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 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 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, 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 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 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 lists every start-up reading.
On dual fuel systems
On a dual fuel 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 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, 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 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 covers the rest of a heat pump decision.