Your Heat Pump in Cold Weather: Defrost Mode, Backup Heat, and What Is Normal

HVAC · 4 min read · By Rodney Jessen · Published November 10, 2026

Heat pumps are a good fit for a lot of Eastside homes, and more of them go in every year. But the first cold snap with a new system can bring some surprises. The outdoor unit steams, the fan stops, the thermostat says AUX, and the air from the vents feels warm instead of hot. Most of that is normal. Here is what to expect and what is worth a call.

How a heat pump heats in winter

A heat pump does not burn fuel. It moves heat from the outdoor air into your home, even when it feels cold outside. As the temperature drops, there is less heat to pull in, so the system works harder and runs longer.

That leads to the first thing new owners notice. A heat pump often runs in long, steady cycles, and the air from the vents feels warm rather than hot. That is how it is designed to work. As long as the house holds the temperature you set, long run times are not a sign of trouble.

Defrost mode: the steam is normal

In cold, damp weather, frost builds up on the outdoor coil. Our wet Pacific Northwest winters are good at making that happen. To clear it, the heat pump runs a defrost cycle. It briefly reverses, sending warm refrigerant to the outdoor coil to melt the frost.

During defrost you may notice:

• A cloud of steam rising from the outdoor unit. Rheem's homeowner guide calls this normal and harmless.
• The outdoor fan stopping for a few minutes.
• A whoosh or hiss at the start and end of the cycle.
• Water dripping from under the unit.
• Cooler air from the vents for a short time, or the AUX light coming on.

A defrost cycle usually lasts a few minutes, and on a damp, near-freezing day it may happen several times. When it is done, the unit goes back to heating on its own.

Backup heat: AUX vs. emergency heat

Many heat pumps have a backup heat source. In many systems it is electric heat strips in the indoor unit. Some homes pair the heat pump with a gas furnace instead, so the furnace handles the coldest days.

• Auxiliary heat, often shown as AUX, turns on by itself when the heat pump needs help. That can be during defrost, on a very cold morning, or after you raise the thermostat several degrees at once. Seeing AUX on a cold day is normal.
• Emergency heat, often shown as EM, is a setting you choose. It usually shuts off the heat pump and runs only the backup heat. It is meant for when the outdoor unit is not working, not for cold weather in general.

Electric backup heat usually costs more to run than the heat pump itself, so leave the thermostat on regular Heat. If AUX is running for hours on a mild day, check the air filter first and then call for service.

Simple habits for cold weather

• Keep the area around the outdoor unit clear of leaves, snow, and plants so air can move through it.
• Make sure water from the roof or a gutter is not dripping onto the unit, where it can freeze.
• Do not cover the outdoor unit. Rheem's guide says never to use a weather cover on it.
• Never chip ice off the coil with a tool. The fins bend and puncture easily.
• Change or check the indoor air filter regularly. A dirty filter makes the whole system work harder.
• Pick a comfortable setting and leave it. Big jumps up and down can bring on backup heat more often.

When to call a professional

Call a heating technician if the outdoor unit stays coated in thick ice after defrost, if it seems stuck in defrost, or if the fan will not spin. Also call if the house will not warm up while the system runs, if AUX or emergency heat runs for long stretches in mild weather, or if you hear grinding or banging.

OutToday serves Bellevue and the Eastside. If your heat pump is acting up during a cold snap, call us at (425) 615-5000 and we will help you sort out what is normal and what needs a fix.