A heat strip is an electric resistance heater fitted to a compatible indoor air handler or other HVAC unit. It heats air as the blower moves it through the system. In a heat-pump installation, it can supply additional heat when needed or provide backup heat. It does not help an air conditioner cool, and not every air conditioner has one.
The useful question is not simply whether your system has “heat.” It is which equipment produces that heat: the heat pump, electric elements, or a separate furnace. That distinction explains both the thermostat display and the running cost.
Where heat strips fit in the system
Electricity passing through a resistance element produces heat. The indoor blower distributes that heat through the ducts. The heat kit must be approved for the air-handler model; it is not a universal accessory chosen just by brand or house size.
A conventional cooling-only air conditioner does not reverse its refrigeration cycle to heat a home. An approved electric heater may provide heating independently in a compatible system. A heat pump, by contrast, moves heat indoors using its refrigeration cycle and may have electric backup. Carrier describes electric heat strips as an optional air-handler feature, not a component present in every unit.
A gas furnace and a hot-water heating coil are different heat sources, not types of electric heat strip. Likewise, do not assume a ductless mini-split contains strip heat just because it has a HEAT setting. Check the installed equipment documentation.
AUX heat and emergency heat are not the same setting
| Thermostat indication | What it generally means | What to do |
|---|---|---|
| HEAT | The system is set to provide heating. | Use the normal heating mode and a comfortable setpoint. |
| AUX HEAT | The controls have requested supplementary heat. | Consider weather, recent thermostat changes, and whether the home reaches its setpoint. |
| EM HEAT / emergency heat | A manually selected backup mode, normally excluding the outdoor heat pump. | Use only as your system instructions direct for a heat-pump problem; arrange service. |
Auxiliary heat can come on during cold weather, recovery from a substantial thermostat increase, or an outdoor-coil defrost cycle. It is not automatically evidence of a breakdown. Emergency heat is not a “faster heat” button for everyday use. See Trane’s explanation of AUX and emergency heat and our guide to when to use emergency heat.
On a dual-fuel system, the backup may be a furnace rather than electric strips. The display alone does not identify the installed heat source.
At what outdoor temperature should heat strips turn on?
There is no single temperature that applies to every installation. Heat-pump capacity, the home’s heat loss, equipment limits, and the control setup all matter. The thermal balance point is where the heat pump’s output matches the building’s heating demand; below it, supplementary heating may be needed.
Natural Resources Canada distinguishes this capacity limit from an economic changeover point and the equipment’s minimum operating temperature in its heat-pump guide. Do not copy a neighbor’s auxiliary-heat lockout setting or disable backup heat merely to suppress the AUX indicator. Have an installer confirm the settings for your equipment and climate.
How much electricity do heat strips use?
Use the installed heater’s rating and actual run time, not the cooling system’s tonnage, to estimate consumption:
Energy (kWh) = active heater capacity (kW) × operating time (hours).
Cost = energy (kWh) × your electricity price per kWh.
For a clearly labeled example, a 10 kW heater running at full output for 30 minutes uses 5 kWh. At an assumed rate of $0.20 per kWh, that is $1.00 for the heater alone. Two total hours at that output would use 20 kWh and cost $4.00. These are arithmetic examples, not typical household bills; they exclude the blower, any simultaneous heat-pump use, and other charges.
A staged heater may operate only part of its total capacity. The nameplate rating therefore does not mean it draws full power all day. Ask the installer which stages are enabled if the thermostat’s energy report does not separate them.
Resistance elements convert electricity into heat rather than moving outdoor heat indoors. A functioning heat pump can deliver heat with less electricity under suitable operating conditions. Comparing electric backup with a gas furnace also requires local fuel prices and equipment performance, not a blanket claim that one is always cheaper.
When AUX heat deserves attention
- After a large temperature increase: return to a steady, comfortable setting and observe normal operation. Large setbacks followed by rapid recovery can call for backup on some controls; use heat-pump-compatible scheduling.
- During cold weather or brief defrost: supplementary heating may be expected. Record the conditions before assuming a fault.
- Persistent AUX in mild weather, rising bills, or a house that will not warm: arrange HVAC diagnosis. Note the outdoor temperature, thermostat setting, room temperature, and how long the indication remains.
- Smoke, an electrical burning smell, or repeated breaker trips: stop operation and seek qualified help. Do not repeatedly reset the breaker or open electrical compartments. For smoke or fire, leave the area and contact emergency services.
What you can maintain—and what needs a professional
Keep return and supply grilles clear and maintain the user-accessible filter as the equipment manual directs. Do not close off numerous vents to “concentrate” heat. Photograph accessible equipment labels and keep service records so a technician can identify the installed heater without guesswork.
Heat-kit installation, element testing, wiring, sequencers, limit controls, circuit sizing, and internal cleaning are professional work. The thermostat being OFF is not electrical isolation, and an installation may have more than one power supply. Never bypass an overheat limit or substitute a larger breaker.
Before approving a replacement quote, request the exact air-handler model, approved heat-kit part number and kW rating, required electrical work, labor, and warranty terms. A product listing or low purchase price does not establish compatibility. There is no reliable universal installed price or replacement age for all systems.
Research and scope
This explanation draws on the manufacturer and government references linked above. It is not a report of hands-on testing or an electrical installation guide. Your installed equipment manual and a qualified installer determine which heat kit, settings, and service procedures are appropriate.
