Heat Pump Emergency Heat: What It Is, When To Use It, And How To Avoid Huge Bills

Heat pump emergency heat is one of the most confused features on a heat pump system, and using it wrong is one of the fastest ways to double or triple a winter electricity bill. Your thermostat has a setting labeled "EM Heat" or "Emergency Heat," and flipping it on during a cold snap might feel like the right call. In most cases, it isn't.

Key Takeaways

• Emergency heat mode has the heat pump shut off completely and runs only the backup heat source. That means the system has electric resistance strips, or it has a gas furnace, or it has an oil furnace. Auxiliary heat, by contrast, comes on automatically alongside the heat pump when heating demand is high.

• On most thermostats, the EM heat setting should only be switched on when the outdoor heat pump unit is physically broken, iced solid or unsafe to run - not simply because outdoor temperatures drop below freezing.

• Heat pumps and ductless mini splits that are sized right for cold climates and have inverter compressors can heat well even when it is well below 20°F outside, so they do not need emergency heat mode nearly as much as older heat pumps do.

• Using emergency heat can raise your energy bills a lot because electric resistance heating uses far more power than a heat pump does when it is running normally.

How a Heat Pump Heats Before Emergency Heat

A heat pump system does not make warmth the way a gas furnace or electric furnace does. In regular heating mode, it is pulling heat from the outside air with refrigerant and a compressor cycle, then it is sending that heat energy into the home through the air handler or indoor unit. Even when it is cold outside, there is still usable thermal energy in the air.

Heat pumps can heat homes efficiently even when outdoor temperatures are low. A typical unit that is not built for cold climates might keep up well down to about 30–35°F, while cold-climate models and many ductless mini splits can still deliver strong heat when the temperature outside is at 5°F or lower. That's why your thermostat offers separate options: heat mode is when the heat pump does the work, aux heat is when the backup helps the heat pump, and EM heat is when you get backup only.

Auxiliary vs Emergency Heat: What's the Difference?

Auxiliary heat:

• You don't have to do anything - auxiliary heat comes on automatically when it's needed.

• Auxiliary heat mode turns on backup electric heat strips (or a furnace in dual-fuel setups) when the heat pump can't meet the thermostat setpoint fast enough, or when outdoor temperatures drop below the system's balance point.

• Auxiliary heat helps heat pumps when the weather is cold. It runs together with the heat pump, and it fills in the gap.

• On the thermostat, it is showing up as "Aux Heat," "Heating Stage 2," or a flame icon with a plus sign.

Emergency heat:

• With emergency heat, you have to turn it on yourself - you pick it at the thermostat on purpose.

• It has the outdoor unit shut off completely and runs only the backup heat.

• For times when the heat pump is broken, not for normal cold weather, emergency heat is what you use.

Simple example: At 28°F a heat pump system may run all the time, and auxiliary heat gives the heat pump automatic help during times of high demand by turning on electric strips now and then. Emergency heat should only enter the picture if the heat pump itself fails or must be shut down. Both modes cost more than normal heat pump operation does, but pure electric strip heat is the most expensive per hour.

What Is Emergency Heat Mode?

With emergency heat mode, the thermostat setting has your heating system rely 100 percent on backup heat and completely go around the heat pump. It has the heat pump shut off completely. The compressor goes quiet. The outdoor fan goes quiet. The reversing valve goes quiet. All of it goes quiet. Electric resistance heating is used in emergency heat mode, or in dual-fuel systems, the gas or oil furnace takes over.

Step by step, here's what happens:

1. A signal from the thermostat turns off the compressor and the outdoor fan.

2. It has the electric resistance heat strips inside the air handler turn on (or it calls the backup furnace).

3. All of the warmth now comes from the backup source until you switch EM heat off.

Emergency heat uses electric resistance heating or gas furnace - whichever backup your system has. In homes with dual-fuel setups, emergency heat mode usually means only the gas furnace runs.

Some advanced control boards or smart thermostats may also trigger EM heat automatically as a safety backup if they detect a compressor lockout or a defrost problem. But under normal operation, it's a manual switch. Emergency heat operates only when the heat pump is broken, either because you've told the thermostat so or because the controls detected a critical fault.

When Should You Use Emergency Heat?

• The outdoor heat pump unit is clearly damaged - fan not turning, loud grinding noise, refrigerant leak (look for oily residue or hissing sounds near the line set), or a panel blown off in a storm.

• With ice completely covering the outdoor unit and no defrosting happening, and a technician unable to come right away, you may need emergency heat.

• If the thermostat is showing a compressor or outdoor unit fault code, and the house is getting cold.

• Your HVAC technician has specifically told you to run EM heat while you are waiting on parts.

• Outdoor temperatures drop below 32 degrees but the system still keeps the house warm, even if it has to run for a longer time.

• You feel cooler air from the vents compared to a gas furnace - that is typical for heat pumps.

• On very cold mornings, the thermostat display may show "Aux Heat" from time to time - that's normal auxiliary heat, not an emergency.

• You'd want to warm the home faster. That speed comes at 2–3× the energy cost.

How Emergency Heat Affects Comfort and Costs

Efficiency and cost: Electric resistance heating is less efficient than heat pumps. A heat pump running at a COP (coefficient of performance) of around 3.0 delivers three times the heat per kilowatt-hour compared to electric resistance heating at COP 1.0. At the U.S. average electricity rate of roughly $0.18/kWh, producing 6,000 kWh of thermal energy costs about $360 with a heat pump at COP 3 versus $1,080 with resistance heat alone. Using emergency heat can significantly increase energy bills - and it can lead to higher utility costs over time if the underlying problem is not fixed.

System wear: Emergency heat components are designed as intermittent backup, not a full-season primary heating system. Running strips or a backup furnace full-time for weeks can accelerate wear and reveal electrical load limitations, like tripped breakers.

Properly sized inverter mini splits (like many Zone Air systems we sell) often keep a home comfortable without any electric strip backup. To earn ENERGY STAR's Cold Climate designation, a heat pump must show a COP of at least 1.75 at 5°F and keep at least 70% of its 47°F heating capacity at 5°F - far more efficient than pure resistance heat on your energy bill.

How to Tell If Your Heat Pump Is in Aux or Emergency Heat

To know which mode you're in, the easiest way is to check your thermostat display and look at the outdoor unit.

Quick homeowner check: Confirm the thermostat is set to "Heat," not "EM Heat." If you see EM heat lit and there's no actual emergency, switch back to regular heat mode and monitor comfort for the next hour. If switching out of EM heat causes tripped breakers, loud noises, or strictly cold air, return to EM heat and schedule a service call.

When Emergency Heat Appears Unexpectedly

If you never touched the emergency heat setting but the system is acting like it's on backup-only heat, something in the equipment or controls may be wrong. Frequent use of emergency heat shows there may be heat pump issues.

• Outdoor unit lockout due to low ambient temperature in older heat pumps

• Failed or failing compressor, outdoor fan motor or defrost control board

• A heat pump that isn't working right, with thermostat wiring issues that have the emergency circuit on by default

• Outdoor unit iced over because the defrost cycle isn't working correctly

• A refrigerant leak that drops heating capacity until the system locks out

• Check that the thermostat isn't manually set to EM heat

• Inspect the outdoor unit for excessive ice, debris or a tripped disconnect

• Make sure dirty filters aren't blocking airflow - replace any dirty air filter

Call a qualified HVAC professional when:

• Breakers are tripping when you try to exit EM heat

• The outdoor unit makes grinding or buzzing sounds, or never starts

• If emergency heat has been running for more than a day with no obvious reason, your electricity bill is shooting up.

How to Cut Emergency and Aux Heat Use

• Keep setpoints moderate in winter (68–70°F when occupied)

• Avoid big, fast temperature jumps that force aux heat on. Use 2–3 degree setbacks overnight instead of 8–10 degree drops

• Seal air leaks around doors, windows and attic access to reduce heating demand

• Add insulation in attics or knee walls so the balance point moves lower and aux heat comes on less

• Replace or clean dirty filters monthly during peak season

• Keep the outdoor unit raised above likely snow levels and clear at least 2–3 feet around it so it can draw in outside air

• Schedule annual service to check refrigerant levels, defrost operation and electrical connections

• Upgrading to a modern variable-speed, cold-climate heat pump or adding ductless mini splits in problem rooms can dramatically cut the need for electric strip backup

• As of late 2026, the federal 25C tax credit for heat pumps applied only to systems placed in service before the end of 2025 (and 2025 claims needed a QMID), so homeowners now should focus on state and utility rebates and on choosing higher-efficiency models that qualify for local incentives

How Ductless Mini Splits Change Emergency Heat

Variable-capacity compressors ramp up as the temperature drops, keeping indoor temperatures comfortable while using far less electricity than electric resistance heating. Units meeting NEEP's cold-climate specification need a variable-capacity compressor, HSPF2 of at least 8.5 and SEER2 of at least 15 for non-ducted systems, and a COP of at least 1.75 at 5°F at maximum capacity. That level of efficiency means most homes stay warm without strip heat even during freezing weather.

Choice tips: Size the system by room and climate, choose 115V vs. 230V based on panel capacity, and verify breaker sizes and line-set routing. Pre-charged quick-connect line sets, like those on Zone Air DIY systems, let you or a handyman connect indoor and outdoor units without a vacuum pump, EPA 608 card or gauges while keeping the manufacturer's warranty. You can buy Zone Air from us, so weigh this accordingly - and check local codes, incentives and installer recommendations before buying.

FAQ About Heat Pump Emergency Heat and Aux Heat

Why Does My Heat Pump Blow Cold Air in Winter?

Heat pumps deliver air around 90–100°F, which feels cooler than a furnace's 120°F output but still heats the home. During a defrost cycle, the system briefly switches to cooling mode to melt ice off the outdoor coil. You may feel cool air for a few minutes. That's normal operation when the system switches to cooling mode for defrost, not a reason to switch to emergency heat.

When Does Aux Heat Usually Come On?

There's no single fixed temperature. Many older single-stage heat pumps call for aux heat around 30–35 degrees, while high-efficiency variable-speed and cold-climate models may not need much aux heat until outside temperatures drop into the low teens or lower, depending on home insulation and thermostat settings.

Related guides

• Best DIY mini split heat pump brands
• DIY mini split buying guide: sizing, voltage and true DIY explained
• Brands with pre-charged quick connect mini splits
• Easiest DIY mini split install: how pre-charged line sets work

Specifications, prices, and warranty terms above are manufacturer- or retailer-published as of October 2026, are subject to change, and should be verified before purchase. MiniSplits on Sale is a Zone Air® dealer and is not affiliated with or endorsed by any other brand named here.

Related reading: our MRCOOL Universal vs DIY 4th Gen cold climate comparison.