Side by Side

Auxiliary heat (AUX) — automatic. The thermostat decides. When the heat pump can't keep pace with heat loss, the strips come on to make up the shortfall while the heat pump keeps running and keeps contributing its efficient output. You don't control it directly, and you generally shouldn't want to.

Emergency heat (EM HEAT) — manual. You select it. The heat pump stops entirely and the strips carry the whole load alone. It exists for when the heat pump is broken, iced solid, or being serviced.

The trap is that "emergency" reads like "extra help for an emergency," and a hard freeze feels like one. It actually means "the heat pump is out of action." Selecting it during cold weather removes the efficient half of your system at the worst possible moment.

Rule of thumb: if the heat pump is working, never touch EM HEAT. AUX already handles cold weather automatically. If the outdoor unit is silent, iced over, or blowing cold air, that's when EM HEAT is the right call — and you should be booking a service visit at the same time.

Why Emergency Heat Can Cost More Than Auxiliary

This is counterintuitive: emergency heat can be more expensive than auxiliary heat even though auxiliary uses more total kilowatts.

When AUX runs, you're drawing heat pump plus strips — more power, but the heat pump is still doing efficient work, so more of that power becomes heat. When you force EM HEAT, you draw less total power but every watt comes from strips at COP 1.0. Per unit of heat delivered into the house, emergency heat is worse.

In practice EM HEAT also tends to run longer to hold the same temperature, because strips alone often can't match the combined output. Longer runtime at worse efficiency is how a bill gets away from you.

Reading Your Thermostat

  • "AUX" or "AUX HEAT" lit — automatic, expected below your balance point (typically 30–40°F on a standard heat pump, 5–15°F on cold-climate models).
  • "EM HEAT" or "EMERGENCY HEAT" selected — manual, and it stays on until you change it back. Worth checking you didn't leave it on from a previous cold spell.
  • "Heat Stage 2" on some brands — same thing as AUX.
  • AUX lit in mild weather — not normal. Usually a missing outdoor lockout setting, a large thermostat setback, low refrigerant, or a defrost fault.

Ecobee, Nest and most smart thermostats log auxiliary runtime separately in their monthly report, which is the fastest way to see how much of your bill it accounts for. Our auxiliary heat cost calculator turns that number into dollars and flags whether it's running more than it should.

One Case Where People Leave It On By Accident

A homeowner switches to EM HEAT during a freeze, the weather warms up, and nobody switches it back. The system then runs on resistance strips through an entire mild spring at three-plus times the necessary cost, with no obvious symptom other than a bill that never comes down.

If your bills stayed high after the cold ended, check the thermostat mode before anything else. It's free and it's a common find.

Reducing How Often AUX Runs

You can't sensibly avoid AUX altogether — below the balance point it's what keeps the house warm. But you can stop it running when it shouldn't:

  • Configure the outdoor lockout so strips can't engage above a set temperature. Often never set at installation.
  • Keep setbacks to 2–3°F, not the 7–10°F recommended for furnaces — a deep setback triggers strip-driven recovery. Our heat pump bill guide covers this trap in detail.
  • Change the filter — restricted airflow makes the system fall behind and call for help.
  • Air sealing and insulation lower the balance point itself, so strips engage at a colder outdoor temperature. See our insulation savings calculator.