Aux Heat vs Emergency Heat on a Heat Pump Thermostat

Aux heat comes on automatically when a heat pump needs help, such as during defrost, after a big setpoint jump or on cold nights, while emergency heat is a manual setting that locks out the heat pump compressor and runs the backup alone. The thermostat decides on its own when to add aux; emergency heat only happens when someone in the household selects it. ENERGY STAR's heat pump guidance notes that in very cold climates a heat pump is often paired with a backup heat source.

Both words show up on the same thermostat because they usually use the same equipment. Honeywell Home's support page says it is common for the emergency heat and the auxiliary heat to be the same heat source: electric heat strips or a fossil-fuel furnace.

What does "Aux Heat On" mean on a heat pump thermostat?

When the thermostat shows "Aux Heat On," the heat pump compressor is still running and the backup is adding heat on top. Honeywell describes aux as the backup that turns on to help the compressor hold your setting. Trane lists three common triggers:

• Cold weather. The outdoor temperature drops below 40 degrees or below an aux trigger temperature set in the thermostat.
• Defrost. The heat pump reverses for a few minutes to melt frost off the outdoor coil, and aux warms the air so the registers don't blow cool. Our defrost cycle guide explains what you'll see and hear outside.
• A big setpoint change. Trane says raising the temperature by more than 3–4 degrees can bring on aux, because the thermostat is trying to catch up quickly.

In Minnesota the cold-weather trigger matters most. Every heat pump has a balance point, the outdoor temperature where its output falls to your house's heat loss. Below it, aux fills the gap every day. A drafty 1920s story-and-a-half in Minneapolis reaches that point at a warmer temperature than a tight 2010s two-story, so the same heat pump leans on aux more in the older house. Our post on whether heat pumps work in Minnesota winters walks through how that crossing point is found.

What is emergency heat on a heat pump thermostat?

Emergency heat (often "Em Heat" or "EM HT") only runs when you move the thermostat's system setting to it. Honeywell's support page says that in that position the compressor does not run; only the emergency heat runs. Trane's advice is to use it only in a true emergency, meaning the heat pump has failed and it's cold outside.

Good reasons to switch to emergency heat:

1. The outdoor unit won't start, or the breaker for it keeps tripping.
2. The outdoor coil stays encased in ice after a defrost cycle.
3. You hear grinding or scraping from the outdoor fan.
4. The outdoor unit is damaged, by a falling branch or a snowplow, say.

Then call for service, and switch back to normal heat once the heat pump is repaired. A thermostat left on emergency heat keeps the heat pump locked out all winter, and nothing on the thermostat reminds you.

Are aux heat and emergency heat expensive to run?

With electric heat strips as the backup, both aux and emergency heat are expensive. A resistance heater turns one kilowatt-hour into one kilowatt-hour of heat. A heat pump moves more heat than that from the outdoor air in most Minnesota weather, which is the whole point of owning one. Trane puts it plainly: aux heat with electric strips uses more energy than the heat pump, though it brings the temperature back up faster. Running on emergency heat alone for a week in January shows up on the electric bill.

With dual fuel, the backup is your gas or propane furnace. Aux on a dual-fuel system usually means the furnace takes over below the switchover temperature, and emergency heat means the furnace runs by itself. That's normal operation, not a warning sign, and it costs what furnace heat costs. Our dual-fuel guide explains how to set that switchover from your own gas and electric rates.

When is aux heat running constantly a problem?

Long aux runs on a -10°F night are expected. Aux that runs most of the day at 40°F is not. Trane says aux that comes on and stays on above 40 degrees can mean a problem with the heat pump or the thermostat. The usual causes we find:

• Low refrigerant from a leak, so the heat pump runs but moves little heat.
• A defrost problem: a sensor or board that keeps the unit in defrost, or a reversing valve that sticks.
• A frozen or buried outdoor unit. Drifts, roof runoff and a unit set on the ground in a snow pile all block airflow.
• Airflow inside. A clogged filter or closed registers make the heat pump struggle and the thermostat call for help.
• Thermostat settings that bring aux on too early or at too warm an outdoor temperature.

On all-electric systems, a heat pump that quietly stopped working can hide for weeks, because the strips keep the house warm. A jump in the electric bill is often the first sign.

Which thermostat settings control when aux heat runs?

Most heat pump thermostats have installer settings that decide when aux may run and when the compressor stops. ecobee's threshold settings are a clear example:

• Aux Heat Max Outdoor Temperature prevents auxiliary heat from running above a set outdoor temperature.
• Compressor Min Outdoor Temperature sets the lowest outdoor temperature at which the compressor operates.
• Compressor to Aux Temperature Delta or Runtime sets how far behind the house must fall, or how long the compressor must run, before aux comes on.

Set too warm, the aux lockout lets strips run on mild fall days. Set the compressor lockout too warm on a cold-climate heat pump and you give up heat pump hours you paid for. Set it too cold on an older standard heat pump and the compressor runs where it can barely keep up. These numbers should come from the heat pump's specs and your house, not the factory default. Our smart thermostat and zoning page covers thermostats that handle heat pumps and dual fuel.

What you can check, and when to call

1. Look at the thermostat mode. If it says emergency heat and nobody chose it on purpose, set it back to heat.
2. Check the air filter and make sure supply and return grilles are open.
3. Look at the outdoor unit. Brush snow off the top and clear drifts from the sides by hand. Never chip ice off the coil.
4. Watch one defrost cycle. If the coil clears and the fan restarts, that part is working.

Call if aux runs for hours in weather above about 40°F, the outdoor unit is iced solid, the electric bill jumped without a cold snap to explain it, or the house won't hold temperature even with aux on. A service call comes with a written, flat-rate price before any work starts. We're open 7 days, 7am–9pm, and a heat pump that has quit in a cold snap is a no-heat call, which gets priority. See our heat pump service page, or the local pages for heat pumps in Minneapolis and heat pumps in Saint Paul, or call 612-429-3465.

Common questions about heat pumps

Sources

• ENERGY STAR energystar.gov
• Trane manufacturer documentation trane.com