The first winter with a heat pump can be confusing. The outdoor unit may stop its fan, release a cloud of steam, and make a loud whoosh. Most of the time, that is defrost mode, and it is normal. This guide explains why heat pumps defrost, what other behavior is normal in cold weather, and which signs point to a problem.
Why your heat pump runs in defrost mode
In heating mode, the outdoor coil is colder than the outside air because it is absorbing heat. When the air is near or below freezing and has moisture in it, frost forms on that coil. Frost blocks airflow and reduces how much heat the unit can collect.
To clear it, the heat pump briefly reverses. The reversing valve switches the refrigerant flow, as described in the U.S. Department of Energy's air-source heat pump guide. Warm refrigerant runs through the outdoor coil and melts the frost. Then the system switches back to heating.
Many units use demand defrost, which starts a cycle only when sensors detect frost. Older or simpler units use timed defrost, which runs after a set amount of operating time. DOE recommends choosing a unit with demand-defrost control because it reduces how often the system defrosts.
What normal defrost looks like
According to Trane's explanation of the heat pump defrost cycle, these signs are normal during defrost:
- Steam from the outdoor unit. This is melting frost turning to vapor. It is not smoke.
- The outdoor fan stops while the compressor keeps running.
- A whoosh or hiss when the refrigerant reverses direction at the start and end of the cycle.
- Changes in compressor sound.
- Water dripping or draining from under the unit.
- A short cycle. Trane says its defrost cycles usually last 3–5 minutes. Times vary by brand.
Indoors, you may feel cooler air from the vents for a few minutes, or your backup heat may turn on to keep the air warm. In a dual-fuel system, the furnace may run during defrost.
Other normal heat pump behavior in winter
- Long run times. Heat pumps often run longer and at lower output than a furnace. A variable-speed unit may run almost nonstop in cold weather.
- Warm, not hot, air. Air from a heat pump feels cooler than air from a gas furnace, even when the system is heating properly.
- A light coat of frost on the outdoor coil between defrost cycles.
- Backup heat on the coldest days. Electric heat strips or a furnace may run when the heat pump cannot keep up alone.
- An "aux heat" light on the thermostat during defrost or after you raise the temperature several degrees.
Signs that something is wrong
- Ice that keeps building on the coil or covers the whole unit, even after defrost cycles.
- Constant defrost. Trane lists a unit that seems to switch into defrost constantly, or cycles that get longer and longer while ice builds up, as reasons to call for service.
- Ice on the top of the unit or fan blades. This often comes from water dripping off a roof edge.
- Grinding, scraping or banging from the outdoor fan.
- The fan does not restart after a defrost cycle.
- The house will not hold temperature in moderate weather, or the system runs on backup heat all the time.
- Smoke, a burning smell, or a tripped breaker.
- Hissing or bubbling at the refrigerant lines when the unit is not defrosting, or oily spots on the lines.
- A thermostat stuck on emergency heat. Emergency heat turns off the heat pump and uses only backup heat.
What you can safely check yourself
- Watch one full cycle. If steam and noise stop within several minutes and the fan restarts, the unit is likely defrosting normally.
- Check the thermostat. Make sure it is set to HEAT, not EM HEAT or emergency heat, unless your technician told you to use it.
- Replace the air filter. ENERGY STAR's maintenance checklist recommends checking filters monthly. A clogged filter reduces airflow and heating output.
- Clear snow by hand or with a broom. Keep snow away from the sides and top of the outdoor unit and from the drain area underneath.
- Look up. If a roof edge drips onto the unit, note it for your technician. A gutter or diverter can fix it.
- Check the breaker once. If a breaker for the heat pump or air handler has tripped, you may reset it one time. If it trips again, leave it off and call for service.
What not to do
- Do not chip, scrape or hammer ice off the coil. The fins and refrigerant tubes are easy to puncture.
- Do not pour hot water on the unit. It can refreeze in the base and around the fan.
- Do not remove panels or touch wiring. The unit carries high voltage even when the fan is still.
- Do not add refrigerant or open refrigerant lines. EPA requires technicians who service equipment that can release refrigerant to be certified.
- Do not cover the outdoor unit while it is in use.
If you see smoke, flames or sparks, turn off the system at the thermostat, keep people away from the unit, and call 911. If you have a dual-fuel system and smell gas, leave the house on foot and call 911 and your gas utility from outside.
When to call a professional
Call for service if ice keeps building after defrost, the fan does not restart, the unit makes grinding or banging noises, a breaker trips more than once, or the house cannot hold temperature. In sub-zero weather, losing heat can freeze pipes, so call quickly. Krinkie's offers heat pump service and 24-hour emergency heating service.
Common questions
Why is steam coming from my heat pump in winter?
It is usually defrost mode. The unit warms its outdoor coil to melt frost, and the melting frost turns to vapor that looks like steam.
How long should a heat pump defrost cycle last?
It varies by brand, but Trane says its cycles usually last 3 to 5 minutes. Call for service if cycles keep getting longer or the unit seems to defrost constantly while ice builds up.
Should I switch my heat pump to emergency heat when it's very cold?
Usually not. Emergency heat turns off the heat pump and runs only the backup heat source. Use it if the heat pump has failed or your technician tells you to, and call for service.