If you have a heat pump, your thermostat probably has two words that confuse a lot of people: Aux (auxiliary heat) and Emergency Heat (sometimes "Em Heat" or "E-Heat"). Both usually use the same thing, electric heat strips inside your indoor air handler. The difference is when and why they turn on.
Understanding this matters because electric heat strips use a lot more electricity than the heat pump itself. A heat pump moves heat from outside into your home, so it often delivers two to three times as much heat for the same electricity. Heat strips make heat the way a toaster does. Run them all winter and your power bill can climb fast.
Safety first
- Smell gas (rotten eggs)? Don't touch switches or the thermostat. Get everyone out, then call your gas company or 911 from outside.
- Carbon monoxide alarm going off? Get everyone outside into fresh air and call 911. Don't go back in to check the furnace.
- Turn the power off at the furnace or air-handler switch before you open any panel.
- Never bypass a safety switch (door switch, float switch, pressure switch, limit switch). They shut the system off for a reason.
What to check
Aux heat: the automatic helper
Aux comes on by itself when the heat pump needs help: on a very cold morning, when you bump the thermostat up several degrees at once, or during a defrost cycle. Seeing Aux for a little while is normal.
Short bursts of Aux on cold mornings are normal.
Emergency heat: the manual backup
Emergency heat is a setting you choose. It turns the heat pump off completely and heats the house with the strips only. It's meant for when the heat pump has failed and you're waiting for a repair, or the outdoor unit is frozen or damaged.
Check which one is on right now
Look at the thermostat mode. If it says Emergency Heat or Em Heat as the mode, someone switched it on, and it won't switch back by itself. Change it back to regular Heat. If it says Heat and you see Aux as a status message, the system is just asking for help, which is usually fine.
Watch whether Aux runs all the time
If Aux shows up for most of the day, even in mild weather, the heat pump probably isn't doing its job. The outdoor unit may not be running, may be iced up, or may be low on refrigerant, so the strips are carrying the whole load. Check the outdoor unit: is the fan spinning and the unit running?
Don't make big jumps on the thermostat
Raising the setpoint several degrees at once, like from 62 to 70 in the morning, tells many thermostats to bring on Aux heat to get there faster. Small, steady changes (or a smart thermostat with heat pump settings) keep the strips off more often.
Watch your power bill
A sudden jump in your electric bill during a normal-weather month is often the first sign that a heat pump has been running on strips. If your bill jumps and Aux shows up a lot, have the heat pump checked.
How to keep your heat pump off the heat strips
The cheapest heat a heat pump system makes is from the heat pump itself. Every hour the strips carry the load is an hour you're paying for resistance heat. A few things keep that to a minimum.
Keep the outdoor unit healthy. A heat pump that's low on refrigerant, has a dirty outdoor coil, or has a weak outdoor fan can't keep up, so the thermostat calls for Aux more and more. Most people don't notice until the power bill arrives.
Use the right thermostat settings. Many smart thermostats have heat pump settings that limit when Aux can run, and some let you set an outdoor temperature below which Aux is allowed. If your thermostat was installed as a gas furnace thermostat, it may be bringing on the strips too early. That's worth having checked.
Be careful with big setbacks. Turning the heat way down at night and way up in the morning saves money on a gas furnace, but on a heat pump it can trigger the strips every morning. Smaller setbacks of a couple of degrees, or letting the thermostat recover slowly, usually work better.
A fall heat pump check covers the refrigerant charge, the outdoor coil and fan, the defrost controls and the heat strips, so you go into winter knowing the system will heat efficiently.
When to call a pro
- Aux runs most of the day in mild weather.
- The outdoor unit isn't running while the thermostat is calling for heat.
- You've had to use Emergency Heat because the heat pump stopped working.
- Your power bill jumped and you can't explain why.
- You smell a burning odor when the heat strips come on (a little dust smell the first time each fall is normal; a strong or ongoing smell is not).
Still not working?
Frost Fire Heating & Cooling: $59 diagnostic (waived with repair), same-day service, no weekend surcharge, 4.8-star rated. Licensed & Insured. Serving Raleigh, Durham, the Triangle, Clayton and surrounding areas.
Common questions
Is it bad to run emergency heat?
It won't hurt the system, but it uses much more electricity than the heat pump. Use it only while you're waiting for a heat pump repair.
Why does my thermostat say Aux heat?
The heat pump is asking for backup heat, usually because it's very cold outside, you raised the temperature quickly, or it's defrosting. Occasional Aux is normal; Aux all day is a sign something's wrong.
Will emergency heat turn off on its own?
No. Emergency Heat is a mode you set, and it stays on until you switch the thermostat back to regular Heat.
Can I get my heat pump checked on a weekend?
Yes. Frost Fire offers same-day service when we can, with no weekend or Sunday surcharge.
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