Heat Pump Auxiliary Heat: What It Is, When It’s Normal, and What It Costs

By /

It’s 14°F outside, your heat pump has been running for two hours, and your thermostat just lit up with “AUX HEAT.” Before you call anyone — here’s what that actually means.

In most cases, you can relax. Auxiliary heat switching on during a cold snap is normal behavior, not a malfunction. (Most thermostats display this as “AUX HEAT” or “AUX” — Ecobee, Nest, and Honeywell each use slightly different labels, but the function is identical.) But “most cases” isn’t “always” — knowing the difference between expected operation and something worth investigating could save you a service call.

This guide covers what heat pump auxiliary heat is, when it’s normal versus a problem, how it differs from emergency heat, typical temperature thresholds, running costs, and clear signs it’s time to call a technician. For the full picture on heat pump types, costs, and whether one makes sense for your home, the heat pumps for homeowners guide covers it all — this article focuses specifically on auxiliary heat behavior.

Quick answer: Auxiliary heat is the electric resistance backup built into most heat pumps. It switches on automatically when outdoor temperatures drop below the heat pump’s balance point and it can’t maintain your set temperature alone. Seeing “AUX HEAT” in cold weather is normal — the time to investigate is if it runs frequently when it’s above 40°F outside, or runs continuously for hours without your home warming up.

This page contains affiliate links and may include sponsored content. As an Amazon Associate, I earn from qualifying purchases at no extra cost to you. Amazon links are labeled #ad; other affiliate links may be marked as “(paid link).” Affiliate Disclosure.

Table of Contents

What Is Auxiliary Heat on a Heat Pump?

Understanding heat pump auxiliary heat starts with understanding what a heat pump does. It moves heat from the outdoor air into your home; it doesn’t generate heat by burning fuel, it transfers it. That process is highly efficient when outdoor temperatures are moderate, but as it gets colder outside, there’s less heat energy in the air to transfer, and the heat pump has to work harder to maintain the same output.

At some point — typically somewhere between 25°F and 40°F depending on the system — the heat pump alone can’t keep up with demand. Your thermostat senses that the indoor temperature is falling or struggling to rise, and it switches on auxiliary heat to close the gap.

Auxiliary heat is electric resistance heating: essentially the same technology as a baseboard heater or a toaster coil, just integrated into your air handler. It produces heat reliably regardless of outdoor temperature, but it’s significantly less efficient than the heat pump itself. According to the U.S. Department of Energy, heat pumps deliver two to four times more heat energy per unit of electricity than resistance heating — which is exactly why aux heat costs more per hour to run. Crucially, it runs alongside the heat pump rather than replacing it — the heat pump keeps running, and the aux strips add the extra heat on top.

Auxiliary Heat vs. Emergency Heat: These Are Not the Same Thing

This is the most common source of confusion, and most articles gloss over it. Auxiliary heat and emergency heat use the same physical components (the electric resistance strips), but they operate completely differently, and mixing them up can cost you real money.

Mode What it does Who controls it Heat pump running?
Auxiliary heat Adds supplemental electric resistance heat when the heat pump can’t keep up alone Thermostat — automatic Yes — heat pump and aux run together
Emergency heat Bypasses the heat pump entirely and runs pure electric resistance heat You — manual switch on thermostat No — heat pump is completely off

When your thermostat displays “AUX HEAT,” that’s automatic and expected in cold weather. The system decided to add supplemental heat, and the heat pump is still your primary source.

When your thermostat displays “EM HEAT” or “EMERGENCY HEAT,” someone — probably you — manually switched to emergency heat mode. In this mode, the heat pump is completely shut off and you’re running pure resistance heat. It costs roughly two to three times as much to operate, and you should only use it if the heat pump itself is genuinely broken or a technician has told you to use it during service.

Red flag: If your thermostat shows “EM HEAT” and you didn’t manually turn it on, that’s worth a call to your HVAC technician. Some older or malfunctioning systems can get stuck in emergency heat mode without user input.

At What Temperature Does Auxiliary Heat Kick In?

The temperature at which heat pump auxiliary heat kicks in varies by system type — mostly whether you have a standard or cold-climate model. The technical term for the outdoor temperature where your heat pump’s output exactly matches your home’s heating demand is the balance point. Below that temperature, aux heat supplements the load to make up the difference.

Outdoor temp System behavior Aux heat running? Action
Above 40°F Heat pump handles it alone No — investigate if yes Call tech if aux runs regularly at this temp
30°F–40°F Heat pump primary, may get some help Occasionally normal Watch your bill; normal if brief
20°F–30°F Heat pump struggling; aux kicks in regularly Yes — normal No action needed
10°F–20°F Aux heat running often; defrost cycles more frequent Yes — normal and expected No action; monitor for ice that won’t clear
Below 10°F Standard systems lean heavily on aux; cold-climate models still performing Yes — expected for standard systems Normal unless home can’t reach set temp
Below 0°F Standard systems near their limit; aux carrying most load Heavy use — expected If you can’t maintain 65°F, system may be undersized

System-type balance points: standard models engage aux around 35°F–40°F; mid-range units around 25°F–35°F. For a heat pump in extreme cold — below 10°F — cold-climate models (Mitsubishi H2i, Bosch IDS, Daikin Fit) maintain output down to 5°F or -13°F with far less aux reliance.

The threshold also shifts based on how well-insulated your home is, whether your ducts leak, the system’s size relative to your heating load, and how large a temperature gap there is between the outdoor reading and your thermostat setting.

If you’re asking about heat pump performance in extreme cold weather, the honest answer depends heavily on whether you have a standard or cold-climate model. One important caveat: if you have a dual-fuel system — a heat pump paired with a gas furnace — your “aux heat” source is the furnace, not electric resistance strips. In that case, the cost and efficiency math is entirely different (gas is typically cheaper per BTU than electric resistance in most of the US). The thresholds above apply to all-electric systems with resistance strips as backup.

In a typical cold-climate home in Pennsylvania or Ohio, a heat pump running in extreme cold conditions — below 25°F–30°F — will display AUX HEAT regularly. (These thresholds are generalizations; your system’s actual balance point depends on your home’s insulation, duct condition, and how well the system was sized at install.) That’s normal operation, not a malfunction. If aux is coming on regularly when it’s 45°F outside, that’s a different conversation.

What Is the Heat Pump Defrost Cycle — and Why Does It Look Alarming?

The heat pump defrost cycle is one of the most misunderstood parts of normal system operation. You step outside on a cold morning and see steam — or what looks like smoke — rising from your outdoor unit. Your first instinct is probably wrong: it’s not burning, it’s defrosting.

On cold, humid days, frost can accumulate on the outdoor coil of your heat pump. If enough builds up, it blocks airflow and seriously hurts efficiency. So the system has a built-in defrost cycle that periodically reverses its operation to melt the frost off the coils. During the defrost cycle:

  • The outdoor unit temporarily reverses and runs in cooling mode (even though it’s cold outside)
  • The auxiliary heat strips turn on to keep warm air flowing through your house while the outdoor unit defrosts
  • Steam or vapor rises from the outdoor unit — this is the frost melting, not smoke
  • The cycle typically lasts 5 to 15 minutes
The steam is normal. Many homeowners see vapor rising from the outdoor unit during a defrost cycle and worry the unit is on fire or broken. It’s water vapor from melting frost — the same thing you’d see rising from a warm driveway after light rain. The unit is working correctly.

A normal heat pump defrost cycle runs every 30 to 90 minutes in cold, humid conditions and completes within 15 minutes. In practice, most homeowners barely notice it once they know what to expect: the indoor airflow continues, the house stays warm, and the unit clears itself. If your outdoor unit seems to be running a constant defrost cycle, defrosting for 30+ minutes at a stretch, or accumulating heavy ice that doesn’t clear, those are signs worth investigating.

Signs the defrost cycle may be a problem:

  • Ice builds up on the outdoor unit and doesn’t clear within 15–20 minutes of defrost starting
  • The bottom of the unit is blocked by a solid sheet of ice, not just light frost
  • Defrost cycles seem to run constantly rather than periodically
  • You’re losing noticeable heat output indoors during defrost
  • The unit sounds different during defrost than it used to

If any of these apply, it could indicate a refrigerant issue, a dirty coil, a malfunctioning defrost board, or a blocked drain.

What Does Heat Pump Auxiliary Heat Cost to Run?

This is where the financial reality check lives. Auxiliary heat is electric resistance heating, which converts 1 kWh of electricity directly into 1 unit of heat (a COP, or coefficient of performance, of 1.0). Your heat pump in moderate conditions runs at a COP of 2.5 to 4.0, meaning it delivers 2.5 to 4 units of heat per unit of electricity consumed.

When heat pump auxiliary heat kicks in, you’re running resistance strips at COP 1.0 alongside whatever the heat pump is contributing. Total system efficiency drops. Here’s what that means in real dollars at common electric rates:

Aux heat capacity At 12¢/kWh At 18¢/kWh At 24¢/kWh
5 kW (small system) $0.60/hr $0.90/hr $1.20/hr
10 kW (typical) $1.20/hr $1.80/hr $2.40/hr
15 kW (larger home) $1.80/hr $2.70/hr $3.60/hr

In homeowner forums, the pattern behind January bill spikes is almost always the same: aux heat running 4–6 hours a night, not 30 minutes. Running a heat pump in extreme cold weather is inherently more expensive than moderate-temperature operation, but it shouldn’t feel like a runaway cost. A 10 kW system running aux heavily at 18¢/kWh can add $25–$40 per night. Brief use on the coldest nights is expected; running constantly in mild weather is not.

Reality check: Aux heat running occasionally on the coldest nights is a normal operating cost of a heat pump in a cold climate. The problem is when it runs constantly in mild weather — that usually means the heat pump isn’t carrying its share. Also worth knowing: raising your thermostat by 3°F+ in one go triggers aux heat even if it’s not that cold outside. Raise the temperature 1–2 degrees at a time to keep the heat pump as your primary source.

Is My Heat Pump Auxiliary Heat Running Too Much?

Enter your outdoor temp and how long aux heat has been running.

Current outdoor reading

If the checker flags your situation as worth watching or worth a call, the next section covers the specific warning signs that technicians look for.

When Auxiliary Heat Is Actually a Problem

Aux heat running occasionally in cold weather is expected. These situations are not:

Aux heat runs when it’s above 40°F outside

Unless your thermostat is set unusually high or you just made a large temperature adjustment, aux heat shouldn’t be needed when it’s 45°F outside. The honest answer is: the exact threshold varies enough by system that it’s hard to draw a hard line — but trust your instincts if it feels like it’s running more than it used to for the same weather conditions. If it’s running regularly in mild weather, the heat pump may be undersized, have a refrigerant leak, or have a failing compressor.

Aux heat runs continuously for hours without stopping

Aux heat is designed to supplement, not carry the full heating load. If it’s been running solidly for 4–6+ hours and your home still isn’t reaching the set temperature, something is wrong: either with the heat pump itself or with a much larger heat loss issue in the house.

Your bill suddenly jumps without a weather explanation

If your electric bill is more than 30–40% higher than the same month last year — with comparable weather — and you’re seeing aux heat run frequently, that’s worth investigating. A refrigerant charge issue, a failing reversing valve, or a stuck defrost board can all cause the aux strips to compensate for a heat pump that isn’t performing correctly.

You see “EM HEAT” and didn’t turn it on

Emergency heat should only activate if you manually select it. If it’s displaying on its own, call your HVAC technician.

Ice that won’t clear from the outdoor unit

A thin layer of frost on the outdoor coil is normal in cold, humid conditions. A solid block of ice that doesn’t clear after a defrost cycle is not normal, and it can permanently damage the unit if left unaddressed.

What to Tell a Technician If You Call

HVAC technicians bill by the hour. Walking in with specific information typically turns a 90-minute diagnostic visit into a 30-minute one. Note down:

  • The outdoor temperature when aux heat seems to run more than expected
  • Whether you’re seeing “AUX HEAT” or “EM HEAT” on the thermostat
  • How long aux heat runs continuously before cycling off
  • Whether the outdoor unit has visible ice buildup and whether defrost cycles seem to clear it
  • Any change in heating output — rooms that used to get warm that now stay cold
  • Your electric bill compared to the same period last year

This gives the technician enough to make a diagnosis without charging you an hour of diagnostic time to establish basics.

Frequently Asked Questions

Auxiliary heat is the backup electric resistance heating built into most heat pump systems. When your thermostat displays “AUX HEAT,” it means outdoor temperatures have dropped low enough that the heat pump can’t maintain your set temperature on its own, so the system automatically adds supplemental heat to close the gap. It’s a normal function, not an error.
Yes, in cold weather — that’s what it’s designed to do. Most heat pumps begin relying on aux heat somewhere between 25°F and 40°F depending on the system. Seeing it activate on cold nights is expected. The concern is when aux heat runs in mild weather above 40°F, or runs continuously without your home reaching the set temperature — those patterns suggest a system issue worth investigating.
You can’t fully prevent it in cold weather without affecting your system’s ability to heat your home. Some thermostats allow you to set an outdoor temperature lockout, which delays aux heat from engaging until a specific threshold. Practically speaking, the best way to reduce aux heat use is to improve your home’s insulation, seal air leaks, raise your thermostat gradually rather than all at once, and keep the outdoor unit clear of ice and snow.
Not necessarily. Heat pumps are designed to run for long cycles, especially in cold weather. Unlike a furnace that blasts heat and shuts off, a heat pump runs more continuously at a lower intensity. If it’s running all night and your home is maintaining temperature, that’s normal operation. If it’s running all night and your home is still cold, or if aux heat is on the whole time, that’s a sign the system may be struggling or undersized.
For standard heat pumps, efficiency drops significantly below 25°F–30°F, and they lean heavily on aux heat below 20°F. At 0°F and below, most standard systems are near their practical limit. Cold-climate heat pumps (Mitsubishi H2i, Bosch IDS, Daikin Fit) are engineered to operate down to -13°F with significantly higher efficiency. If you’re in a Zone 5 or 6 climate, a cold-climate model makes a real difference in how much you rely on aux heat in January.
Yes. As temperatures fall, the heat pump’s efficiency (COP) drops and it relies more on electric resistance aux heat, which has a COP of 1.0, much less efficient than the heat pump itself at 2.5–4.0 in mild weather. In a very cold month, your bill will be noticeably higher than in fall or early spring. That said, a cold-climate heat pump in a well-insulated home will still typically cost less to operate than electric resistance heating alone, even in extreme cold.

How to Reduce Auxiliary Heat Usage

If aux heat is running more than you’d like — or your bill is higher than expected — these changes can reduce how often it activates without sacrificing comfort:

  • Raise the thermostat gradually. A jump of 3°F+ at once triggers aux heat even in mild weather. Raise 1–2 degrees at a time and the heat pump handles it without the strips.
  • Improve insulation and seal air leaks. The better your home holds heat, the longer the heat pump stays within its efficient range before aux kicks in. Attic insulation and door/window sealing are the highest-ROI fixes.
  • Keep the outdoor unit clear. Snow packed against the unit or debris blocking airflow hurts efficiency and forces earlier aux engagement. A quick check after heavy snowfall is worth it.
  • Use your thermostat’s aux lockout setting. Many thermostats (including Nest, Ecobee, and Honeywell) let you set a minimum outdoor temperature below which aux is allowed to run. If yours never locks out, it may be engaging earlier than necessary.
  • Consider a cold-climate upgrade. If you have a standard heat pump and heavy aux heat use is a consistent winter problem, a cold-climate model (Mitsubishi H2i, Bosch IDS, Daikin Fit) maintains much higher efficiency at low temperatures — often running little to no aux heat down to 5°F.

Conclusion

Seeing “AUX HEAT” on your thermostat during a cold snap is normal — your heat pump auxiliary heat is doing exactly what it’s built to do. The key is knowing when that behavior crosses from expected operation into something worth investigating. If aux heat runs in mild weather, runs non-stop without your home warming up, or you’re seeing emergency heat you didn’t turn on yourself, those are the signals that warrant a call. Everything else: check the outdoor temp, give it time, and watch the bill.

Ask a Licensed HVAC Technician

Not sure if your aux heat behavior warrants a service call? JustAnswer connects you with a licensed HVAC technician who can assess your system — outdoor temps, duration, thermostat readings, any unusual sounds — and tell you whether it’s normal operation or worth an in-person diagnostic visit.

Ask an Expert

This article is for general informational purposes. Heat pump behavior varies by model, installation quality, and home characteristics. If you’re unsure whether your system is operating correctly, a diagnostic visit from a licensed HVAC technician is the most reliable answer. Acara Institute is not affiliated with any HVAC installer, manufacturer, or service provider.

Leave a Comment

Your email address will not be published. Required fields are marked *

Scroll to Top