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

Emergency Heat vs Auxiliary Heat

Understanding the difference between emergency heat and auxiliary heat prevents confusion and unnecessary energy waste.

Auxiliary heat (AUX heat) is the backup heating that the system activates automatically when the heat pump alone cannot meet the thermostat's demand. This happens when outdoor temperatures are very low and the heat pump's output drops below the home's heating needs, or when the thermostat detects a large gap between the set temperature and the current temperature (like after a setback recovery). The heat pump continues running alongside the auxiliary heat, and the system returns to heat-pump-only operation once the temperature demand is satisfied. Auxiliary heat activation is normal and requires no action from the homeowner.

Emergency heat (EM heat) is a manual thermostat setting that shuts off the heat pump compressor entirely and runs only the backup heating system. The heat pump does not contribute any heating when emergency heat is engaged. This setting exists for situations where the outdoor unit is physically damaged, completely iced over, or otherwise incapable of operating safely.

How Emergency Heat Works Mechanically

When you select the EM heat setting on your thermostat, the thermostat sends a signal to the heat pump's control board that disables the compressor contactor and reversing valve, preventing the outdoor unit from running. Simultaneously, it activates the backup heat circuit, which powers the electric resistance strips in the air handler or fires the gas furnace burners in a dual fuel system.

The indoor blower fan runs normally, pushing air across the heated strips or furnace heat exchanger and distributing it through the ductwork. The system operates exactly like a traditional electric furnace or gas furnace, with no involvement from the heat pump's refrigerant cycle.

Electric resistance strips are rated in kilowatts, typically 5 kW, 10 kW, 15 kW, or 20 kW depending on the system size. A 10 kW strip package produces approximately 34,000 BTU per hour, which is enough to heat a well-insulated 1,500-square-foot home in moderate cold but may struggle during extreme cold. Larger homes or colder climates may have multiple strip packages totaling 15 to 20 kW.

Why Emergency Heat Is So Expensive

The cost difference between heat pump operation and emergency heat comes down to the coefficient of performance (COP). A heat pump operating at a COP of 3.0 produces 3 units of heat for every unit of electricity consumed. Electric resistance strips operate at a COP of exactly 1.0, producing 1 unit of heat per unit of electricity. The heat pump is three times more efficient, meaning emergency heat uses three times as much electricity to produce the same amount of heat.

At the national average electricity rate of $0.16 per kWh, the cost per 100,000 BTU of delivered heat is approximately $1.56 with the heat pump (at COP 3.0) and $4.69 with electric resistance strips (at COP 1.0). Over the course of a cold day, this difference amounts to $20 to $40 in additional energy cost.

Dual fuel systems with a gas furnace backup are less expensive to run on emergency heat because natural gas is cheaper per BTU than electricity. The cost per 100,000 BTU from a 96% AFUE gas furnace is approximately $1.25 at average gas prices, which is actually cheaper than the heat pump at very cold temperatures where the heat pump's COP drops below 2.0. This is one of the key advantages of the dual fuel configuration.

What to Do When You Need Emergency Heat

If you need to switch to emergency heat, follow these steps to manage the situation effectively.

First, switch the thermostat to EM heat to ensure your home stays warm. Do not wait for a technician appointment if it is cold outside.

Second, lower the thermostat set temperature by 2 to 3 degrees to reduce energy consumption while running on backup heat. Every degree of setback reduces heating cost by approximately 3% during emergency heat operation.

Third, call an HVAC technician and explain the symptoms. Mention any unusual sounds, visible damage, or ice buildup on the outdoor unit. Ask for emergency or priority scheduling, as most HVAC companies offer expedited service for no-heat calls during winter.

Fourth, check if the problem is something you can resolve without a technician. A tripped breaker, a clogged filter, or debris blocking the outdoor unit can sometimes be fixed without professional help. Review the troubleshooting steps in our heat pump not heating guide before committing to a service call.

Fifth, switch back to normal heat pump operation as soon as the repair is complete. Do not leave emergency heat engaged after the heat pump is fixed, as some homeowners forget and run on expensive backup heat for weeks before noticing the high utility bill.