Emergency Heat on a Heat Pump: How It Works, When to Use It, and Costs

Emergency heat on a heat pump can be a lifesaver when winter strikes and equipment fails. Yet many homeowners confuse it with normal heating or auxiliary heat. This guide explains what emergency heat is, when to use it, how much it costs, and how to stay safe and efficient in U.S. climates.

How Heat Pumps Heat A Home

A heat pump moves heat rather than creating it. In heating mode, it extracts heat from outdoor air and transfers it indoors via a refrigerant cycle. This process uses electricity efficiently, delivering two to four units of heat for each unit of energy.

Most air-source heat pumps lose capacity as outdoor temperatures drop. To maintain comfort in freezing weather or during defrost cycles, many systems include backup heat. Backup may be electric resistance heat strips or a gas or oil furnace in a dual-fuel system.

Modern cold-climate heat pumps maintain strong output at low temperatures. Some perform near rated capacity down to 5°F or below. Still, backup remains important during extreme conditions or equipment faults.

For a primer on heat pump operation and efficiency, see U.S. Department of Energy and ENERGY STAR.

What Is Emergency Heat On A Heat Pump?

Emergency heat is a manual thermostat mode that forces the system to use its backup heat source only. In most all-electric systems, that means the electric resistance heat strips heat the home while the outdoor heat pump unit is shut off.

In a dual-fuel setup, emergency heat tells the system to run the gas or oil furnace exclusively and lock out the heat pump. This bypasses the compressor during a failure, extreme icing, or other emergency.

Emergency heat is not designed for everyday use. It is intended for short-term operation when the heat pump cannot run safely or has failed. Because it uses less efficient heat sources, it is typically more expensive.

Emergency Heat Vs Auxiliary Heat

Homeowners often confuse emergency heat with auxiliary heat. The difference matters for comfort and cost, and affects what to do in cold weather.

Auxiliary Heat

Auxiliary heat (Aux Heat) is automatic. The thermostat or control board adds supplemental heat when the heat pump alone cannot meet the setpoint quickly or during a defrost cycle. The heat pump continues running, and backup heat assists as needed.

Aux heat may engage briefly after large thermostat setbacks or during extreme cold. This is normal. It should stage on and off in response to demand, minimizing electricity or fuel use.

Emergency Heat

Emergency heat is manual. Selecting “EM Heat” or “Emergency” locks out the heat pump and runs only the backup heat. Use it when the outdoor unit is iced over, damaged, or tripping breakers, or when advised by a professional.

Key point: If the heat pump is functioning, let the system decide when auxiliary heat is needed. Only choose emergency heat when the heat pump should be off for safety or repairs.

When To Use Emergency Heat (And When Not To)

Use Emergency Heat In These Situations

• Compressor failure or outdoor unit fault: The outdoor fan or compressor will not run, repeatedly trips breakers, or makes severe grinding or buzzing noises.
• Heavy ice buildup: The outdoor unit is encased in ice and cannot defrost, or the coil is blocked with snow after a storm.
• Physical damage: The unit was struck by debris, flood water, or a fallen branch, or refrigerant lines are damaged.
• Service direction: An HVAC technician asks to keep the heat pump off until a repair is completed.
• Dual-fuel lockout: In rare, extreme cold with a known low balance point, a dual-fuel system may be set to furnace-only for protection.

Avoid Emergency Heat In These Situations

• It is just very cold: Most modern heat pumps can heat effectively in the teens or single digits. Let the controls engage auxiliary heat only if needed.
• Faster warmup: Emergency heat may deliver hotter air, but it is expensive. A short-term need for quicker warmup is better served by auxiliary staging.
• Routine operation: Do not use emergency heat for days or weeks as a substitute for a working heat pump. Schedule repairs promptly.

Rule of thumb: If the outdoor unit is healthy, stay in normal Heat mode and let the system stage automatically. If the outdoor unit is unsafe or nonfunctional, switch to Emergency and call for service.

Costs, Energy Use, And Efficiency

Emergency heat relies on backup sources that are less efficient than a running heat pump. Electric resistance heat has a coefficient of performance (COP) of 1, meaning one unit of electricity becomes one unit of heat. A heat pump often delivers two to four units of heat per unit of electricity.

How Much Emergency Heat Costs

Electric heat strips are commonly 5 to 20 kilowatts (kW) and may be staged. Cost per hour equals strip kW multiplied by your electricity rate. According to the U.S. Energy Information Administration (EIA), average U.S. residential electricity prices are roughly 16–17¢/kWh, varying by state.

*Estimated at $1.00–$1.50 per therm and 90–95% AFUE; actual cost varies by gas rate and efficiency. Source: EIA.

Takeaway: A 10 kW strip can cost roughly $1.60 per hour at $0.16/kWh, and more where electricity rates are higher. Running emergency heat all day can add $38+ per day at that size.

Heat Output Comparison

A typical 3-ton heat pump provides about 36,000 BTU/h at 47°F. At 17°F, output may fall to 18,000–28,000 BTU/h depending on model. A 10 kW strip provides ~34,000 BTU/h consistently, but at much higher operating cost per delivered BTU compared with a functioning heat pump.

Cold-climate models narrow the gap at low temperatures. Look for HSPF2 and capacity at 5°F when comparing units. See NEEP cold-climate specifications for performance listings.

Thermostat Settings, Staging, And Lockouts

Thermostats control when auxiliary or emergency heat engages. Understanding staging avoids unnecessary emergency heat use and saves money.

Staging And Differential

• Staging: Many systems use multi-stage heat strips (e.g., 5 kW + 5 kW). The thermostat brings on strips stepwise if the setpoint is not met within a set time or temperature differential.
• Differential: The temperature difference between room and setpoint affects when auxiliary heat activates. Smart thermostats can adjust to minimize strip use.

Outdoor Lockout Temperatures

• Aux heat lockout: Prevents auxiliary strips from energizing above a chosen outdoor temperature, relying on the heat pump until colder weather.
• Compressor lockout: In dual-fuel systems, sets a temperature below which the furnace runs exclusively to protect the compressor or optimize cost.

Experienced installers program lockouts based on local climate, electric and gas rates, and the system’s balance point. Ask an HVAC pro to review these settings for your home.

Thermostat Labels

• Em Heat: Manual emergency heat mode; locks out the heat pump.
• Aux Heat: Automatic supplemental heat while the heat pump still runs.
• Heat: Normal heating mode; heat pump runs and stages auxiliary if needed.

Popular thermostats may also display “Defrost” or “Heating Stage 2” when auxiliary heat is active automatically. Consult your thermostat manual for exact terminology.

What To Do If The Heat Pump Freezes Or Fails

If the outdoor unit is iced over or not running, protect the equipment and keep safe while awaiting service. Use emergency heat appropriately to maintain comfort.

Step-By-Step Actions

1. Inspect safely: From a distance, check if the outdoor fan or compressor runs. Listen for abnormal noises. Turn off power at the disconnect if there are sparks, burnt smells, or flooding.
2. Switch to Emergency Heat: On the thermostat, select Em Heat. This locks out the outdoor unit and uses backup heat.
3. Clear snow and debris: Gently remove snow around the outdoor unit. Keep at least two feet clear on all sides and above.
4. Do not chip ice: Do not pry ice from the coil or fins. Let a technician handle defrost or repairs.
5. Call a licensed HVAC technician: Describe symptoms, any error codes, and recent weather conditions.
6. Use modest setpoints: Set the thermostat to a comfortable but reasonable temperature to limit electric consumption.

Defrost Cycles Are Normal

Heat pumps periodically reverse cycle to defrost the outdoor coil. During defrost, the system may blow slightly cool air; auxiliary heat often engages briefly to temper supply air. This behavior is expected and not a reason to use emergency heat.

Regional Considerations In The U.S.

Climate and utility rates influence how often auxiliary or emergency heat is needed and how much it costs. A few regional themes help frame expectations.

Northeast And Upper Midwest

Cold-climate heat pumps are increasingly common and can heat reliably in subzero weather. Auxiliary heat sizing and outdoor lockouts may be tuned carefully to reduce operating costs during cold snaps. Dual-fuel systems are also common where gas is inexpensive.

Southeast And Mid-Atlantic

Air-source heat pumps dominate, and backup strips handle occasional freezes. Electricity rates vary. Avoid large thermostat setbacks on cold nights to reduce auxiliary use. Keep vegetation and lint away from outdoor coils to improve defrost performance.

Pacific Northwest

Milder winters and hydroelectric power can make heat pumps very cost-effective. Emergency heat may be rarely needed. Ductless and ducted inverter systems often run without strips except during defrost or unusual cold spells.

Mountain West And Plains

Wide temperature swings challenge staging strategies. Smart thermostats with outdoor sensors and learning algorithms help limit strip use. Snow management and wind protection around outdoor units are important.

Safety And Equipment Health

Emergency heat is safe when used properly, but a few precautions protect both people and equipment.

• Electrical load: Electric strips draw high current. Do not run other heavy loads on the same circuit. Ensure breakers and wiring are rated for installed strip size.
• Generator sizing: Most portable generators cannot power large heat strips. If using backup power, consider enabling only a 5 kW stage and the blower, or rely on a gas furnace in dual-fuel systems.
• Ventilation for dual-fuel: If emergency heat runs a gas or oil furnace, ensure flue venting is clear and carbon monoxide alarms are healthy.
• Short-term use: Do not rely on emergency heat for extended periods. Address root causes quickly to avoid high bills and stress on electrical components.

Generator And Load Planning

Tip: Ask an electrician and HVAC pro to set up a “generator mode” or stage lockout to limit strips during outages.

Maintenance To Avoid Emergency Heat

Basic maintenance reduces the risk of failures that force emergency heat. Many steps are DIY-friendly and cost little compared to emergency operation.

• Filters: Replace or clean filters every 1–3 months. Restricted airflow stresses compressors and heat strips.
• Outdoor coil care: Keep the unit level, clear of leaves, lint, and snow. Maintain two feet of clearance. Gently hose the coil when dirt accumulates.
• Defrost function: If frost persists, have a technician check defrost sensors, boards, and refrigerant charge.
• Ducts and airflow: Seal and insulate ducts, especially in attics or crawlspaces. Low airflow can trigger auxiliary heat more often.
• Thermostat updates: Calibrate or update firmware on smart thermostats to improve staging logic.
• Professional servicing: Schedule annual checks for refrigerant levels, electrical connections, and heat strip staging.

Sizing Backup Heat And The Balance Point

Backup heat sizing should match the home’s heat loss and the heat pump’s capacity at low temperatures. Oversized strips raise breaker sizes, wiring costs, and generator needs.

Balance point is the outdoor temperature where heat pump output equals the home’s heat loss. Below that point, auxiliary heat may be needed. A contractor can estimate the balance point from capacity tables and a Manual J load calculation.

Fine-tuning thermostat lockouts around the balance point can reduce costs without sacrificing comfort. Avoid all-or-nothing settings unless using dual-fuel optimization.

Common Myths About Emergency Heat

• “Use emergency heat below 35°F.” False. Modern heat pumps often heat well below that. Only use emergency heat when the pump should be off.
• “Emergency heat warms faster.” It may blow hotter air, but total warmup time depends on home heat loss. The extra cost is usually not worth it.
• “Aux heat means something is broken.” Not necessarily. Short bursts during defrost or large setbacks are normal.
• “Emergency heat is safer.” It is for emergencies; using it routinely can raise bills and mask problems needing repair.

How To Read Your Energy Use And Bills

Because emergency heat can be costly, tracking usage helps avoid surprises. Smart thermostats and some air handlers log runtime by stage. Utility portals often show hourly or daily consumption.

Watch for: Sudden kWh spikes on cold days, long Stage 2/Strip runtimes, or sustained “Em Heat” indicators. These patterns can reveal stuck relays, miswired thermostats, or oversized temperature setbacks.

Defrost, Ice, And Cold-Weather Tips

In freezing, humid weather, frost is expected. Efficient defrost requires good airflow and correct sensors. Persistent ice signals a control or refrigerant issue.

• Raise the unit: Mount the outdoor unit several inches above grade to prevent snow blockage.
• Wind baffles: In windy sites, approved baffles can reduce frost by limiting cold air recirculation. Consult the manufacturer.
• Drainage: Ensure defrost meltwater has somewhere to go. Ice can build under the unit and contact the coil.
• Setback strategy: Use modest setbacks (2–4°F). Large overnight setbacks often trigger auxiliary heat in the morning.

Environmental And Comfort Considerations

Emergency heat increases energy use and emissions compared to normal heat pump operation. Running the compressor whenever possible delivers more heat per kilowatt-hour and improves comfort with steadier supply temperatures.

Cold-climate, variable-speed systems reduce the need for strips, maintain airflow, and deliver more even temperatures. If emergency heat is used frequently in normal cold weather, consider a performance evaluation or system upgrade.

Troubleshooting Quick Guide

Selecting Or Upgrading Thermostats And Controls

For better control of auxiliary and emergency heat, consider thermostats with outdoor sensors, intelligent staging, and lockout settings. Professional setup ensures proper wiring for multi-stage strips and dual-fuel changeover.

• Outdoor temperature sensors: Enable precise lockouts and balance-point control.
• Adaptive recovery: Pre-warms the home gradually to avoid strip engagement after setbacks.
• Dual-fuel control: Manages compressor vs furnace by temperature or utility rates.
• Runtime reporting: Helps identify excessive auxiliary or emergency heat usage.

Key Takeaways For Using Emergency Heat Wisely

• Emergency heat is a manual backup mode used when the outdoor unit should be off.
• Auxiliary heat is automatic supplemental heat that assists a running heat pump.
• Costs add up fast with electric strips; a 10 kW heater can cost ~$1.60/hour at $0.16/kWh.
• Use Em Heat during failures, severe icing, or when directed by a technician.
• Optimize controls with staging, outdoor lockouts, and modest thermostat setbacks.
• Maintain the system to reduce strip runtime and prevent emergencies.

FAQs: Quick Answers

What Is Emergency Heat On A Heat Pump?

It is a thermostat mode that locks out the compressor and uses the backup heat source only, typically electric strips or a gas furnace in dual-fuel systems.

Is Emergency Heat The Same As Auxiliary Heat?

No. Auxiliary heat is automatic assistance while the heat pump runs. Emergency heat is a manual, backup-only mode for failures or unsafe conditions.

When Should Emergency Heat Be Used?

Use it when the outdoor unit is damaged, iced over, or malfunctioning, or when a technician instructs you to keep the heat pump off.

Does Emergency Heat Cost More?

Yes. Electric resistance heat is far less efficient than a running heat pump. Expect significantly higher operating costs per hour.

Will Emergency Heat Hurt My System?

Short-term use is safe if installed correctly. However, it can mask underlying issues. Prolonged use stresses electrical components and increases bills.

How Do I Turn On Emergency Heat?

On most thermostats, switch from Heat to Em Heat or Emergency. The display should confirm the mode and the outdoor unit should stop running.

What Size Heat Strips Do I Have?

Look at the air handler nameplate or installation documents. Common sizes are 5, 10, 15, and 20 kW, often in stages. An HVAC tech can verify.

Can A Portable Generator Run Emergency Heat?

Usually not for large strips. It may handle a 5 kW stage and blower, but confirm generator capacity and consider lockouts to limit load.

Do Cold-Climate Heat Pumps Eliminate Strips?

They reduce reliance on strips but still include backup for defrost and extreme events. Proper sizing and controls minimize use.

For further reading on performance, costs, and efficiency, see DOE Energy Saver, ENERGY STAR, and the EIA’s electricity price data.