Heat Pump Aux Heat Running Constantly: Normal, Costly, or a Fault?

Heat Pump Aux Heat Running Constantly: Normal, Costly, or a Fault?

By EnergyBS Editorial Team | November 13, 2026

Aux heat that runs for hours is normal only when it is colder outside than your heat pump's balance point, which for many houses sits somewhere below freezing. Above about 40°F (4°C), Carrier says long AUX runs may point to a fault. Every hour of strip heat costs its kilowatt rating times your electricity price: a 10 kW strip at the August 2026 US residential average of 18.16 cents per kWh (EIA) costs about $1.82 an hour, three times what the heat pump would spend on the same heat at a coefficient of performance (COP) of 3.

That price gap is why a thermostat stuck on AUX shows up on the bill within a single billing cycle, and why the outdoor temperature at the moment you see the light is the first thing to write down. The same AUX indicator means three different things at 45°F, 25°F and 0°F, and the table below sorts them.

What the AUX light is actually reporting

On a heat pump thermostat, AUX (or AUX HEAT, or a second-stage heat icon) means the thermostat is energizing its auxiliary heat output. In most ducted systems that output switches on electric resistance coils, called heat strips or a heater kit, mounted in the indoor air handler. In a dual fuel system it lights a gas or propane furnace instead, and the cost question changes completely; our dual fuel heat pump cost guide covers that case.

Two features of the light trip people up.

First, AUX normally runs alongside the heat pump, not instead of it. Carrier describes auxiliary heat as automatic: it "runs alongside your heat pump to boost heating power." If the outdoor unit is humming and the fan on top is turning while AUX is lit, the compressor is still doing most of the work and the strips are filling a gap.

Second, AUX is not emergency heat. Emergency heat (EM HEAT, EHEAT) is a mode you select by hand. Carrier's page says it "shuts off the heat pump entirely and uses only the backup heating source," and that you should switch to it only if the heat pump has broken down. A thermostat left on EM after a scare will run the strips all day by design. If your display says EM rather than AUX, switch the mode back to HEAT before reading further.

Symptom plus outdoor temperature: a decision table

Read the outdoor temperature from a weather app or your thermostat's outdoor sensor, then find the row that matches what you see. The temperatures here are reference points from the sources named in the last column, not a universal rule: Carrier states plainly that "there is no single outdoor temperature at which every Carrier heat pump activates auxiliary heat."

The table carries most of the diagnosis. If your case fits rows one to three, the system is working as designed and the decision is about money and settings. Rows four to nine point to a setting or a part.

Three patterns that are normal, and how long each should last

Defrost: minutes, with steam

When a heat pump heats in cold, damp air, frost builds on the outdoor coil. Carrier notes that when the outdoor unit develops frost or ice during heating, "the system can enter a defrost cycle," which reverses the refrigerant flow for a short time to melt it. During those minutes the indoor coil briefly gets colder, so many systems turn the strips on to keep the vents from blowing cool air. Carrier says auxiliary heat "assists during the defrost cycle to prevent cold air from blowing out of your vents while the outdoor unit de-ices itself."

The cost of this is small because the time is short. If a defrost lasts, say, 8 minutes, a 10 kW strip running through it uses about 1.3 kWh, or roughly 24 cents at the US average price. Even several defrost cycles a day add only a dollar or two. If AUX comes on with every defrost and goes off again within minutes, there is nothing to fix. Our defrost cycle explainer covers what the steam and the noises mean. What is not normal is a coil that stays white with ice between cycles, which leaves the strips carrying the house; that case belongs in the fault section below.

Recovery after a setback: the morning spike

The second normal pattern is the most expensive one people create themselves. A thermostat that drops to 62°F overnight and jumps back to 70°F at 6:30 a.m. sees an 8-degree error. Carrier lists this as a designed trigger: auxiliary heat "helps raise the indoor temperature quickly if you increase your thermostat setting by more than 2-3 degrees at once."

The strips run until the house reaches the setpoint. On a cold morning, with the heat pump already near its limit, that can mean half an hour or more of strip heat. The savings from the setback come from lower heat loss overnight, and the recovery hands part of them straight back at resistance prices. Whether a setback wins depends on how much heat the house saves overnight against how much of the recovery runs on strips. In a well-insulated house with a small setback, recovery can be done by the compressor alone; with a large setback on a cold morning, it usually cannot.

Three ways to keep the saving without the strip spike:

• Use a setback of 2 to 3 degrees at most, which keeps the morning jump inside the range Carrier names.
• Use the thermostat's own recovery feature if it has one built for heat pumps, which starts warming earlier so the compressor can do the job slowly.
• Skip the setback on the coldest nights of the year, when the heat pump cannot recover without help in any case.

Our heat pump blowing cold air guide covers the comfort side of this recovery; the point here is the bill.

Below the balance point: hours, by design

The third normal pattern is the one people mistake for a fault. A heat pump's heating capacity falls as the outdoor air gets colder, while the house's heat loss rises. The outdoor temperature where the two lines cross is the thermal balance point. Carrier defines it as "the outdoor temperature at which a heat pump's heating capacity intersects with" the home's demand. Below that point, the heat pump alone cannot keep up, and the strips make up the difference for as long as the cold lasts. In a deep cold snap, that can be all day and all night.

To see how much strip heat is normal, here is an illustration with round numbers we chose to keep the arithmetic visible. It is not a measured house.

• House heat loss: 36,000 Btu/h at 5°F, falling in a straight line to zero at 65°F. That is 600 Btu/h for each degree of difference.
• Heat pump capacity: 36,000 Btu/h at 47°F and 22,000 Btu/h at 17°F, also in a straight line, about 467 Btu/h per degree.

Setting the two lines equal: 600 × (65 − T) = 22,000 + 466.7 × (T − 17). That gives T ≈ 23.4°F (about -4.8°C). This house's balance point is about 23°F.

The kW column divides the gap by 3,412 Btu/h per kW. At 15°F, AUX in this house should be lit a good part of every hour, because the strips are supplying almost a third of the heat. At 5°F they supply more than half. A homeowner watching AUX run "constantly" at 5°F is seeing a correctly working system.

Your own balance point depends on your house and your equipment. The capacity numbers come from the outdoor unit's extended performance data; the heat loss comes from a load calculation. Our Manual J sizing guide explains the load side, and the cold climate performance data page shows how capacity and COP fall with temperature. ENERGY STAR's cold climate label sets a floor at the cold end: a certified cold climate unit must reach a COP of at least 1.75 at 5°F. A heat pump with a balance point near 0°F needs strips only on the coldest days; one with a balance point near 35°F leans on them for weeks.

The settings that decide when the strips may run

Most heat pump thermostats let the installer set two outdoor temperature limits. Both need an outdoor temperature reading to work.

Auxiliary heat lockout (an upper limit). Above this outdoor temperature, the strips are not allowed to run, whatever the indoor error. This is the setting that stops AUX on a mild morning after a setback.

Compressor lockout (a lower limit). Below this outdoor temperature, the compressor is shut off and the strips carry the whole house. Set too high, this one alone makes AUX run "constantly" in ordinary cold weather, with the outdoor unit silent.

The Bonneville Power Administration (BPA), the US federal power marketer in the Pacific Northwest, publishes thermostat support sheets for its PTCS heat pump specification. They give a concrete reference for both limits:

Two details in those sheets matter for anyone diagnosing a constant AUX light.

First, on the Honeywell T6 Pro, ISU 130 must be set to 1, meaning a wired outdoor sensor is installed, or "strip heat lockout will not function." A thermostat with no outdoor reading cannot apply an outdoor lockout at all. If your thermostat shows no outdoor temperature, assume the lockouts are not working.

Second, BPA says that if ISU 356 does not appear as an option, the strip heat lockout is not configured. The menu itself is a test.

Those values are one utility program's specification, written for the Pacific Northwest. They are not a code rule and they are not right for every house. A 35°F aux lockout is sensible when the balance point is below 35°F, as it should be for a heat pump sized to carry most of the winter load. If your house's balance point is higher (an undersized heat pump, or a leaky house), a 35°F lockout will leave the house short of heat between the balance point and 35°F. That is a sizing problem showing itself, and the answer is a load calculation, not a higher lockout.

A compressor lockout of 5°F or lower, or disabled, keeps the heat pump running in cold weather where it is still cheaper than strips. Below the outdoor unit's rated operating range, the manufacturer's limit applies whatever the thermostat says. Carrier, for example, lists heating operation down to -13°F for the Infinity 23 (27VNA3) and -23°F for the Infinity 21 (27VNA1).

Other settings change how quickly AUX comes on once the outdoor limits allow it: an indoor temperature "droop" or delta (how far below setpoint the room must fall), a timer (how long the compressor runs alone before help is added), and whether the strips may run at the same time as the compressor. Names and defaults differ by brand, and we did not find a published default we could cite for each. Our backup heat guide has a worksheet for writing down every installed value; ask the installer to fill it in.

Faults that keep the strips switched on

When the outdoor temperature is above the balance point and AUX still runs for long stretches, the heat pump is not producing the heat it should, or the controls are calling for strips when they should not. Carrier's own list for AUX above 40°F names four causes: a dirty filter, a refrigerant leak, a faulty sensor, or a thermostat raised too fast. The full list in practice, roughly from cheapest to most expensive to check:

A clogged filter or blocked return. Low airflow across the indoor coil cuts the heat pump's output, so the room falls behind and the thermostat adds strips. This is the one cause you can fix in five minutes. Replace the filter and give the system a day.

A setting, not a part. A compressor lockout set at 30°F instead of 5°F, an outdoor sensor that was never enabled, or a schedule with a large setback all produce constant AUX on a healthy machine. The BPA table above gives the menus to check.

An outdoor unit that is not running. If AUX is lit and the outdoor fan is still, check the outdoor disconnect and the breaker at the panel. A tripped breaker that trips again when reset is an electrical fault for a technician, not something to keep resetting. Our heat pump electrical requirements guide explains how those circuits are sized.

Low refrigerant charge. A system that has leaked refrigerant still runs, but delivers less heat per hour. The compressor runs long, the room lags, and AUX fills in, even on mild days. A refrigerant leak is never "topped up" as routine; the leak needs finding. Low charge often shows in the cooling season too, as long runtimes and weak cooling.

A faulty outdoor temperature sensor. A sensor that reads 10°F on a 40°F day can lock the compressor out or let the strips run outside their lockout. Compare the thermostat's outdoor reading with a local weather report.

A defrost problem. A failed defrost control or sensor can leave the coil iced. An iced coil moves little heat, and the strips carry the load. The clue is a coil that stays white for hours rather than clearing after each defrost.

Wiring that calls strips with every heating call. If the auxiliary terminal at the air handler is tied to the same call as the compressor, the strips run every time the heat pump does. This is a wiring check for a technician, but you can spot the symptom: AUX, or warm strip air, appears at the very start of every heating cycle, even on a mild afternoon.

A heater relay stuck closed. The strips stay energized even when the thermostat is satisfied or off. The vents blow hot air and the meter keeps spinning with no call for heat. Switch the system off at the thermostat; if hot air continues from the vents, turn off the air handler's breaker and call for service. This one carries a fire risk as well as a cost.

An undersized heat pump or undersized strips. If the outdoor unit runs all the time and the house still cannot hold temperature well below the balance point, either the heat pump or the strip kit is too small for the design load. That is not a repair but a design limit, and it shows on the coldest days of every winter.

For what each of these repairs tends to cost, see our heat pump repair cost guide.

What an hour of aux heat costs

Electric resistance heat turns every kWh into 3,412 Btu of heat, no more and no less. Carrier describes it as "100% efficient but much more expensive to operate than a heat pump, which can be 300% efficient." That makes the cost arithmetic simple.

• Strip heat cost per hour = strip kW × price per kWh.
• Heat pump cost for the same heat = strip kW ÷ COP × price per kWh.

We worked it for three prices, each read on November 13, 2026:

• United States: 18.16 cents per kWh, the US average residential price for August 2026 in EIA's Electricity Monthly Update (released October 23, 2026). It is an average that folds in fixed charges, so your marginal price can be higher or lower, and winter months differ.
• Quebec: 11.142 cents per kWh, Hydro-Québec's Rate D price for energy above the first block of 40 kWh times the days in the billing period, in force from April 1, 2026. An electrically heated home in winter uses well above the first block, so aux kWh land at this price.
• Ontario: 13.2 cents per kWh, the Ontario Energy Board's Regulated Price Plan tiered price above 1,000 kWh a month in winter, from November 1, 2026. That is the commodity price only; delivery and regulatory charges come on top, and the Ontario Electricity Rebate comes off, so check your own bill's per-kWh total.

The COP columns use 3.0 (a mild winter day), 2.0 (hard cold) and 1.75, the ENERGY STAR cold climate minimum at 5°F. The heat pump numbers show what the same heat would cost if the heat pump could supply it.

Three things stand out.

One hour of 15 kW strip heat at the US average costs $2.72, the same as three hours of that heat output from the heat pump at COP 3. That is the whole case for the lockout settings above.

Even at the cold climate floor of COP 1.75, the heat pump does the job for 57% of the strip cost. Every hour the compressor can carry instead of the strips saves money, all the way down to the unit's rated limit. That is why BPA's spec lets the compressor run to 5°F or below.

Quebec's low price makes the dollar gap smaller but not the ratio. A 10 kW strip still costs three times the heat pump at COP 3, about C$1.11 against C$0.37 an hour.

Two details that change the hourly figure

Voltage. A resistance heater rated at 240 volts puts out less on a 208-volt supply, common in apartments and some commercial buildings. Output scales with the square of the voltage: (208 ÷ 240)² = 0.751. A 10 kW kit at 208 volts gives about 7.5 kW and costs 75% as much per hour, but also delivers 75% as much heat, so it runs longer to do the same work. The cost per unit of heat does not change. On a 240-volt circuit, a 10 kW strip draws 10,000 ÷ 240 = 41.7 amps and a 15 kW strip 62.5 amps, which is why they sit on their own breakers.

Peak pricing. Some rates price winter peaks far above the average. On Hydro-Québec's Rate Flex D, energy during winter peak events costs 46.463 cents per kWh, from April 1, 2026. A 10 kW strip running through a peak event costs about C$4.65 an hour, more than four times its cost on Rate D. Morning recovery from a setback tends to land in the same hours as winter peaks, so on Flex D the setback spike is especially costly. Ontario readers on time-of-use pricing pay the on-peak rate for the same reason; our Ontario electricity rates explainer lays out the periods.

A day of aux: a normal cold snap against a stuck strip

The hourly table turns into daily numbers with one more step. Two illustrations, using the house from the balance point section:

A normal 15°F day. The strips fill a gap of 2.6 kW on average. Over 24 hours that is 2.618 × 24 = 62.8 kWh of strip heat. At the US average price that is $11.41 for the day; at Hydro-Québec's Rate D upper price it is C$7.00. This is the cost of the house being colder than the heat pump can carry, and it is normal. At 5°F the same house uses 137.9 kWh of strip heat, or $25.04 a day at the US average.

A stuck strip on a 40°F day. Suppose a 10 kW strip runs for a total of 12 hours a day, doing work the heat pump could do at a COP of 3. The strips use 120 kWh; the heat pump would have used 40 kWh for the same heat. At 18.16 cents that is $21.79 against $7.26, an extra $14.53 a day. Over a 30-day billing period, the fault adds about $436.

The second case is the one worth catching early. A normal cold snap is expensive but ends with the weather. A fault on a mild day runs every day until someone finds it, and it costs most in the shoulder months, when the heat pump should be doing almost all the work for very little. Our guide to heat pump electricity use shows what a full season should look like for several climates, so you can tell whether a bill is out of line.

Proving it with your meter and runtime history

You do not need tools to tell a strip problem from a cold snap. You need two numbers from the same hour: the outdoor temperature and the house's electricity use.

Hourly meter data. Many utilities with smart meters show hourly or 15-minute use online. Strip heat is easy to spot because it comes in large blocks. A 10 kW strip that runs for a full hour adds 10 kWh to that hour. The heat pump's own draw is much smaller and changes gradually with temperature; the maximum input is on the outdoor unit's nameplate or spec sheet. Suppose a house uses 3 or 4 kWh an hour at 40°F on most days and suddenly shows 12 kWh hours at the same temperature, those hours almost certainly include strip heat.

Thermostat runtime history. Many Wi-Fi thermostats keep a log of how long each stage ran each day: compressor, aux, fan. Compare aux runtime with the outdoor temperature for the same days. Aux hours should be near zero on mild days, should rise as the weather cools past the balance point, and should fall again when it warms. Aux hours that stay high through a mild spell point to rows four to nine of the decision table.

A one-day lockout test. If you suspect the settings, ask your installer to confirm the outdoor lockout values against the BPA references above, or the equivalent for your thermostat. With a lockout in place, a day above that temperature should show no strip hours at all. If the house then cannot hold its setpoint on a mild day, the heat pump is underperforming and the strips were hiding it. That is useful information for the technician, not a reason to remove the lockout.

A quick vent check. During a heating cycle with AUX off, the supply air from a working heat pump should feel warm but not hot. With strips on, it is noticeably hotter. If you feel hot air with AUX not shown and the thermostat satisfied, that points to the stuck-relay row, and the system should be shut off until it is checked.

Before you book a service call

A technician's time is billed by the hour, so a written record saves money. Bring these to the call:

• The outdoor temperature each time you saw AUX running for a long stretch, and how long it ran.
• Whether the outdoor unit and its fan were running at the same time.
• Any pattern by time of day: mornings only (recovery), all night in cold snaps (balance point), or all day in mild weather (fault).
• Photos of the thermostat's installer settings screens showing the compressor and aux lockouts, the outdoor sensor setting and any droop or timer values.
• A screenshot of the hourly meter data for one problem day and one normal day at a similar temperature.

Ask the technician to check refrigerant charge only through the manufacturer's method and to find the leak if the charge is low, to check the defrost control if the coil was iced, and to confirm that the auxiliary wiring only energizes the strips on a second-stage call. Ask for the balance point, or the outdoor temperature at which the installer expects strips to start, written on the invoice. That number tells you in future whether AUX on a given morning is working as designed.

For a dual fuel system the same diagnosis holds, but the backup is a furnace and the cost per hour depends on gas or propane prices; the heat pump hub links our other guides on equipment, sizing and running costs.

Frequently Asked Questions

What does AUX heat on my thermostat mean?

It means the thermostat is calling for the backup heat source, usually electric resistance strips in the indoor air handler, sometimes a furnace in a dual fuel system. It normally runs together with the heat pump to add heat during defrost, after a large setpoint increase, or when it is colder than the heat pump can handle alone.

Is it normal for aux heat to run all the time?

Only in weather colder than your heat pump's balance point, when the strips are filling a real gap. In our illustration house, with a balance point near 23°F, strips supplied about 30% of the heat at 15°F and over half at 5°F. AUX running for long stretches on days above about 40°F is not normal.

How much does aux heat cost per hour?

Multiply the strip kW by your electricity price. At the August 2026 US average of 18.16 cents per kWh, 5, 10 and 15 kW strips cost about $0.91, $1.82 and $2.72 an hour. At Hydro-Québec's Rate D upper price of 11.142 cents, the same strips cost about C$0.56, C$1.11 and C$1.67 an hour.

Should I turn off aux heat to save money?

Do not disable it without a plan. Below the balance point the house needs that heat, and a system with the strips switched off may not hold a safe indoor temperature. The better step is to have outdoor lockouts set so the strips can only run in cold weather, and to avoid large setbacks that trigger them in mild weather.

What is the difference between aux heat and emergency heat?

AUX switches on automatically, usually alongside the heat pump. Emergency heat is a mode you choose by hand that shuts the heat pump off and runs only the backup. Use emergency heat only when the heat pump itself has failed, and switch back once it is repaired.

Can a low refrigerant charge make aux heat run more?

Yes. A system low on refrigerant delivers less heat per hour, so the thermostat adds strip heat to keep up, even in mild weather. Carrier lists a refrigerant leak among the causes of AUX running above 40°F. The leak needs to be found and repaired, not just topped up.