Emergency Heat vs Auxiliary Heat: What the Difference Is and Why It Matters

This is probably the most misunderstood pair of words in home heating, and the confusion is not the homeowner’s fault. The thermostat says “EM HEAT” in a spot right next to HEAT and COOL, as if it were just another mode. The word “emergency” sounds like something you’d reach for during a bad winter storm. Both impressions are wrong, and acting on them can add hundreds of dollars to a single month’s bill.

Here is the short version. Your heat pump is the cheap heat. The strips are the expensive heat. Auxiliary heat is your system automatically blending in a little expensive heat when it needs to. Emergency heat is you manually switching off the cheap heat entirely and running only the expensive kind.

The rest of this guide explains why resistance heat costs what it does, what the aux indicator on your thermostat is actually telling you, when emergency heat genuinely is the right call, and how to tell whether your aux heat is running far more than it should — which is usually a symptom of something fixable.

The actual difference, side by side

Both terms describe the same physical equipment: a bank of electric resistance heating elements, usually 5 to 20 kilowatts, sitting in the air handler downstream of the indoor coil. In a dual-fuel setup, the backup is a gas or propane furnace instead. What differs is who decides when that backup runs, and whether the heat pump keeps running at the same time.

There is one more practical difference worth knowing. In many systems, selecting emergency heat also triggers an alert light or a service reminder, precisely because the manufacturer expects it to be a fault condition rather than a comfort preference. If your thermostat has been sitting on EM HEAT since last February and nobody noticed, that light is trying to tell you something.

Auxiliary heat is your system helping itself. Emergency heat is you telling it to stop trying.

— the distinction that saves people the most money

Why backup heat costs so much

The gap comes down to one number: coefficient of performance, or COP. It is simply how many units of heat you get out per unit of electricity you put in.

Electric resistance heat has a COP of about 1.0. Every watt of electricity becomes one watt of heat. That sounds like perfect efficiency, and in a narrow sense it is — nothing is wasted. But it is also the ceiling. You can never get more heat out than the electricity you paid for.

A heat pump does not create heat. It moves heat that already exists in the outdoor air into your house, and moving is far cheaper than making. In typical U.S. winter conditions a modern heat pump delivers a COP somewhere between 2 and 4 — two to four units of heat per unit of electricity. That is the whole reason heat pumps exist, and our guide to SEER2, HSPF2 and COP goes deeper into how those numbers are measured.

The catch is that COP falls as it gets colder. There is less heat in 10°F air than in 45°F air, so the compressor works harder for less return. A unit that hits COP 3.5 at 47°F might be at 2.2 around 17°F and closer to 1.7 at 5°F. Even at 1.7, though, it is still nearly twice as good as the strips. That single fact is why running the heat pump plus a bit of aux almost always beats running the strips alone.

What it looks like on a bill

Numbers make this concrete. Below is an illustration, not a measurement. It assumes a house that needs 30,000 BTU per hour of heat on a cold night, and electricity at 18 cents per kilowatt-hour, which is roughly the U.S. residential average in 2026. Your own rate may be 13 cents or 50 cents depending on where you live, so scale accordingly.

Read the first and last rows together. Same house, same comfort, same 30,000 BTU/h delivered. Three times the cost. Leave a system on emergency heat for a two-week cold spell and you have added several hundred dollars to a bill for no comfort benefit at all — because the house is exactly the same temperature either way.

That blended middle row is worth dwelling on too. Even when aux heat is carrying half the load, the total is still meaningfully cheaper than pure emergency heat, because the heat pump is doing the other half at better than 1:1. The compressor earns its keep right down to very low temperatures.

For a fuller picture of what heat pumps cost to buy and run against other options, see heat pump costs in 2026 and heat pump versus furnace.

What the aux / EM HEAT light means

Thermostats label these two states differently, which adds to the confusion. Here is how to read yours.

“AUX HEAT” or “AUX” appearing during a heating cycle means the strips are running right now, alongside the heat pump. It comes and goes. Seeing it during a 15°F morning is normal. Seeing it flash on for a couple of minutes during a defrost cycle is normal — the system is warming the supply air so you don’t feel a cold draft while the outdoor coil melts its frost. Seeing it lit for hours at 40°F outside is not normal.

“EM HEAT” displayed as the system mode means someone selected emergency heat and the compressor is locked out. It does not come and go. It stays until a person changes it. On many thermostats it is also accompanied by a red indicator or an alert, because the manufacturer treats it as an abnormal state.

Some thermostats blur the line. A few older models label anything second-stage as “AUX,” and a handful of smart thermostats show a small flame or “heating +” icon instead of the word. If you are not sure what your display is telling you, the model’s manual is the place to check — and it is worth knowing before a cold night, not during one.

Smart thermostats make this far easier. Most log runtime by stage, so you can see exactly how many hours of aux heat you used last month and at what outdoor temperatures. That report is the single most useful diagnostic tool a heat pump owner has, and it is free.

When emergency heat is the right call

There is a real use for the setting. It exists so you can keep your house warm when the heat pump genuinely cannot run — and running the compressor when it shouldn’t can turn a repair into a replacement.

Legitimate reasons to switch to emergency heat:

• The outdoor unit is physically damaged. A tree limb came down on it, a vehicle hit it, the fan blade is bent, or the cabinet is crushed. Running it risks destroying the compressor.
• The outdoor coil is encased in solid ice and defrost is not clearing it. Light frost that clears in a normal defrost cycle is fine. A unit that looks like a block of ice, with the fan frozen in place, needs the compressor off and a technician out.
• The compressor has failed or is making grinding, screeching or hard-start noises. Our noise guide covers which sounds are normal and which mean stop.
• A technician told you to. Some faults — a leaking reversing valve, a failed defrost board, a suspected refrigerant leak — are safest handled with the compressor locked out until the part arrives.
• A refrigerant leak is suspected. Running a system low on charge can burn out a compressor. Systems built since 2025 use A2L refrigerants (R-454B or R-32) which are mildly flammable; leak handling belongs to an EPA-certified professional.

In every one of these cases, emergency heat is a bridge to a service call, not a destination. Set it, call a licensed technician, and switch back to HEAT once the system is repaired. Expect the bill to be high for however many days it takes.

When people reach for it wrongly

Now the other list. These are the reasons people actually give for switching to emergency heat, and none of them hold up.

“It’s really cold out.” This is the most common one and the most expensive. Cold weather is when your heating demand is highest, which means it is precisely when the efficiency of your heat source matters most. Switching off a heat pump running at COP 2 and replacing it with strips at COP 1 during the coldest week of the year is the worst possible timing. The system was designed to handle cold weather by blending in aux heat automatically. Let it.

“The house isn’t warming up fast enough.” Emergency heat will not warm your house faster in any meaningful way, because the strips were already available to the thermostat as auxiliary heat. All you have done is remove the compressor’s contribution. In many systems the total output actually drops when you switch to EM HEAT, because you lose the heat pump’s share.

“The outdoor unit is steaming and making noise.” That is a defrost cycle. The system briefly runs in reverse to melt frost off the outdoor coil, which produces steam, a whooshing sound and sometimes a brief pause in warm air. It is completely normal, it lasts a few minutes, and it happens several times on a damp, cold day. Our explainer on how heat pumps work covers the defrost cycle in detail.

“There’s frost on the outdoor coil.” Light frost in cold, damp weather is expected. The system clears it on its own schedule. Only ice that covers the whole unit and never clears is a problem.

“I read it’s a good idea overnight.” It is not. Whatever you read was probably about a furnace, or was simply wrong.

Balance point, and how it gets set

Balance point is the outdoor temperature at which your heat pump’s output exactly matches your home’s heat loss. Above it, the heat pump alone can hold your setpoint. Below it, something has to help.

For a typical air-source heat pump in a typical American house, the balance point lands somewhere around 30 to 40°F. For a well-insulated home with a properly sized cold-climate unit, it can be down in the teens or lower. For a leaky older house with an undersized system, it might be 45°F, which means aux heat runs a lot more than the owner expects.

Two things determine it: how much heat your house loses at a given outdoor temperature, and how much heat your unit can produce at that same temperature. That is why correct sizing is the foundation of the whole conversation. A load calculation done properly gives your installer both curves, and the point where they cross is your balance point.

Where it shows up in settings is the auxiliary heat lockout — an outdoor temperature above which the thermostat is simply not allowed to use the strips. If your balance point is 32°F, there is no scenario above roughly 40°F where resistance heat helps you, so locking it out saves money with zero comfort cost. Many installations ship with no lockout configured at all, meaning the strips are theoretically available at 55°F.

One more nuance: some thermostats and utilities distinguish a thermal balance point (where capacity meets load) from an economic balance point (where the heat pump stops being cheaper than the backup fuel). For an all-electric system those are effectively the same, since the strips are always more expensive. For dual-fuel, they can be quite different — which brings us to the next section.

Dual-fuel systems and switchover

A dual-fuel or hybrid system pairs a heat pump with a gas or propane furnace instead of resistance strips. The thermostat picks whichever makes sense based on outdoor temperature. Above the switchover (or changeover, or crossover) temperature, the heat pump runs. Below it, the heat pump locks out and the furnace takes over.

Here the “emergency heat” setting means something slightly different: it runs the furnace alone, with the compressor locked out. And because a modern gas furnace is a far more capable backup than a strip heater, the cost penalty is smaller — but it is still not free, and it still means you are not using your cheapest heat source.

Where should switchover be set? Honestly, it depends on your gas price, your electricity price and your equipment, and defaults are frequently wrong. Surveys of thermostat products have found factory defaults anywhere from 35°F to 50°F, and contractor defaults often land around 35°F. Those numbers were reasonable for a single-stage heat pump in an era of cheap gas. With a variable-speed cold-climate heat pump and today’s rates, the crossover often belongs much lower.

The honest answer is that the ideal switchover is a math problem involving your two fuel prices and your unit’s capacity curve, and it changes when rates change. If your utility raised electric rates 10% this year, last year’s setting is no longer optimal. It is worth revisiting every couple of years rather than setting once and forgetting.

Is your aux heat running too much?

Occasional aux heat is normal. Constant aux heat is a symptom. Here is how to tell the difference and what usually causes it.

Signs your aux is running more than it should:

• The AUX indicator is lit at outdoor temperatures well above 40°F
• It comes on every single morning during your recovery period
• Your smart thermostat’s runtime report shows aux hours approaching or exceeding compressor hours
• Your winter electric bill is dramatically higher than neighbors with similar homes and similar equipment
• The system runs constantly and still never quite reaches setpoint

The usual causes, roughly in order of how often we see them:

1. Too aggressive a thermostat setback

   The most common and the easiest to fix. A deep overnight setback means an eight-degree gap at 6 a.m., which is a direct invitation to the strips. Shrink the setback to 2–4°F or flatten the schedule entirely. Details in our thermostat settings guide.

2. No aux lockout configured

   If nothing tells the thermostat “don’t use strips above 40°F,” it will use them whenever it sees a gap, even on a mild day. Ask your installer to set one based on your balance point.

3. Restricted airflow

   A clogged filter, a dirty indoor coil, closed registers or crushed ductwork all reduce heat delivery. The room stays cold, the thermostat sees a gap, and it calls for aux. Start with the maintenance checklist — a $20 filter fixes this surprisingly often.

4. Low refrigerant charge

   A system losing refrigerant loses capacity, so it falls behind sooner and calls for backup more. This needs a technician to diagnose by superheat and subcooling, and a leak has to be found and repaired, not just topped up.

5. A failed sensor or control board

   A bad outdoor temperature sensor can make the thermostat think it’s 10°F when it’s 40°F. A failed defrost sensor can leave the coil frosted, killing capacity. Both are technician jobs and both produce exactly this symptom.

6. The system is undersized for the house

   If the unit was sized for a smaller heat load than the house actually has — or the house got a poorly insulated addition — the heat pump simply cannot keep up and aux fills the gap all winter. This is the hardest and most expensive cause to fix. Our cold-climate heat pump roundup covers units that hold capacity where standard models fade.

Work down that list in order. The first three are things you or your installer can address in an afternoon. The last three need a technician with gauges. What you should not do is accept a high aux runtime as “just how heat pumps are,” because it usually isn’t.

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