The key difference
| AUX Heat | Emergency Heat | |
|---|---|---|
| Triggered by | Thermostat automatically | You manually switch it on |
| Compressor | Running (strips supplement it) | Disabled entirely |
| Terminal | W2 | E / Em |
| Normal use? | Yes (expected on cold days) | No (only for broken compressors) |
| Cost vs. heat pump only | Moderate increase | High: strip-only heat |
| Turns off automatically? | Yes, when demand drops | No: stays on until you flip it back |
When does AUX heat come on?
Your thermostat fires AUX when one of two conditions is met:
1. Outdoor temperature below the balance point. Below roughly 30–40°F (varies by system), the heat pump’s efficiency drops to the point where the strips help more than they hurt. Some thermostats let you set this crossover explicitly. Honeywell calls it the balance point lockout (ISU 3120 on the T10 Pro). Ecobee calls it Compressor Min Outdoor Temperature (default 35°F, adjustable down to 0°F or disabled).
2. Temperature differential too large. If the gap between your set point and the actual room temperature is wide enough that the heat pump can’t catch up within a reasonable window, the thermostat stages up to AUX. Most units are factory-set to 2–3°F differential. Ecobee calls this the Compressor to Aux Temperature Delta.
Seeing AUX fire on the coldest days of the year is normal. Seeing it fire every hour on a 40°F day when you just bumped the thermostat 2°F is not: the heat pump may not be performing, or the staging settings need tuning.
When to use Emergency heat
One reason: the heat pump outdoor unit is physically broken or out of service. That means the compressor isn’t running, a refrigerant line is leaking, or you’re waiting on a repair and it’s below freezing.
Emergency heat is not for:
- Cold snaps where AUX is already helping (the system is handling it)
- When AUX seems to run a lot (that’s a balance-point or staging question, not an EM situation)
- Saving energy (it uses more, not less)
If you’re in EM heat and the repair is done, turn it off. The thermostat will not switch itself back.
How to reduce AUX heat use
If AUX fires more than you’d like, the solution is usually a thermostat that exposes balance-point and lockout controls. Not every thermostat does.
Three that do, and what to look for:
ecobee Premium / ecobee SmartThermostat: Two independent controls: Compressor Min Outdoor Temperature (locks out compressor below X°F, forcing AUX to handle it alone) and Aux Heat Max Outdoor Temperature (blocks AUX above Y°F, keeping the heat pump solo). Requires a wired or internet-sourced outdoor temp.
Nest Learning Thermostat (4th gen): Heat Pump Balance setting with four modes from Max Comfort (heat pump + AUX together freely) to Max Savings (delays AUX as long as possible). Also supports a manual AUX lockout temperature when Heat Pump Balance is turned Off. Uses Wi-Fi weather, not a physical sensor.
Honeywell T10 Pro: Balance point lockout via ISU 3120 in the installer setup menu; Backup Heat Differential via ISU 3090 (2–15°F gap before AUX stages up). Supports a wired outdoor sensor (C7089U1006 to S terminals) for accuracy.
→ For a side-by-side breakdown of these controls, see Why Your Smart Thermostat Keeps Firing the AUX Strips (and the Ones That Let You Stop It).
Per-brand guides
Nest thermostats: The Nest shows “Aux” during normal AUX operation and “Em” when you’ve manually switched to Emergency heat. The AUX/Em question is one of the most common Nest confusion points. For a full walkthrough of every Nest heat mode, see What is Auxiliary and Emergency Heat (EMER) On A Nest Thermostat?.
Honeywell thermostats: Honeywell uses the same W2/E wiring conventions. For what the “AUX” light means specifically on Honeywell models, see What Does Auxiliary Heat Mean On a Honeywell Thermostat?.
Honeywell heat pump wiring (if you’re diagnosing): Honeywell Thermostat Wiring For Heat Pumps covers the O/B, Y1/Y2, W2, and E terminal layout step by step.
Nest wiring: Nest Thermostat Wiring For Heat Pump covers terminal layout including the reversing valve (O/B orientation); also see What is O or B Orientation On Nest Heat Pump?.
Emergency heat not working?
If you’ve switched to Emergency heat and the strips aren’t firing:
- Check the E/Em terminal has a wire connected. Some installs leave it unwired if they never tested EM mode.
- Confirm the thermostat is actually in Emergency heat mode (display should say Em or E-Heat, not just showing a high setpoint).
- Check the air handler breaker. Strips are typically on a separate 240V breaker from the compressor.
- If the strips are confirmed wired but still not heating, the elements or sequencer may have failed.
For a full step-by-step diagnosis: Heat Pump Emergency Heat Not Working: Troubleshooting and Fixes.
When to call a pro
- Refrigerant leak (hissing, ice on lines, compressor won’t start): refrigerant handling requires EPA 608 certification.
- Compressor failure confirmed: strips can run temporarily but the system needs repair.
- EM heat fires by itself without you touching the switch: this points to a control board or wiring fault, not a thermostat setting.
- Line voltage (240V) at the air handler: if you’re not comfortable with the electrical side, stop and call.
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FAQ
Is it bad if AUX heat runs a lot? Occasional AUX on cold days is normal and expected. If AUX is running constantly at moderate temperatures (above 40°F), the heat pump may not be performing correctly. Check for refrigerant issues, a dirty coil, or a staging configuration that’s set too aggressively.
Can I leave the thermostat in Emergency heat? You can, but it’ll cost you. Electric strip heat runs at COP 1.0: one unit of heat per unit of electricity. A working heat pump at 35°F runs at COP 2.0 or better. Leaving it in EM when the compressor is fine is throwing money away.
My thermostat shows “AUX” — should I switch to Emergency heat? No. AUX means the system is working correctly and adding strip heat as needed. Switching to EM would disable the compressor and make the system less efficient. Leave it alone.
What temperature does AUX heat kick in? It depends on your thermostat’s balance-point setting and the size of your heat pump relative to your home. Most factory defaults stage in AUX around 35–40°F outdoor temp or when there’s a 2–3°F gap between setpoint and room temp. You can adjust both on ecobee, Nest 4th gen, and Honeywell T10 Pro.
Curious what leaving a system in Emergency heat (or a stuck-AUX lockout) actually costs over a season? Run your setpoints through the Thermostat Savings Calculator to see the dollar difference.
