When to Use Emergency Heat on a Heat Pump: Signs, Settings, and Savings

When temperatures plunge or a system struggles, many ask when to switch to emergency heat on a heat pump. This guide explains what emergency heat really does, when to turn it on, and how to avoid unnecessary energy costs. Learn the difference between EM Heat and auxiliary heat, how to diagnose issues like ice buildup, and ways to stay comfortable efficiently in American winters.

What Is Emergency Heat On A Heat Pump?

Emergency heat, often labeled EM Heat on thermostats, is a manual mode that bypasses the heat pump’s outdoor unit and uses the backup heater alone. For most all-electric systems, that backup is electric resistance “heat strips.” In dual-fuel systems, it is usually a gas furnace.

Because electric resistance heat converts electricity to heat at 100% efficiency (COP of 1), it is reliable but expensive compared with a heat pump, which typically delivers two to three units of heat per unit of electricity in mild to moderate cold.

Key point: Emergency heat is intended for equipment failure or special circumstances, not routine cold-weather operation. In normal use, the thermostat manages heating automatically without manual EM Heat.

Emergency Heat Vs. Auxiliary Heat Vs. Defrost

Confusion often starts with terms. Auxiliary heat (AUX) is the automatic backup that your thermostat calls when the heat pump alone cannot meet demand quickly or during defrost. Emergency heat is a manual override that forces the backup on and locks the heat pump off.

During a winter defrost cycle, the heat pump reverses briefly to melt frost off the outdoor coil. You may feel cooler air for a few minutes; the system may engage AUX heat temporarily to maintain indoor comfort. Light frost is normal and not a reason to switch to EM Heat.

Remember: AUX is automatic support; EM Heat is manual backup. If the heat pump runs and heats (even slowly), there is rarely a need to choose EM Heat.

When To Switch To Emergency Heat: Clear Triggers

There are only a handful of cases where turning on emergency heat on a heat pump is the right call. These involve safety, equipment protection, or maintaining heat while the outdoor unit cannot operate.

Mechanical Failure Or Outdoor Unit Not Running

Switch to EM Heat if the outdoor unit will not run, makes unusual grinding or screeching sounds, or trips the breaker repeatedly. Locking the heat pump off prevents further damage while the home stays warm. Call an HVAC professional promptly.

Severe Ice Buildup Blocking The Outdoor Unit

Rime or a thin frost layer is normal. However, a solid block of ice encasing the outdoor unit, fan blades, or coil is not. If vents are blocked by ice or snow, switch to emergency heat, gently clear snow from around the unit, and schedule service to check defrost controls and sensors.

Thermostat Or Sensor Malfunction

If the system short-cycles, shows thermostat errors, or fails to reach temperature with the outdoor unit struggling, temporarily select EM Heat to maintain comfort. This protects the system while a technician diagnoses control board, sensor, or thermostat issues.

While Awaiting Repair In Freezing Weather

If a technician cannot arrive same-day and indoor temperatures are dropping, EM Heat can safely maintain warmth. Monitor energy use, set a reasonable temperature, and revert to normal heat once the heat pump is repaired.

Extreme Cold? Let The Thermostat Decide

In very cold weather, do not assume EM Heat is necessary. Modern heat pumps, especially cold-climate models, work effectively below 0°F. Leave the thermostat in normal heat mode; it will automatically call for auxiliary heat if needed. Choose EM Heat only if the heat pump cannot operate or is unsafe.

When Not To Use Emergency Heat

There are many times when EM Heat is unnecessary and costly. Avoid turning it on in these situations:

• It’s just cold outside. The system should manage with heat pump and AUX automatically.
• Light frost on the outdoor coil. Defrost cycles handle this without EM Heat.
• Slow warm-ups after a big setback. The thermostat may bring on AUX; manual EM Heat is not required.
• Minor temperature swings. Give the system time; verify vents and filters are clear.

Using EM Heat in these scenarios usually increases energy costs significantly without improving comfort.

Temperature Guidelines And Common Myths

A popular myth says to switch to emergency heat below a specific temperature, such as 30°F or 25°F. That rule of thumb is outdated. Performance depends on system design, home insulation, and outdoor conditions.

Modern guidance: Leave your thermostat in Heat/Auto and let it manage staging. Only select EM Heat when the heat pump cannot run or is ineffective due to malfunction or severe icing.

Dual-fuel note: In gas + heat pump systems, controls may automatically switch to gas at a set temperature. That is not EM Heat; it is normal economic switchover.

How To Turn On Emergency Heat And What To Expect

If you must use EM Heat, follow these steps to stay safe and control costs. The exact interface varies by thermostat brand and model.

• Set thermostat to EM Heat. Many thermostats have a mode selector labeled Heat, Cool, Off, and EM Heat. Choose EM Heat to lock out the outdoor unit.
• Set a moderate target temperature. Choose the lowest comfortable setpoint to reduce energy use, often 66–70°F while home.
• Expect slower ramp-up. Electric resistance heat warms steadily but may take time. Limit big setbacks.
• Watch for a brief “hot-dust” smell. Heat strips can burn off dust the first few minutes. Persistent burning smells or smoke indicate a problem; turn off and call for service.
• Monitor your electrical panel. Large heat strips (10–20 kW) draw high current. If breakers trip, call a pro—do not keep resetting.

Revert to normal heat once the outdoor unit is clear and operating or after repairs are completed.

Costs: What Emergency Heat Really Costs

Electric resistance backup is reliable but energy-intensive. Estimating cost helps decide when to use it. The math uses power (kW) multiplied by your electricity rate ($/kWh).

Example: At $0.16 per kWh (near the recent U.S. residential average), a 10 kW heat strip costs roughly $1.60 per hour. Many homes have 10–15 kW strips; some larger systems have 20 kW.

By comparison, a heat pump delivering 20,000 BTU/h with a COP of 2.5 might draw about 2.3–2.5 kW, costing roughly $0.37–$0.40 per hour at the same rate. Bottom line: emergency heat can cost several times more per hour than normal heat pump operation.

For dual-fuel systems, gas may be cheaper than electric strips depending on local rates. Controls usually handle switchover automatically based on outdoor temperature or economic balance points.

Troubleshooting Before You Flip To Emergency Heat

Before selecting EM Heat, a few quick checks can restore normal heating and save energy. These apply when the system runs but lags, not when there is clear failure or icing.

• Check the thermostat mode and setpoint. Ensure Heat is selected and the setpoint is above room temperature by a few degrees.
• Replace or clean the air filter. A clogged filter starves airflow, reduces capacity, and can trigger AUX unnecessarily.
• Open supply vents and clear returns. Blocked vents reduce output and comfort in key rooms.
• Inspect the outdoor unit’s clearance. Remove snow drifts and debris at least two feet around the cabinet. Keep the top clear for airflow.
• Listen during defrost. A swooshing sound and brief cooler air can be normal; do not force EM Heat for standard defrost.
• Verify breakers. If the outdoor unit’s breaker is tripped, do not repeatedly reset. Call a technician.
• Observe frost. Light, even frost is normal; thick ice indicates a fault. Use EM Heat and schedule service if iced solid.

If comfort cannot be maintained after these checks and the outdoor unit appears stressed, it is reasonable to use EM Heat temporarily.

Cold-Climate Heat Pumps And Dual-Fuel Options

Technology has advanced rapidly. Inverter-driven, cold-climate heat pumps maintain meaningful capacity at very low outdoor temperatures, often below 0°F. They use variable-speed compressors and enhanced vapor injection to deliver heat efficiently in deep cold.

For homes with frequent subzero events or limited electrical capacity for large heat strips, dual-fuel systems pair a heat pump with a gas furnace. Controls switch to gas when the heat pump becomes less efficient or cannot meet load, avoiding expensive electric resistance operation.

Upgrade benefits: improved comfort, quieter operation, lower winter bills, and less reliance on emergency heat. If EM Heat sees frequent use, a system assessment or upgrade can pay off.

Efficiency, Comfort, And Setbacks In Winter

Heat pumps provide steady, even heat and perform best with modest temperature changes. Large setbacks can trigger AUX or EM Heat unnecessarily as the system rushes to catch up.

• Use small setbacks. Night or away setbacks of 2–3°F are usually fine. Larger drops can erase savings if AUX runs heavily during recovery.
• Optimize fan settings. Auto is typically best. Continuous fan can reduce comfort in cool homes unless paired with variable-speed systems.
• Leverage smart thermostats. Features like heat pump balance or adaptive recovery limit AUX use while maintaining comfort.
• Seal and insulate. Air sealing, attic insulation, and duct sealing reduce runtime, making the system less likely to call AUX.
• Humidity matters. Dry winter air can feel cooler. Maintaining indoor humidity around 30–40% can improve comfort without raising the thermostat.

Tip: If certain rooms stay cool, address airflow or insulation rather than raising the whole-house setpoint, which can increase AUX or EM Heat usage.

Safety And Maintenance Considerations

Safety and reliability go hand-in-hand. A few proactive steps reduce the chance of emergency heat being needed and protect equipment during cold snaps.

• Schedule annual service. Have a technician test defrost controls, check refrigerant charge, inspect heat strips, and verify electrical connections before winter.
• Keep the outdoor unit clear. Maintain drainage and grade to prevent ice dams, and avoid placing the unit under roof drip lines without a proper cover or diverter.
• Know your breaker labeling. Many systems have separate breakers for the air handler and heat strips. Understand their ratings to avoid repeated trips.
• Watch for unusual odors. A brief dusty smell is normal when strips first heat. Persistent burning odors or smoke require immediate power-off and service.
• Use correct filters. Overly restrictive filters can harm airflow and trigger inefficiency and AUX usage.

Proper maintenance minimizes surprises and preserves efficiency, meaning fewer circumstances where emergency heat is necessary.

Frequently Asked Questions About Emergency Heat

Common questions often arise during cold snaps. These concise answers provide quick guidance for typical households using heat pumps in the U.S.

• Does emergency heat warm the house faster? Sometimes, but at higher cost. It may reduce warm-up time after big setbacks, but using smaller setbacks is more efficient.
• How long can EM Heat run? It can run continuously, but costs add up quickly. Use it only until the heat pump is safe to operate or repairs are done.
• Is EM Heat the same as AUX? No. AUX is automatic backup used with the heat pump. EM Heat is a manual mode that disables the heat pump.
• Should EM Heat be used during defrost? No. Defrost is normal. The thermostat may briefly use AUX to offset cool air; manual EM Heat is not needed.
• What if my thermostat has no EM Heat option? Some thermostats hide EM Heat or lack a manual mode. If the outdoor unit fails, turn the system to Heat and raise the setpoint; many controls will rely on AUX automatically. Call a pro to verify configuration.

Practical Scenarios: Decide Whether To Switch

These realistic examples illustrate when to stay in normal heat and when to move to emergency heat on a heat pump, helping prevent costly mistakes.

• Scenario 1: 25°F outside, home warms slowly, light frost on unit. Stay in normal heat. Replace the filter, ensure vents are open, and give the system time. AUX may assist automatically.
• Scenario 2: 18°F outside, outdoor unit encased in thick ice. Switch to EM Heat, clear snow around the unit safely, and schedule service to inspect defrost controls.
• Scenario 3: 35°F outside, breaker trips when the outdoor unit starts. Use EM Heat to maintain comfort and call a technician. Do not repeatedly reset the breaker.
• Scenario 4: 5°F outside, cold-climate heat pump installed, home reaches setpoint. Normal heat is appropriate. Controls will use AUX only if needed.
• Scenario 5: Heat pump compressor failed awaiting parts. EM Heat until repair, with moderate setpoints to manage cost.

How To Reduce Reliance On Emergency Heat

Reducing EM Heat usage saves money and improves comfort. Focus on efficiency upgrades, smart controls, and preventive care that help the heat pump handle more hours on its own.

• Weatherize. Seal gaps around doors, windows, and penetrations. Boost attic insulation to recommended R-values.
• Balance airflow. Have a contractor measure static pressure, adjust dampers, or add returns to improve distribution.
• Tune controls. Ask a pro to set heat pump lockout and balance points appropriately. Use thermostat features that limit AUX.
• Upgrade equipment. Variable-speed, cold-climate heat pumps retain capacity in deep cold and reduce backup heat usage.
• Right-size heat strips. Oversized strips can drive unnecessary AUX. Proper sizing meets peak load without waste.

These steps often provide faster payback than expected, especially in regions with high electricity rates.

Energy Performance Basics: Why Heat Pumps Usually Win

Heat pumps move heat rather than create it. In moderate cold, they deliver two to four units of heat per unit of electricity. Even in deeper cold, high-quality systems maintain a COP well above 1, meaning they still beat resistance heat on cost per unit of heat.

Electric resistance offers certainty at a price. It is excellent for backup or emergency use but rarely the cheapest main heat source if the heat pump is available and healthy.

Takeaway: Reserve EM Heat for failure, icing, or service windows; let the heat pump carry the load the rest of the time.

Quick Decision Guide: When To Switch To Emergency Heat

• Yes—Use EM Heat Now if the outdoor unit will not run, makes alarming noises, is encased in thick ice, or trips breakers. Call for service.
• Maybe—Temporarily Use EM Heat if you are awaiting repair and need immediate comfort, or if a thermostat/control fault prevents safe heat pump operation.
• No—Stay In Normal Heat for routine cold weather, light frost, slow recovery after a setback, or minor comfort swings. Let AUX engage automatically if needed.

Knowing when to switch to emergency heat on a heat pump prevents damage, preserves comfort, and avoids bill shock. With good maintenance, efficient settings, and modern equipment, most homes can minimize EM Heat and enjoy steady, affordable warmth all winter.