Why Does Your Thermostat Say "Aux Heat"?
Every January, half of heat-pump-owning Arkansas googles this at 6 AM. Short version: aux heat is your system's backup muscle — normal in bursts, expensive as a lifestyle.
Quick Answer
A heat pump moves heat from outside air — and below about 35°F there's less heat to grab, so electric backup coils (auxiliary heat) kick in to close the gap. Seeing AUX during a cold snap, after a big thermostat setback, or during defrost is normal. Seeing it on mild days, constantly, or with a power bill that made you sit down means the heat pump itself isn't pulling its weight.
Normal: cold snaps, brief bursts | Problem: aux running in 45°+ weather or nonstop
What to Know, In Order
1. Aux heat and emergency heat aren't the same button
AUX comes on automatically to assist the heat pump. EM HEAT is the switch YOU flip to run backup only — for when the outdoor unit is broken or iced solid. If you flipped EM HEAT during a cold snap and forgot it, that alone explains a shocking bill: flip it back to normal heat.
2. Don't crank the thermostat — creep it
Jumping the setpoint from 62° to 70° practically orders the aux coils on. Raise it one or two degrees at a time and the heat pump does the work at a third of the cost. If you run schedules, keep setbacks modest in winter — heat pumps hate big morning catch-ups.
3. Cold snap? Let it ride
When Arkansas drops into the teens, aux assist is the system working as designed — every heat pump in the neighborhood is doing the same thing. Judge your system by the mild days, not the polar ones.
4. Aux on mild days = the heat pump is loafing
If backup heat runs when it's 45-50° outside, something's dragging: low refrigerant, a failing compressor or reversing valve, a lazy defrost board, or a thermostat mis-wired to call for aux too early. Each one quietly doubles or triples your heating cost until it's fixed.
5. The bill is the smoke alarm
Electric strip heat costs roughly three times what heat-pump heat costs. A winter Entergy bill that jumps way past last year's — without a brutal cold snap to blame — is usually aux heat running behind your back. Bring us the bill; it's diagnostic gold.
Iced-over outdoor unit?
If aux is on because your outdoor unit is wearing a shell of ice, that's its own problem with its own playbook — see Heat Pump Frozen Over? Use EM HEAT as the bridge and get the ice diagnosed, not chipped.
Ready for a pro to take it from here?
Aux-heat-runs-constantly calls usually end at a refrigerant charge, a defrost control, or a thermostat wiring fix — an hour's work that pays for itself on the next Entergy bill. Our heating repair crews run same-day calls across Central Arkansas — including heating repair in Bryant, heating repair in Benton and heating repair in Little Rock.
Aux Light Living On Your Thermostat?
$149 flat diagnostic, $75 back as a repair credit. Our heating repair techs put gauges on the system and tell you exactly why the backup is carrying the load.
Call 501-307-5959