Auxiliary heat vs emergency heat: what your heat pump is doing

Updated August 31, 2026 · Claude AC · 2 sources cited

Contents (6)

The mechanical difference in one paragraph
What the strips cost per hour
Three times aux heat is doing its job
When aux running is a fault, not a feature
Emergency heat: one legitimate use
Frequently asked questions

Two abbreviations on a heat pump thermostat cause more confusion, and more surprise bills, than anything else in residential HVAC: AUX and EM. They energise the same electric resistance elements sitting in your air handler. They are not the same setting, they are not triggered the same way, and only one of them is ever something you should select yourself.

The mechanical difference in one paragraph

A heat pump does not make heat, it moves it — the same refrigeration cycle described in our guide to heat pumps versus air conditioners, run backwards. Moving heat is cheap: for every kilowatt-hour of electricity it consumes, a modern air-source unit delivers roughly 2 to 3.5 kWh of heat into the house at typical winter temperatures. Electric strip heat makes heat by pushing current through a coil of wire. It delivers exactly 1 kWh of heat per kWh of electricity, forever, no matter the weather. That ratio — three to one, sometimes four to one — is the entire story behind every complaint about a heat pump costing a fortune to run.

Auxiliary heat means the strips are running alongside the compressor, automatically, because the thermostat decided the heat pump alone was not keeping up. Emergency heat is a manual mode that locks the compressor out entirely and heats the house with the strips only. Auxiliary is a decision your control makes several times a winter. Emergency is a decision you make when the compressor is broken.

What the strips cost per hour

Residential strip banks are usually sized 5, 10, 15 or 20 kW, and the whole bank rarely runs at once — most systems stage them. The arithmetic is unforgiving and easy to check against your own rate:

Strip stage running	At $0.14/kWh	At $0.20/kWh	At $0.30/kWh
5 kW	$0.70/h	$1.00/h	$1.50/h
10 kW	$1.40/h	$2.00/h	$3.00/h
15 kW	$2.10/h	$3.00/h	$4.50/h
20 kW	$2.80/h	$4.00/h	$6.00/h

For comparison, the compressor and blower of a 3-ton heat pump typically draw somewhere between 1.5 and 3.5 kW while heating — call it $0.30 to $0.70 an hour at $0.20/kWh, while delivering more heat than a 10 kW strip bank does in mild cold. A thermostat that quietly holds 10 kW of resistance heat on for six hours a night has added roughly $12 a night to your bill in a $0.20 market. Over a two-week cold snap that is a bill people phone the utility about.

Three times aux heat is doing its job

1. Below the balance point

A heat pump's capacity falls as the outdoor temperature drops, while the house's heat loss rises. The temperature where the two curves cross is the balance point — commonly somewhere between 25 and 35 °F for a conventional system in an average house, lower in a tight house with a generously sized unit. Below it, the heat pump genuinely cannot keep up on its own and the strips are supposed to fill the gap. Cold-climate models shift that crossing point well down; the Department of Energy notes that cold-climate air-source heat pumps keep working below 5 °F, with a backup source reserved for temperatures below that.

2. Recovery from a setback

This is the most common and most expensive trigger, and it is entirely avoidable. Most thermostats call for auxiliary heat when the room is more than about 2 °F below setpoint and the gap is not closing. Set a heat pump back 8 °F overnight and the morning recovery is a straight invitation for the strips to run for an hour. The counter-intuitive rule that follows: heat pumps do not reward deep setbacks the way a furnace does. Either keep a narrow schedule, or use a thermostat with genuine heat-pump recovery logic that ramps early using the compressor alone. The same "let it run steadily" logic we describe for summer thermostat settings applies with more force in winter.

3. Defrost

When the outdoor coil frosts over, the system briefly reverses into cooling to melt the ice — outdoor fan off, a plume of steam that neighbours occasionally mistake for a fire — and energises the strips so the indoor supply air does not blow cold for those few minutes. Aux light on for five to twelve minutes, every 30 to 90 minutes in damp cold weather, is normal. Aux light on continuously at 45 °F outside is not.

When aux running is a fault, not a feature

If the AUX indicator is lit in mild weather, treat it as a diagnostic. The usual causes, roughly in order of how often they turn up:

A thermostat wired or configured as a conventional system. A replacement smart thermostat set to "furnace" staging will happily run strips as first-stage heat. This is the single most common cause after a DIY thermostat swap.
Aggressive setback schedules, as above — free to fix.
Low refrigerant charge or a dirty outdoor coil. Both cut heating capacity, which raises the effective balance point, which calls the strips earlier every night. A leak that would merely reduce comfort in July becomes a visible bill in January.
A stuck reversing valve or a failed defrost board, leaving the machine running defrost cycles it does not need.
No outdoor temperature lockout configured. Many controls can be told to forbid strip heat above, say, 40 °F. It is a five-minute setting that pays for itself in one cold week.
A genuinely undersized heat pump, or a house that lost its envelope — the strips are then covering a design problem, not a mechanical one.

Emergency heat: one legitimate use

Emergency heat exists for the night the compressor fails, the outdoor unit is buried in ice, or a technician tells you to lock it out until a part arrives. It is not a "very cold outside" mode. Selecting EM heat at 20 °F does not make the house warm faster — it removes the only efficient heat source you own and leaves you paying resistance rates for every degree. If a household member switched to EM heat in December and nobody switched it back, that alone can double a winter electric bill. It is worth checking the thermostat before you go looking for anything more exotic.

Two practical notes. Systems with a gas or oil furnace as backup instead of strips — dual fuel — behave differently: the control switches to the fossil burner at a set outdoor temperature rather than adding electric heat, and the cost comparison depends on local gas versus electricity prices. And in the coldest states, a cold-climate model plus a decent envelope is what keeps the strips idle; if you are shopping in a place like Minnesota, the low-temperature capacity figure matters far more than the headline efficiency number explained in our SEER2 guide. Our heat pump section covers the equipment side.

Frequently asked questions

Is it bad if auxiliary heat comes on?

Not in itself. Aux heat during a defrost cycle, or on a genuinely cold morning below the balance point, is the system working as designed. What matters is frequency and duration: brief and cold-weather-correlated is fine, constant and mild-weather is a fault worth chasing.

Should I use emergency heat when it gets very cold?

No. Emergency heat locks out the compressor, which is still delivering two to three times more heat per kilowatt-hour than the strips even in the twenties. Use EM heat only when the heat pump itself is broken or has been locked out by a technician.

Why does my heat pump blow lukewarm air?

Because it is supposed to. Supply air from a heat pump typically arrives around 90 to 105 °F against a 98.6 °F hand — it feels cool even while it is heating the house. A furnace delivers 120 to 140 °F in short blasts. Longer, gentler run times are the normal signature, not a malfunction.

Can I turn the auxiliary heat off completely?

Most thermostats allow an outdoor lockout rather than a full disable, which is the sensible version: strips stay available below the temperature you nominate. Disabling backup heat entirely leaves you with no defrost tempering and no margin in a cold snap, and in some installations the strips are also the freeze protection for the house.

How do I know my balance point?

Watch the thermostat over a cold week and note the outdoor temperature at which aux first engages on a steady evening — not during recovery, not during defrost. That observed number is more useful than any manufacturer chart, and if it sits above about 35 °F on a system in reasonable condition, ask a technician to check charge, airflow and coil cleanliness before accepting it.