Heating Guide · Central Texas

Aux Heat vs Emergency Heat: What's the Difference?

Aux heat is automatic: your thermostat turns on backup heat alongside the heat pump when the heat pump can't keep up by itself. Emergency heat is a manual setting that shuts the heat pump off and runs only the backup heat. Seeing "AUX" on a cold morning is normal. Emergency heat is for when the outdoor unit is broken, and it's the most expensive way to heat your house.

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AUX
Automatic, Works With the Heat Pump
EM
Manual, Heat Pump Shut Off
~35°F
Where Heat Pumps Start Needing Help
2-3x
Strip Heat Cost vs Heat Pump (Mild Weather)
Side by Side

Aux Heat vs Emergency Heat

Both use the same backup heat, usually electric resistance strips in the air handler. The difference is who turns it on, and whether the heat pump keeps running.

AUX

Auxiliary Heat

Normal, automatic, short bursts
Turned on byThe thermostat, automatically
Heat pump running?Yes, both run together
What makes the heatHeat pump + backup strips
When it's normalCold mornings, defrost, big set-point jumps
Cost to runHigher, but only while it's on
What you doNothing. Let it work
Aux heat is the heat pump asking for help. On a 30-degree morning or right after you bump the thermostat up five degrees, that's exactly what it should do. The warning sign is AUX running all day when it's 50 degrees out.
EM

Emergency Heat

Manual override for a broken heat pump
Turned on byYou, at the thermostat
Heat pump running?No, it's locked out
What makes the heatBackup strips only
When it's normalOutdoor unit broken, iced solid, or damaged
Cost to runHighest, every hour it's on
What you doUse it until the repair, then switch back
The name is misleading. It isn't for emergencies in general, and it won't heat faster. It's a way to keep the house warm when the outdoor unit can't run. Leave it on for days and the electric bill will tell you about it.

Have a gas furnace paired with your heat pump? That's a dual-fuel system. On dual fuel, both aux heat and emergency heat use the gas furnace instead of electric strips, so the cost math is different, but the rule is the same: aux is automatic, emergency heat is for a broken heat pump.

Reading Your Thermostat

When Aux Heat Comes On, and When It Shouldn't

The first four are normal. The last two mean the heat pump isn't doing its share, and you're paying for it.

Normal: Cold Mornings

Below about 35°F, a heat pump pulls less heat out of the outside air. The thermostat brings on aux heat to make up the gap. In Central Texas that's mostly early mornings in December through February.

Normal: Defrost Cycle

When the outdoor coil frosts over, the heat pump briefly runs in reverse to melt it. Most systems turn the strips on during defrost so you don't feel cold air. A few minutes of AUX every hour or so in damp, cold weather is expected.

Normal: Big Set-Point Jump

Raise the thermostat from 64 to 70 and many thermostats call for aux heat to close the gap fast. Small changes of a degree or two usually let the heat pump handle it alone.

Normal: Coming Out of a Setback

If your schedule drops the temperature overnight or while you're at work, the morning or evening recovery can trigger aux heat for the same reason as a big set-point jump.

Not Normal: AUX All Day in Mild Weather

If it's 45 or 50 degrees outside and AUX barely turns off, the heat pump is struggling. Common causes are low refrigerant, a failing reversing valve, or a defrost problem. The house stays warm, so people miss it until the bill shows up.

Do ThisCall for a heat pump repair. It's costing you every day.

Not Normal: Outdoor Unit Silent While AUX Runs

Warm air inside, but walk outside and the heat pump isn't running at all. The strips are carrying the whole house. Usually a tripped breaker, a failed capacitor, or a bad contactor on the outdoor unit.

Do ThisCheck the outdoor breaker once. If that's not it, call.
Using It Right

When to Use Emergency Heat

Emergency heat is a stopgap. Here's how to use it without running up a big bill.

Emergency heat buys you time, not a fix

Electric heat strips turn one unit of electricity into one unit of heat. A heat pump in mild weather moves two to three units of heat for every unit of electricity it uses. That's why an hour on emergency heat costs so much more than an hour on a working heat pump.

So use it when you have to, get the heat pump fixed, and switch back.

Running on emergency heat right now? Call (512) 200-1740. We repair heat pumps in Liberty Hill and across Central Texas, same day on most calls.
1

Switch to EM heat if the outdoor unit can't run

It's encased in ice, it's making a grinding or screeching noise, it was damaged, or its breaker keeps tripping. Running a damaged heat pump can turn a repair into a replacement.

Broken outdoor unit = EM heat
2

Don't use it just because it's cold

A working heat pump plus automatic aux heat will handle a Central Texas cold snap. Switching to EM heat on a freezing night doesn't make the house warmer, it just stops the cheaper heat source.

3

Call for the repair the same day

Every day on emergency heat is a day at the highest heating cost your system has. The sooner the heat pump is back, the sooner the bill comes down.

4

Switch back to Heat after the repair

Easy to forget. If you leave the thermostat on EM heat, the heat pump stays off even though it's fixed.

Common Questions

Aux Heat & Emergency Heat Questions

Is it bad if my thermostat says aux heat?
No. Aux heat coming on during cold mornings, defrost cycles, or after you raise the temperature a few degrees is normal. It's only a problem if AUX stays on for hours in mild weather, or if the outdoor unit isn't running while it's on.
Does emergency heat heat the house faster?
No. It shuts off the heat pump and uses only the backup heat, which is already available through aux heat when it's needed. You get the same backup heat without the heat pump helping, at a higher cost.
Why is my electric bill so high when aux heat runs?
Electric heat strips are simple resistance heaters, like a giant hair dryer. They make one unit of heat for each unit of electricity. A heat pump in mild weather moves two to three times that much heat for the same electricity. When the strips carry more of the load, the bill goes up fast.
How do I keep aux heat from running so much?
Make small thermostat changes instead of big jumps, keep the filter clean, and get the heat pump checked before winter. Many smart thermostats also have an aux heat lockout temperature that keeps the strips off above a set outdoor temperature. Setting it right depends on your system, so have your technician set it rather than guessing. Our $89.95 Cooling & Heating Inspection covers the heat pump and the strips.
What if my heat pump blows cold air instead of warm?
That's a different problem with its own list of causes, from a defrost cycle to a stuck reversing valve. Walk through our heat pump blowing cold air guide before you switch anything to emergency heat.
Do you service heat pumps and heat strips near me?
If you're in or around Liberty Hill, yes. We service heat pumps, heat strips, and dual-fuel systems across Leander, Georgetown, Cedar Park, Round Rock, Burnet, and Bertram.

Stuck on
Emergency Heat?

Same-day heat pump repair on most calls, flat-rate pricing, and real techs who actually know heat pumps. Call or request service online.

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