Emergency Heat: When to Use It on a Heat Pump and What It Costs

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A heat pump thermostat often has a setting that most homeowners never touch until something goes wrong: “EM HEAT” or “Emergency Heat.” On a frigid January morning along the Colorado Front Range, a glowing “AUX” light or a sudden jump in the electric bill can send people searching for what that setting does and whether they should be using it. The short version is that emergency heat is a manual backup mode meant for times when the outdoor heat pump unit cannot do its job. Used at the wrong time, it can quietly multiply heating costs. Used at the right time, it keeps a house warm while a technician is on the way. This guide explains the difference between emergency heat and auxiliary heat, when each one should run, why resistance heat costs more, how cold-climate heat pumps change the picture, what the thermostat wiring has to do with it, and when it is time to call a professional.

Part of our Heating, Cooling & Ventilation guide — start there for the full picture →

What Emergency Heat Means on a Heat Pump

A heat pump heats a home by moving heat from the outdoor air to the indoor air using refrigerant, a compressor and two coils. ENERGY STAR describes a heat pump as moving heat rather than converting it from fuel, which is why an air-source heat pump can deliver up to three times more heat energy to a home than the electrical energy it consumes. That efficiency is the whole reason to own one.

Because the amount of heat available in outdoor air falls as the temperature drops, most ducted heat pump systems include a second heat source. In many homes that backup is a bank of electric resistance heating elements, often called heat strips, mounted in the indoor air handler. In a “dual fuel” home, the backup may be a gas furnace instead.

Emergency heat is a thermostat mode that shuts the heat pump’s compressor off entirely and runs only that backup source. The outdoor unit stops heating, and the house is warmed by the strips or the furnace alone. It is a manual override, which means a person has to select it, and the system will stay in that mode until someone switches it back to normal heat.

Emergency Heat vs Auxiliary Heat

The two terms are easy to confuse because they often use the same hardware. The difference lies in who turns the backup on and whether the heat pump keeps running.

Auxiliary heat (automatic)

Auxiliary heat, shown as “AUX” on many thermostats, is backup heat that the thermostat or control board brings on automatically. Common triggers include:

• The indoor temperature falling more than a set number of degrees below the setpoint, often after someone raises the thermostat by several degrees at once.
• Outdoor temperatures low enough that the heat pump alone cannot keep up with the heat loss of the house.
• Defrost cycles, when the outdoor unit briefly reverses and the system adds backup heat so cold air does not blow from the registers.

In auxiliary mode the heat pump usually keeps running alongside the backup. The strips or furnace simply top off what the heat pump is already delivering, and they shut off once the house catches up. Seeing “AUX” for short periods on cold mornings or during defrost is normal.

Emergency heat (manual)

Emergency heat removes the heat pump from the equation. The compressor is locked out and the backup source carries the entire heating load. On a system with electric strips, that means every bit of heat comes from resistance elements, with no help from the far more efficient compressor.

A simple way to remember the difference: auxiliary heat is the system helping itself; emergency heat is a person telling the system to stop using the heat pump.

When to Use Emergency Heat (and When Not To)

Emergency heat is designed for situations where the outdoor unit is not working or should not be running. Reasonable reasons to switch it on include:

• The outdoor unit has failed. If the compressor or outdoor fan stops working, the house can still be heated with emergency heat until a repair is made.
• The outdoor unit is physically damaged or buried. A fallen limb, heavy drifted snow, ice that has encased the coil and is not clearing, or a damaged refrigerant line are reasons to stop running the compressor.
• The outdoor unit is making grinding, banging or screeching noises. Continuing to run a compressor that sounds wrong can turn a moderate repair into a major one.
• A technician has advised it. Sometimes a contractor will ask homeowners to use emergency heat while waiting for a part.

Situations where emergency heat is usually not the right answer:

• Normal cold weather. Many homeowners switch to EM heat during a cold snap because they assume the heat pump “can’t handle it.” The system’s automatic auxiliary heat is designed to handle that. Switching to emergency heat simply removes the efficient part of the system.
• A defrost cycle. Steam or a cloud of vapor rising from the outdoor unit and a brief pause in heating are normal parts of defrost, not a reason to change modes.
• Lukewarm-feeling air. Heat pumps deliver air that is warm but not as hot as a gas furnace’s output. That gentler feel is normal and does not by itself mean something is wrong.

If emergency heat does get turned on, it is worth setting a reminder to switch it back to normal heat once the outdoor unit is repaired. Forgetting to do so is one of the most common reasons for a surprisingly high winter electric bill.

Why Emergency Heat Costs More to Run

Electric resistance heat converts electricity into heat at roughly one unit of heat for each unit of electricity. A heat pump moves heat that already exists outdoors, which is why ENERGY STAR says a certified air-source heat pump can deliver up to three times more heat energy than the electricity it consumes. When emergency heat locks the compressor out, the home loses that multiplier.

The practical result is that running a house on electric strips alone can cost several times what the same heat would cost from a heat pump in mild weather. The exact difference depends on outdoor temperatures, the efficiency of the specific heat pump at those temperatures, local electric rates, how well the house is insulated and sealed, and how many kilowatts of strip heat are installed. Households with a gas furnace as the backup will see a different trade-off that depends on local gas and electric prices.

Utility bills offer a rough reality check. ENERGY STAR notes that nearly half of the energy used in a home goes to heating and cooling, so a heating mode that runs at lower efficiency for weeks can show up clearly on a monthly statement. A bill that jumps sharply in a single month, without a matching cold snap, can be a sign that emergency heat was left on or that auxiliary heat is running far more than it should.

Thermostat habits that trigger backup heat

ENERGY STAR’s heat pump guidance points out that, unlike a furnace or boiler, heat pumps do not save energy by turning the temperature down when people are away or asleep, and it recommends picking a comfortable temperature and leaving it there. Large setbacks force the system to recover several degrees at once, which is one of the common triggers for auxiliary strips. ENERGY STAR also suggests setting the system to “heat” or “cool” rather than “auto” to avoid unwanted mode switching during unseasonable weather. Many modern heat pump thermostats include “smart recovery” or adaptive recovery features designed to warm the home gradually so the strips stay off.

Cold-Climate Heat Pumps and Colorado Winters

Older heat pumps earned a reputation for struggling in cold climates, and that history still shapes how many people use the EM heat setting. The technology has changed. ENERGY STAR states that many newer certified air-source heat pumps provide space heating even in very cold climates thanks to advanced compressors and refrigerants, and that its cold-climate certification requires third-party verified performance testing down to 5°F. ENERGY STAR also notes that a cold-climate heat pump keeps working below 5°F, but that pairing it with a backup energy source heats the home most efficiently when temperatures drop even lower.

That description fits the Front Range well. Denver, Colorado Springs, Fort Collins and the foothill communities see many winter days that are cold at night but mild by afternoon, punctuated by arctic outbreaks that can push temperatures below zero. A well-sized cold-climate heat pump can carry most of that season on its own, with backup heat filling gaps during the coldest stretches. Higher-elevation homes in the foothills and mountains see longer periods of deep cold and may lean on backup heat more often.

Homeowners comparing systems for an older house can find more detail in the guides to heat pump installation and heat pump vs furnace trade-offs. Sizing matters here. ENERGY STAR advises that contractors verify system size with a Manual J load calculation, and an undersized heat pump will call for backup heat much more often than a correctly sized one.

Ductless systems

ENERGY STAR describes ductless mini-splits as one or more indoor head units mounted on a wall or ceiling, connected to an outdoor unit that extracts heat from the air even when it is cold. These systems often have no built-in resistance backup and no emergency heat setting at all. Homes heated by mini-splits typically rely on another heat source, such as baseboard heaters or a furnace, if the outdoor unit fails. If a mini-split stops heating, the guide to mini-split repair covers common causes.

Defrost Cycles: Normal Behavior That Looks Like a Problem

In heating mode, the outdoor coil is colder than the outdoor air, so moisture in the air can freeze on it. Frost acts as insulation and reduces the unit’s ability to pull heat from outdoors. To clear it, the heat pump periodically runs a defrost cycle, briefly reversing into a cooling-like mode so warm refrigerant melts the frost off the outdoor coil.

During defrost, homeowners commonly notice:

• The outdoor fan stopping while the compressor keeps running.
• A whooshing sound when the reversing valve shifts, both at the start and the end of the cycle.
• Steam or vapor rising from the outdoor unit, which is melting frost and is not smoke.
• The “AUX” indicator lighting up while backup heat tempers the air indoors.

Defrost cycles typically last a few minutes. A unit that stays coated in thick ice for hours, never seems to clear, or develops a solid block of ice at the base is a different matter. Possible causes include a failed defrost control or sensor, low refrigerant charge, a stuck reversing valve, a failed outdoor fan motor, or poor drainage that lets meltwater refreeze around the unit. Clearing snow from around the outdoor unit and keeping roof runoff from dripping onto it are reasonable homeowner steps. Chipping ice off the coil with a tool is not, because the fins and tubing are easy to puncture.

Thermostat Wiring for Auxiliary and Emergency Heat

Heat pump thermostats use more control wires than a basic furnace thermostat. Terminal labels vary by brand, but a typical heat pump setup includes terminals for the compressor, the reversing valve (often labeled O or B, depending on whether the valve energizes in cooling or heating), the indoor fan, 24-volt power and common, and one or more terminals for auxiliary and emergency heat. These are often labeled W2, AUX and E, and in some installations the auxiliary and emergency terminals are jumpered together.

Wiring and configuration problems can produce symptoms that look like equipment failure:

• A thermostat set up for a conventional system instead of a heat pump may run strips constantly or never call for the compressor.
• An incorrect O/B reversing valve setting can make the system cool when heat is called for, which then triggers the strips to compensate.
• A missing or loose auxiliary wire can leave the house cold during defrost or deep cold, while an auxiliary wire landed on the wrong terminal can run strips when they should be off.
• Outdoor temperature lockout settings, when present, may be set so that strips run at temperatures where the heat pump could carry the load alone.

Replacing a thermostat on a heat pump is a common time for these mistakes to appear. The guide to thermostat wiring explains terminal labels in more detail. Because strip heaters run on high-voltage circuits and the air handler contains live electrical components, any work inside the equipment itself belongs to a qualified HVAC technician.

When to Call an HVAC Technician

Emergency heat is a stopgap, not a repair. Signs that a heat pump needs professional attention include:

• The outdoor unit not running at all while the thermostat calls for heat.
• “AUX” displayed almost continuously, even in mild weather.
• The outdoor coil staying heavily iced after defrost cycles should have cleared it.
• Unusual noises from the compressor or outdoor fan.
• Breakers for the heat strips or outdoor unit tripping repeatedly.
• A burning smell beyond the brief dusty odor that strips can produce the first time they run in the fall.
• A sudden, unexplained rise in the electric bill.

Refrigerant problems are one of the common causes of poor heat pump performance. Refrigerant work requires an EPA Section 608 certified technician, and topping off refrigerant without finding and fixing the leak is not a lasting repair. ENERGY STAR’s maintenance checklist recommends annual pre-season check-ups by a contractor, with the heating system checked in the fall, and notes that too much or too little refrigerant makes a system less efficient.

What heat pump repairs commonly cost

Repair prices vary widely by equipment, part availability and season. Very broadly:

• Service call and diagnosis: often in the low hundreds of dollars, sometimes credited toward the repair, and higher for nights, weekends or holidays.
• Thermostat replacement or reconfiguration: often a modest charge for labor plus the cost of the thermostat.
• Capacitor, contactor, defrost board or sensor: commonly a few hundred dollars installed, depending on the part.
• Heat strip element or sequencer repair: often a few hundred dollars or more.
• Refrigerant leak repair: highly variable and can reach well into four figures for coil replacement.
• Compressor or outdoor unit replacement: often a large fraction of the cost of a new system, which is why many owners compare repair against replacement.

Getting a written diagnosis that lists measured readings and the specific failed part makes it easier to compare quotes.

Heat Pumps During a Home Inspection

Buyers evaluating a home with a heat pump should know what a standard inspection does and does not cover. The ASHI Standard of Practice directs inspectors to open readily openable access panels, inspect installed heating equipment and distribution systems, and describe the energy sources and heating systems. It does not require inspectors to determine heat supply adequacy or distribution balance. An inspector may operate the system in heating mode where conditions and the equipment allow, note the presence of backup heat, and flag visible issues such as a damaged outdoor coil, missing insulation on refrigerant lines, poor clearance around the outdoor unit or a unit sitting directly on the ground where snow and meltwater collect.

Some conditions limit what can be tested. Many manufacturers advise against running a heat pump in cooling mode below certain outdoor temperatures, so a winter inspection may not verify cooling, and a summer inspection may not fully test heating. Buyers who want deeper answers can ask an HVAC contractor for a separate evaluation that includes refrigerant pressures, strip heat amperage and defrost operation. The heating and cooling hiring guide walks through how to choose the right professional, and the broader home inspector hiring guide explains what to expect from a general inspection.

References

• Air-Source Heat Pumps — ENERGY STAR (U.S. EPA)
• Heat and Cool Efficiently — ENERGY STAR (U.S. EPA)
• Maintenance Checklist — ENERGY STAR (U.S. EPA)
• Ductless Heating and Cooling — ENERGY STAR (U.S. EPA)
• ASHI Standard of Practice for Home Inspections — American Society of Home Inspectors

Buying a Front Range home with a heat pump and unsure how hard it leans on backup heat? Get in touch with us and we can help you line up an inspection that looks closely at the heating system before closing.