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Backup Heat Strips

What heat strips are

Inside most ducted air handlers is a slot for an electric heater kit: coils of resistance wire that glow when energized, warming the air passing over them. Kits come in steps such as 5, 8, 10, 15 and 20 kW. Each kilowatt adds about 3,412 BTU/h. Because resistance heat turns one unit of electricity into one unit of heat, it costs two to three times as much per unit of heat as the heat pump in mild weather.

When strips turn on

• Auxiliary heat: the thermostat calls for extra heat when the heat pump alone is not raising the temperature fast enough, often during recovery from a setback or on very cold days.
• Defrost tempering: some systems run strips during defrost so cold air does not blow into the house.
• Emergency heat: a manual mode that runs strips only, used if the outdoor unit has failed.
• Lockout settings: many thermostats let installers prevent strips from running above a set outdoor temperature, such as 35°F.

Sizing strips

Strips are usually sized to cover the gap between the heat pump's output at the design temperature and the home's heat loss, sometimes with extra for emergency use. A larger kit offers more emergency capacity but requires a bigger circuit and may push the home's electrical service toward an upgrade. A licensed electrician must check panel capacity and install wiring to the manufacturer's specs. Your HVAC contractor should base the size on a load calculation; the heat pump size calculator gives a starting range.

Electrical considerations

A 10 kW strip at 240 V draws about 42 amps, so the circuit and breaker must be sized accordingly under code. Many air handlers split larger kits into stages on separate breakers. If the house also has an electric range, dryer, EV charger and water heater, total load can exceed a 100 A service. An electrician does a service load calculation before adding equipment. Never upsize a breaker without an electrician's review.

Reducing strip use

• Choose a heat pump with strong output at your design low; see cold-climate heat pumps.
• Use small thermostat setbacks or none; see thermostat settings.
• Ask your installer to set an outdoor lockout so strips run only when truly needed.
• Seal ducts and add insulation to reduce heat loss.
• Keep filters and coils clean so the heat pump delivers full output.

Strips vs dual fuel

Instead of strips, some homes keep a gas furnace as backup. The thermostat switches to the furnace below a chosen outdoor temperature, often set near the cost break-even point. This avoids high electric backup costs and electrical upgrades but keeps a gas connection and its fixed fees. See heat pump vs furnace and COP explained.

How to tell if strips are running

Many thermostats show "Aux" or "Aux heat" when strips are on and "Em heat" in emergency mode. Smart thermostats and some utility apps report runtime. If strips run often on mild days, something may be wrong: a frozen outdoor coil, a failed outdoor unit or a setting issue. See heat pump not heating.

Worked example

Figures below are illustrative examples (2026-10-29), not quotes or guarantees.

A home loses 30,000 BTU/h on a 0°F night and the heat pump delivers 22,000 BTU/h at that temperature. The gap is 8,000 BTU/h, about 2.3 kW. Over a 10-hour night the strips use about 23 kWh, about $3.45 at $0.15/kWh. If the heat pump produced the same 8,000 BTU/h at a COP of 2, it would use about 11.7 kWh, roughly $1.76.

Safety notes

Strips get very hot. Air handler panels should stay closed and the unit should have proper clearances. If you smell burning dust at first use each season, that is common for a short time; a persistent burning smell, tripping breakers or scorch marks need an electrician or HVAC technician right away. Do not open electrical panels or air handlers yourself.

How strips are staged

Most air handlers with electric backup split the heater into stages, for example two or three banks of 5 kW each. A heat-pump thermostat brings on the first stage when the compressor cannot hold setpoint, and only adds more if the gap keeps growing. Good staging matters because each bank draws a large, steady load. A house that only needs a little supplemental heat on a cold morning should see one stage cycle briefly, not the whole bank running for an hour. Ask the installer how many stages are wired and which thermostat terminals control them.

Sizing the backup

Backup is usually sized for one of two jobs. The first is supplemental heat below the balance point, where the strips only cover the shortfall between what the heat pump delivers and what the house loses. The second is emergency heat, where the strips must carry the house alone if the outdoor unit fails. Emergency sizing needs far more capacity and therefore a larger breaker and wire. Many homeowners choose a middle path: enough backup to keep pipes safe and rooms tolerable during a repair, not full comfort on the coldest night. In cold-climate homes with a heat pump rated for low temperatures, a smaller strip may be enough.

Electrical panel impact

A 10 kW strip draws about 42 amps at 240 volts, and a 15 kW strip about 63 amps. That load sits on top of the outdoor unit, the water heater, the range and a car charger if you have one. Before choosing a larger strip, a licensed electrician should run a load calculation on the panel. In some homes the answer is a smaller strip plus a better-insulated attic, which reduces both the backup needed and the bills.

Signs the strips run too often

High winter bills, a thermostat showing "aux" for long stretches on mild days, and warm air that feels very hot at the registers can point to strips running when the heat pump should be enough. Common causes are large setbacks, a lockout temperature set too high, a low refrigerant charge, or a dirty filter and coil. A technician can read the stage history on many smart thermostats and adjust settings without new equipment.

FAQ

Should I use emergency heat when it is very cold?

Usually not. Leave the system in normal heat; it adds strips automatically when needed.

Can I remove strips to save money?

Removing them may leave you without heat if the heat pump fails. Ask your contractor about lockouts instead.

Do ductless mini-splits have strips?

Most do not; homes often use baseboards or another source as backup.

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