Heat Pump Emergency Heat: What It Is, When to Use It, and Costs

Homeowners often see “Emergency Heat” or “EM Heat” on the thermostat and wonder when to use it. This guide explains how a heat pump’s emergency heat works, when to turn it on, what “Aux Heat” means, and how much each option costs. Learn practical steps, safety tips, and smart settings to keep a home warm efficiently during cold weather without overspending.

What Is Heat Pump Emergency Heat?

“Emergency Heat” (also labeled “EM Heat” or “Emer Heat”) is a manual setting that bypasses the outdoor heat pump and runs the backup heat source only. In most homes, that backup is electric resistance heat strips in the air handler. In dual-fuel systems, the backup is a gas furnace.

Key idea: EM Heat is designed for emergencies—like a failed outdoor unit—not for routine cold weather. When enabled, the outdoor unit stops, and the thermostat calls only the backup heat to protect comfort until service can be scheduled.

Backups go by several names: “electric heat,” “strip heat,” “resistance heat,” “W2/Aux,” or “second stage.” All refer to the secondary heat that does not use the heat pump’s efficient refrigeration cycle.

How A Heat Pump Heats Your Home

A heat pump transfers heat from outside air into the house using refrigerant and a compressor. This process can deliver two to four units of heat per unit of electricity, measured as the Coefficient of Performance (COP). At moderate temperatures, a COP of 3 is common.

As outdoor temperatures fall, the heat pump’s capacity and COP decline. Modern cold-climate heat pumps can still heat well below freezing, but there is a point where the backup heat is needed to maintain setpoint or handle quick warmups.

Heat pumps also run a defrost cycle to clear frost from the outdoor coil. During defrost, the system briefly reverses to cooling; auxiliary heat may run to offset the cold air and maintain comfort. This is normal and not a reason to switch to EM Heat.

Emergency Heat Vs. Aux Heat: Key Differences

Homeowners often confuse “Emergency Heat” and “Aux Heat.” They are not the same, and using the wrong one can raise energy bills unnecessarily.

Bottom line: Use Aux Heat automatically. Use EM Heat only if the outdoor unit should be turned off—for example, to prevent damage or when it is not working.

When To Use Emergency Heat

Use “Emergency Heat” sparingly. It is a lifeline for comfort when the heat pump cannot or should not run. If unsure, consult a licensed HVAC technician or the manufacturer’s guidance.

• Outdoor unit failure: Loud grinding, tripped breaker, seized fan, or compressor error codes. Switch to EM Heat and schedule service.
• Severe icing: If the outdoor unit is encased in ice and not defrosting, shut down the heat pump, turn on EM Heat, and call for service.
• Refrigerant or sensor issues: Short cycling, frost in unusual places, or very low airflow. EM Heat can maintain comfort until repair.
• Storm damage or debris: If the outdoor unit is at risk of damage, cut power to it and use EM Heat temporarily.
• Awaiting parts: EM Heat can bridge the gap during repairs in freezing weather.

Do not use EM Heat simply because it is very cold. Modern heat pumps, especially cold-climate models, can operate well below 0°F. Let the thermostat manage Aux Heat automatically during cold snaps.

How To Turn On Emergency Heat On Common Thermostats

Always verify your system type first. If you have a dual-fuel system, EM Heat may engage the gas furnace. For electric-only systems, EM Heat typically uses resistance heat strips. When EM Heat is on, the outdoor unit should be off.

Honeywell Home/Resideo

• Tap Mode until “EM Heat” appears, or open Menu > Mode > EM Heat (model dependent).
• Confirm that “EM Heat” is displayed. The outdoor unit should stop.
• To exit, change Mode back to Heat.

Ecobee

• Go to Main Menu > Settings > Installation Settings > Equipment.
• Ensure the Aux/Heat Pump configuration is correct.
• From the Main screen, tap Mode and select “Emergency Heat” (if wired and enabled).
• Some installers lock out manual EM Heat; if not visible, contact the installer.

Google Nest

• Press the thermostat ring > Settings > Equipment to confirm wiring (W2/Aux present).
• From the Quick View menu, choose “Heat • E” (Emergency) if available.
• In the app, go to Settings > Thermostat > Emergency Heat (if supported by wiring and configuration).
• Nest may manage “Alt Heat” automatically; manual EM Heat appears only when properly wired.

Carrier, Trane/American Standard, Lennox, Rheem/Ruud, Bryant

• Most proprietary touchscreen stats show Mode options including “EM Heat.”
• Select EM Heat to disable the outdoor unit and run backup heat only.
• If unsure, check the thermostat manual or the installer’s setup menu for EM Heat enablement.

Tip: If EM Heat is on and the outdoor unit is still running, turn off power to the outdoor disconnect and call for service—wiring may be incorrect.

Costs: Emergency Heat Versus Heat Pump And Gas

Electric resistance heat (strips) is far less efficient than a heat pump’s normal mode. It can cost two to three times more per unit of heat. Dual-fuel systems may be less expensive than strips, depending on local gas prices and furnace efficiency.

The example below uses typical U.S. averages: electricity at $0.16/kWh and natural gas at $1.20/therm. One therm is 100,000 BTU. Numbers are estimates; check local rates and equipment specs.

For a home needing 50,000 BTU/hour on a cold evening, the difference is significant. A heat pump at COP 2 costs about $1.15 per hour, while electric EM Heat costs about $2.35 per hour. Over a long cold snap, that adds up quickly.

Takeaway: If the outdoor unit is healthy, let the system run normally. Use EM Heat only when the heat pump cannot safely operate.

Troubleshooting: Heat Pump Not Heating And Emergency Heat Questions

Common cold-weather questions often involve “Aux Heat always on,” “Thermostat stuck in EM Heat,” or “Heat pump not heating.” Here is what to check before calling for service.

• Aux Heat showing frequently: Large setpoint bumps, defrost cycles, or extreme cold can trigger Aux. Use gradual setpoint changes, and check that filters are clean.
• Blowing cool or lukewarm air: Heat pumps deliver lower supply temperatures than furnaces. Measure temperature rise across the coil; ensure outdoor defrost is operating.
• Outdoor unit iced over: Some frost is normal. If it becomes a solid ice block or the fan stops for long periods outside of defrost, switch to EM Heat and schedule service.
• High bills after turning on EM Heat: Resistance heat is expensive. Exit EM Heat as soon as the outdoor unit is repaired.
• EM Heat option missing: Not all thermostats display EM Heat unless W2/Aux is wired and configured. Installer lockouts can also hide it.
• EM Heat won’t turn off: Confirm the thermostat mode is “Heat,” not “EM Heat.” Power-cycle the indoor unit if needed. If backup heat stays on, call a technician.
• Breaker trips: Heat strips draw high current. A weak breaker or short can trip. Do not repeatedly reset; have it inspected.

Cold-Climate Performance And Smart Lockouts

Cold-climate heat pumps (often labeled “CC-ASHP”) are engineered for lower ambient temperatures, sometimes down to -5°F or below. They maintain higher capacity and COP than standard models in subfreezing conditions.

Smart thermostats and advanced controls can set balance temperatures or lockout points—outdoor temperatures below which the thermostat prefers backup heat, or above which it prefers the heat pump. Properly tuned, these settings reduce run costs while maintaining comfort.

• Electric-only systems: Set the Aux heat lockout as low as practical to prioritize the heat pump.
• Dual-fuel systems: Set the fossil lockout so the heat pump runs when it is cost-effective; switch to gas when the heat pump’s COP falls below about 1.8–2.2, depending on local rates.
• Weather-adaptive controls: Features like Nest’s Heat Pump Balance or utility-optimized thermostats can minimize backup runtime while avoiding long recovery times.

Pro tip: If recoveries from setback frequently trigger Aux, use smaller setbacks or schedule earlier preheating so the heat pump does more of the work.

Protecting Your System: Safety And Maintenance

Well-maintained systems rely less on emergency heat and run more efficiently. Routine care also prevents nuisance icing and lockouts during cold snaps.

• Airflow: Replace or clean filters every 1–3 months. Keep supply and return vents clear.
• Outdoor unit clearance: Maintain 18–24 inches of free space around the cabinet. Keep coils free of leaves, snow, and debris.
• Defrost function: If defrost is frequent or ineffective, a sensor or board may be failing. Do not defeat defrost; seek service.
• Electrical checks: Heat strips should be staged and controlled by proper relays. Loose connections can overheat; have annual inspections.
• Condensate management: Ensure drains and pans are clear to avoid overflows during defrost.
• Annual tune-up: A professional can verify refrigerant charge, measure temperature rise, test strip elements, and validate thermostat staging and lockouts.

Safety: If there is smoke, burning smell, or repeated breaker trips, turn off power and call a professional immediately.

Backup Heat Options: Electric Strips, Hydronic, Dual Fuel

Most heat pumps pair with electric resistance strips. Some homes use different backups based on local fuel prices and climate conditions. Each option affects EM Heat performance and cost.

• Electric resistance strips: Simple and reliable, but the highest operating cost. EM Heat with strips is the most expensive mode for many households.
• Hydronic coil (boiler-fed): Uses hot water from a boiler through a coil in the air handler. Costs depend on fuel (natural gas, propane, oil) and boiler efficiency.
• Dual fuel (hybrid): A gas furnace provides backup heat. This often lowers operating cost in very cold weather and can be the EM Heat source when selected.

Considerations: Dual-fuel setups require correct thermostat logic to avoid running the heat pump and furnace simultaneously, which can damage equipment and waste energy.

Energy-Saving Tips During A Cold Snap

When temperatures plunge, smart habits keep comfort high while minimizing backup heat usage and cost. Small adjustments can reduce the need for Aux or EM Heat.

• Moderate setbacks: Use small setbacks (2–4°F) to avoid long recoveries that trigger Aux Heat.
• Preheat strategically: Schedule earlier start times before peak cold periods to let the heat pump do more of the work.
• Seal drafts: Weather-strip doors and windows; close fireplace dampers when not in use.
• Fan setting: Use “Auto,” not “On,” to avoid continuous blower operation that can feel cool.
• Zoning: Heat occupied rooms and close doors to unused areas if the system and code allow.
• Space heaters: Use cautiously; they are also resistance heat and can raise electric bills quickly. Keep clear of combustibles.

If a power outage occurs: Turn off EM Heat and the heat pump at the thermostat. After power returns, wait a few minutes before restarting to protect compressors and elements.

Utility Programs, Rebates, And Codes

Utilities across the U.S. offer rebates for high-efficiency heat pumps, smart thermostats, and weatherization. Some also provide demand-response programs that preheat homes and reduce peak strain—often with bill credits.

• Rebates: Check local offers for cold-climate heat pumps, dual-fuel controls, and high-efficiency furnaces where applicable. ENERGY STAR certified equipment may qualify.
• Thermostat optimization: Many utilities support smart thermostats that intelligently limit Aux run time and optimize heat pump performance in cold weather.
• Federal incentives: The Inflation Reduction Act offers tax credits for heat pumps, electrical upgrades, and envelope improvements. State programs may add incentives.

Learn more at ENERGY STAR, the incentive database at DSIRE, and local utility websites for region-specific guidance.

Frequently Asked Questions

Is Emergency Heat The Same As Aux Heat?

No. Aux Heat turns on automatically to help the heat pump meet demand. Emergency Heat is a manual mode that shuts off the outdoor unit and runs only the backup heat source. Use EM Heat only when the heat pump cannot safely operate.

Does Emergency Heat Cost More To Run?

Usually yes, especially with electric heat strips. Electric resistance heat costs roughly two to three times more per unit of heat than a well-performing heat pump. With dual fuel, costs depend on local gas and electric rates and equipment efficiency.

At What Temperature Should Emergency Heat Be Used?

There is no single temperature. EM Heat is not tied to temperature—it’s for equipment failure or protection. Cold-climate heat pumps can operate well below 0°F; rely on thermostat logic and Aux Heat unless a technician advises otherwise.

How Do I Know If My Heat Pump Is In Emergency Heat?

Your thermostat will display “EM Heat,” “Emer Heat,” or “Heat • E.” The outdoor unit should be off. If the outdoor unit runs while EM Heat is enabled, there may be a wiring or control issue.

Why Does Aux Heat Keep Turning On?

Large setpoint changes, defrost cycles, or high heat loss can trigger Aux Heat. Make smaller temperature changes, schedule preheating, and ensure filters and coils are clean. If Aux runs constantly, have a technician check capacity and controls.

Is It Safe To Leave Emergency Heat On For Days?

It is safe if the system is correctly wired and maintained, but it is expensive. If EM Heat uses electric strips, expect higher bills. Return to normal Heat mode as soon as the outdoor unit is repaired.

Can Emergency Heat And The Heat Pump Run Together?

In EM Heat mode, the outdoor unit should be off. If both run simultaneously in EM Heat, turn off the system and call a professional. In normal “Heat” mode, the heat pump and Aux may run together briefly, depending on staging.

What If The Outdoor Unit Is Covered In Ice?

A thin layer of frost is normal and cleared by defrost cycles. If the unit is fully encased in ice or cannot defrost, switch to EM Heat, shut off power to the outdoor unit, and call for service to prevent damage.

How Big Should My Heat Strips Be?

Common sizes are 5–20 kW, matched to home heat loss and duct capacity. Oversized strips can cause short cycling or tripped breakers. A contractor should size strips based on Manual J load calculations and airflow limits.

Can A Smart Thermostat Reduce My Use Of Emergency Heat?

Yes. Smart controls optimize staging, use weather forecasts, and limit Aux run time with lockouts and balance settings. They help the heat pump carry more of the load, especially during scheduled preheating before peak cold.

Key Specs And Features To Check On Your System

• HSPF/HSPF2: Seasonal efficiency metric. Higher is better; HSPF2 is the newer test procedure.
• COP at 5°F and 17°F: Indicates cold-weather performance. Cold-climate units publish robust low-ambient data.
• Capacity at low ambient: Ensure adequate BTU output at design temperatures for your region.
• Thermostat staging: Verify correct wiring of O/B, W2/Aux, and outdoor sensor if used.
• Defrost strategy: Demand-defrost saves energy compared to timed defrost.

How To Read Your Thermostat’s Heat Messages

Thermostats use different labels for similar behaviors. Understanding these terms prevents unnecessary EM Heat use.

• Heating Stage 1: Heat pump only.
• Heating Stage 2 / Aux: Heat pump plus backup heat as needed.
• Defrost Mode: Outdoor unit reverses; Aux may run to keep air warm.
• EM Heat: Backup heat only; outdoor unit off.

Watch-outs: If “Aux” shows constantly in mild weather, there may be a control or capacity issue. If “Defrost” is frequent and prolonged, sensors or boards may require service.

Quick DIY Checks Before Calling For Service

• Filter: Replace a dirty filter to restore airflow and capacity.
• Breaker/fuses: Verify indoor and outdoor breakers are on; check any outdoor fuses.
• Thermostat mode: Ensure “Heat” not “Cool” or “Off.” Confirm schedule and holds.
• Outdoor clearance: Remove snow, leaves, or obstructions around the unit. Do not chip ice off coils.
• Vents: Open and unobstructed; closed registers can reduce airflow and cause icing.
• Condensate switch: A tripped float can stop heating; clear the drain and reset.

If problems persist: Switch to EM Heat to maintain comfort and schedule professional service.

How Installers Optimize Emergency And Aux Heat

Correct setup ensures comfort and reasonable bills during cold weather. Ask installers about these parameters during commissioning or maintenance.

• Staging and timers: Aux Heat delay allows the heat pump time to recover before calling strips.
• Outdoor sensors: Enable lockouts and balance points by temperature.
• Charge and airflow: Proper refrigerant charge and duct airflow prevent icing and low capacity.
• Strip kW staging: Multiple stages reduce spikes and limit overuse.
• Dual-fuel logic: Ensure exclusive operation—no simultaneous furnace and heat pump.

Where To Learn More

For technical overviews and best practices, the U.S. Department of Energy and ENERGY STAR offer consumer resources on heat pumps and controls. Equipment selection guidance for cold climates is cataloged by the Northeast Energy Efficiency Partnerships (NEEP).

• U.S. DOE: Heat Pump Systems
• ENERGY STAR: Heat Pumps
• NEEP: Cold Climate Air-Source Heat Pumps
• EIA: Average Retail Price of Electricity

Remember: Emergency Heat is a helpful backup, not a daily setting. Use it to protect the home when the heat pump is down, then return to normal operation to keep comfort high and costs in check.