How is aux heat different from emergency heat?
Aux heat is backup heat the thermostat adds automatically, alongside the heat pump, when the heat pump can’t keep up or is defrosting. Emergency heat is a setting you choose by hand that shuts the heat pump off and runs only the backup.
Both use the same backup equipment. The difference is who turns it on and whether the heat pump keeps working.
| Aux heat | Emergency heat | |
|---|---|---|
| Turned on by | The thermostat, automatically | You, by hand |
| Heat pump | Keeps running | Off |
| Runs | In short spells, then shuts off | Until you switch back |
| Running cost | Higher than the heat pump alone | Highest, the whole time |
What is the backup heat in a heat pump?
In most Upstate heat pump systems it is electric heat strips inside the indoor air handler. In a dual-fuel system it is a gas or propane furnace.
Heat strips work like a toaster: each kilowatt-hour becomes one kilowatt-hour of heat. A heat pump moves heat from outdoors instead of making it, and in mild weather it delivers two to three times as much heat for the same electricity. That’s why backup heat costs more to run. Lake homes around Keowee Key and The Cliffs at Keowee are often all-electric heat pumps with strip backup. Where Fort Hill Natural Gas reaches, some homes have dual fuel, and the thermostat hands the job to the furnace below a set outdoor temperature.
When does aux heat come on?
Automatically, in three situations: during a defrost cycle, when the heat pump can’t hold the setpoint on a cold morning, and when you raise the thermostat several degrees at once.
In defrost, the backup keeps cold air out of the vents while the heat pump melts frost off the outdoor coil. Most thermostats show “Aux” on the screen while it runs. Short spells like these are normal.
When should you use emergency heat?
Only when the heat pump itself can’t run: the outdoor unit has stopped, has been damaged by a fallen limb or flooding, or is iced solid and not defrosting. Switch to emergency heat to keep the house warm, then book a repair.
A technician may also tell you to use it until a part arrives. Once the outdoor unit is fixed, switch back to Heat.
Why not leave it on emergency heat?
Because emergency heat skips the heat pump, usually the cheapest heat in the house, and runs only the backup the whole time. On electric strips that shows up quickly on the power bill.
Some people switch to it because heat pump air feels cool. That air is warm, just not as hot as furnace air. If the heat pump truly can’t keep up on a mild day, it needs a repair, not emergency heat. See why a heat pump blows cold air.
Why does my aux heat keep coming on?
Short bursts during defrost and on the coldest mornings are normal. Aux heat that runs for long stretches on mild days has a cause, and most of them can be fixed.
- Big setpoint jumps. Raising the thermostat several degrees, including recovery from a deep overnight setback, calls for aux heat.
- Cold snaps. On the coldest nights of the year, some aux heat is expected.
- Low refrigerant. A leak cuts the heat pump’s output, so the strips carry more. A leak search runs $200–$500 and a recharge $200–$650.
- Failing defrost. If the defrost board or sensor fails, the outdoor coil ices over and the heat pump stops making heat. A defrost board runs $250–$650.
- Wrong thermostat setup. A thermostat set up as a conventional furnace, wired so the strips come on with the heat pump, or with aux settings too eager will run the strips far too often. A new thermostat runs $150–$350 installed, but often it only needs its settings corrected.
- A system too small for the house. If the heat pump or the strips are undersized, aux runs for long periods on cold mornings and the house still falls behind.
- A dirty filter or outdoor coil, which cuts output like low refrigerant does.
How often does backup heat run in the Upstate?
Not often. Seneca’s January low averages about 32°F, and most winter nights here stay above the point where a modern, correctly sized heat pump needs much help.
The exceptions are frosty, humid mornings, when the heat pump defrosts more often, and the occasional cold snap. That is why heat pumps do most of the heating in Oconee County, and why aux heat should be the exception, not the rule.
How do you set the thermostat to limit aux heat?
Keep temperature changes small and let the heat pump do the work. A deep setback can cost more with a heat pump than none at all, because the morning recovery runs on strips.
- Set back a couple of degrees at most, or not at all.
- Raise the temperature in small steps, not all at once.
- Use a heat pump thermostat with smart recovery, which starts the heat pump early instead of calling the strips.
- Ask the technician to set an aux heat lockout above a chosen outdoor temperature, if your thermostat has an outdoor sensor or a weather connection.
- Leave the fan on Auto, and change the filter every 1 to 3 months.
A smart thermostat runs $250–$550 installed, and Duke Energy has offered a rebate of about $125 on one; confirm the current amount on duke-energy.com.
When should you call a technician?
Call if aux heat runs most of the day in mild weather, the heat pump won’t run unless you switch to emergency heat, the outdoor unit is iced solid, or your winter power bill jumps for no clear reason.
You can safely check the thermostat settings, the filter, leaves or snow around the outdoor unit, and the outdoor unit’s breaker, reset once. Leave the heat strips, wiring and refrigerant to a licensed technician. A diagnostic visit is $75–$150, and a typical not-heating repair runs $150–$650. Read about heat pump repair in Seneca or get a ballpark price and a licensed local HVAC contractor will call you back.