# What is the difference between auxiliary heat and emergency heat?

> What aux heat and emergency heat mean on a heat pump thermostat, when each should run in Memphis, and the lockout settings that keep strips off the bill.

URL: https://blufflinememphisheatpumps.com/guide/auxiliary-emergency-heat/

Learn

Two settings that both turn on the strips, why one is automatic and the other is a last resort, and how to keep the bill in check.

Last Updated: November 12, 2026

The short answer

Auxiliary heat is automatic. The thermostat adds backup strips (or the furnace on dual fuel) when the heat pump can't keep up, during defrost, or after a big setpoint jump. Emergency heat is a manual switch that turns the heat pump off and runs backup heat alone. Use it only when the outdoor unit is broken.

Key takeaways

- Aux heat comes on by itself; emergency heat is a switch you set.
- Strips cost about $35.80 per million BTU on MLGW against about $10.20 to $14.30 for the heat pump.
- Aux heat during defrost and on the coldest mornings is normal.
- Aux heat on mild days usually means a missing outdoor sensor, no lockout or a failing heat pump.
- Emergency heat is for a broken outdoor unit, not for cold weather.

## Two settings, one set of strips

Most Memphis heat pumps have electric resistance strips in the air handler as backup. The thermostat can turn them on two ways: automatically as auxiliary heat, or by your choice as emergency heat.

[Bluffline Memphis Heat Pumps](https://blufflinememphisheatpumps.com/) sets up the staging on every installation, because it decides whether a heat pump saves money or quietly runs strip heat all winter.

### At a glance

|  | Auxiliary heat | Emergency heat |
| --- | --- | --- |
| Who turns it on | The thermostat, automatically | You, with a switch or menu |
| Heat pump during it | Keeps running | Off |
| When it is meant for | Coldest hours, defrost, big recoveries | A broken outdoor unit |
| Cost | Higher than heat pump alone, for short periods | Highest, for as long as it runs |

## What auxiliary heat is for

The heat pump is the main heat source. Auxiliary heat adds to it in three situations.

### The coldest hours

Every heat pump has a balance point, the outdoor temperature where its output equals the house’s heat loss. Below it, the heat pump falls behind. In a typical Memphis house that is in the 20s or low 30s. The strips fill the gap.

### Defrost

In cold, damp weather the outdoor coil frosts. To clear it, the heat pump briefly reverses into cooling mode, and the air from the registers would turn cool. The thermostat runs the strips during defrost so it doesn’t. The [defrost guide](https://blufflinememphisheatpumps.com/guide/heat-pump-defrost-ice/) explains the cycle.

### Big setpoint jumps

Raising the thermostat several degrees at once, especially after an overnight setback, can bring the strips on to speed the recovery. Many thermostats let you limit this.

## Why it matters on the bill

Strips turn one unit of electricity into one unit of heat. A heat pump delivers two to four. On MLGW’s rates:

| Heat source | Cost per million BTU |
| --- | --- |
| Heat pump, mild day | About $10.20 |
| Heat pump, cold day | About $14.30 |
| Electric strips | About $35.80 |

Every hour the strips run when they don’t need to costs about three times what the heat pump would.

## When aux heat runs too much

Long aux heat runs on mild days usually come from one of these:

- **No outdoor sensor.** The thermostat can’t tell a 50-degree day from a 20-degree one, so it calls the strips whenever the house falls behind.
- **No lockout.** Even with a sensor, the strips need to be locked out above a set temperature, often 30 to 35 degrees.
- **Large setbacks.** A 6 or 8 degree overnight setback means a long morning recovery with strips.
- **A heat pump losing capacity.** An aging compressor, low refrigerant or a dirty outdoor coil delivers less heat, so the strips make up the difference.
- **Strips staged too early.** Some thermostats bring strips on after only a few minutes of the heat pump running.

### What a fix looks like

An outdoor sensor, a lockout set with you, smaller setbacks or a smart recovery setting, and a check of the heat pump’s charge and coil. In a Hernando house on our [service area page](https://blufflinememphisheatpumps.com/service-areas/hernando/), those changes and a new outdoor unit cut the strip hours to the coldest mornings.

## What emergency heat is for

Emergency heat turns the heat pump off and runs the backup heat alone. It exists for one situation: the outdoor unit can’t run.

### When to use it

- **The outdoor unit is broken** and waiting for repair.
- **It is iced solid** after an ice storm and won’t clear through defrost.
- **It is making a grinding or banging noise** and you want it off.

### When not to use it

Cold weather alone isn’t a reason. A correctly sized heat pump keeps working well below freezing, and auxiliary heat comes on automatically when needed. Running emergency heat on a cold night just turns off your cheapest heat source. If you switched to it during a cold snap, switch back to normal heat once the weather breaks.

## What strip hours cost

A 10 kW strip kit running for one hour uses 10 kilowatt-hours. At about 12 cents per kilowatt-hour on MLGW in winter, that is about $1.20 an hour.

### A winter of extra strip hours

| Extra strip hours | Cost at about $1.20 an hour |
| --- | --- |
| One hour each morning for 90 days | About $108 |
| Three hours a day for 90 days | About $324 |
| Strips running most of the day for 60 days | Several hundred dollars more |

Those are the hours a missing sensor or lockout adds on top of the strip use that is genuinely needed. A [Millington](https://blufflinememphisheatpumps.com/service-areas/millington/) ranch that used to heat entirely on strips shows the far end of the scale.

## After a power outage

When power comes back after an outage, many heat pumps wait a few minutes before restarting the compressor to protect it. Some thermostats bring the strips on during that wait, and a house that cooled during the outage will need a long recovery.

### What to expect

- **A burst of aux heat** right after power returns is normal.
- **The heat pump takes over** once its delay ends and the house nears the setpoint.
- **If the outdoor unit iced** during an ice storm, it may need several defrost cycles, or emergency heat, until it clears.

The [ice storm guide](https://blufflinememphisheatpumps.com/guide/heat-pump-ice-storm-power-outage/) covers outages in more detail.

## Testing the staging at start-up

On a new installation, staging should be proven, not assumed. At start-up we:

- Run the heat pump alone and record its supply temperature.
- Call for auxiliary heat and confirm the strips come on.
- Force a defrost cycle and confirm the strips cover it.
- Check the outdoor sensor against a thermometer.
- Set the lockout with you and write it on the start-up sheet. The [commissioning checklist](https://blufflinememphisheatpumps.com/guide/heat-pump-commissioning-checklist/) lists every start-up reading.

## On dual fuel systems

On a [dual fuel](https://blufflinememphisheatpumps.com/dual-fuel-heat-pump/) system, backup heat is the gas furnace, not strips. Auxiliary heat means the furnace takes over below the switchover temperature, and emergency heat usually runs the furnace alone. The [switchover guide](https://blufflinememphisheatpumps.com/guide/dual-fuel-switchover-temperature/) covers where to set the handoff.

## Sizing the strips

The strips should be sized to the gap between the heat pump’s output and the house’s heat loss at Shelby County’s 20-degree design temperature, plus what defrost needs. Many Memphis houses need 5 to 10 kW.

### Why not bigger

A 15 or 20 kW kit draws far more power than the house needs, can push an older panel toward an upgrade and makes every unneeded aux call more expensive. When we replace an [electric furnace with a heat pump](https://blufflinememphisheatpumps.com/electric-furnace-to-heat-pump/), the backup usually drops to a third of the old strip size.

## Reading your thermostat

Most heat pump thermostats show what is running:

- **HEAT or HEAT ON**: the heat pump alone.
- **AUX HEAT** or **HEAT STAGE 2**: the heat pump plus strips, or the furnace on dual fuel.
- **EM HEAT**: emergency heat, heat pump off.
- **DEFROST**: some thermostats show this during the defrost cycle.

The [thermostat setup guide](https://blufflinememphisheatpumps.com/guide/heat-pump-thermostat-setup/) walks through the menus for lockouts, staging and recovery.

## The short version

Let aux heat do its job on the coldest mornings and during defrost.

Make sure an outdoor sensor and lockout keep it off on mild days. Use emergency heat only when the outdoor unit is broken. The [Memphis buying guide](https://blufflinememphisheatpumps.com/guide/memphis-heat-pump-buying-guide/) covers the rest of a heat pump decision.

## Questions about this guide

### Why does my thermostat say AUX HEAT when it is not very cold outside?

Common causes: a setpoint raised several degrees at once, a defrost cycle, a thermostat with no outdoor sensor or lockout, or a heat pump that has lost capacity. A short burst after a big setpoint change is normal; long runs on mild days aren't.

### Should I use emergency heat when it gets really cold?

No. A correctly sized heat pump keeps working well below freezing, and the thermostat adds auxiliary heat automatically when needed. Emergency heat turns the heat pump off and runs strips alone, which costs far more.

### Is it safe to run emergency heat for a few days?

Yes, it is safe, just expensive. If the outdoor unit is broken or iced solid, emergency heat keeps the house warm until it is repaired. Switch back to normal heat as soon as the heat pump works again.

## Related service

- [Service**Electric furnace to heat pump** Strip heat makes the biggest winter bills in Memphis. A heat pump moves heat instead of making it and keeps strips for the coldest hours.](https://blufflinememphisheatpumps.com/electric-furnace-to-heat-pump/)

## More guides

- [Learn**How should a heat pump thermostat be set up in Memphis?** The handful of settings that decide whether a heat pump saves money: the equipment type, the outdoor sensor, the lockout, setbacks and the fan.](https://blufflinememphisheatpumps.com/guide/heat-pump-thermostat-setup/)
- [Learn**Why does a heat pump go through a defrost cycle and ice up in winter?** Frost, steam, a whoosh and a pause are normal. A block of ice that stays isn't. Here is how to tell them apart.](https://blufflinememphisheatpumps.com/guide/heat-pump-defrost-ice/)
- [Learn**What switchover temperature should a dual fuel heat pump use in Memphis?** The two numbers behind the setting, what MLGW's rates say about them, and how to pick a point you can live with.](https://blufflinememphisheatpumps.com/guide/dual-fuel-switchover-temperature/)
- [Decision Guide**How do you buy the right heat pump for a Memphis house?** The whole decision in the order you meet it, from the system you have now to the readings you should get at start-up.](https://blufflinememphisheatpumps.com/guide/memphis-heat-pump-buying-guide/)

## Get an estimate for your house

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