Heat Pump Defrost Mode: Why Your Outdoor Unit Steams in Winter

Short answer: Steam rising from a heat pump's outdoor unit on a cold morning is usually defrost mode, and it's normal. The heat pump briefly reverses to melt frost off the outdoor coil, the outdoor fan stops, you may hear a whoosh, and the vents may blow cooler air for a few minutes. It's a problem when the coil stays covered in thick ice, defrost never seems to end, or the house stops warming.

Safety first: if you smell gas (a rotten-egg smell), leave the house now and call 911 or Southwest Gas from outside. If a carbon monoxide alarm sounds, get everyone out and call 911 from outside. A dusty or oily smell the first time the heat runs each season is normal and can linger for a couple of days.

What defrost mode is

In heating mode, a heat pump pulls heat out of the outdoor air through its outdoor coil. On cool, damp mornings, that coil can get colder than the air around it, and moisture collects on it as frost. Frost blocks airflow and cuts heating.

To clear it, the heat pump runs a defrost cycle. It shifts its reversing valve so the system runs briefly as if it were cooling, sending warm refrigerant to the outdoor coil. The frost melts, and the heat pump switches back to heating.

What you'll see and hear (all normal)

• Steam or "smoke" from the outdoor unit. Warm coil plus melting frost makes vapor. It can look alarming, but it's water vapor, not smoke. There's no smell to it.
• The outdoor fan stops while the compressor keeps running. That helps the coil warm up faster.
• A whoosh or loud click when the reversing valve shifts at the start and end of defrost.
• Water running out from under the unit as the frost melts.
• Cooler air at the vents for a few minutes. Many systems turn on backup electric heat strips during defrost to offset this, so you may not notice it at all.

A defrost cycle usually lasts only a few minutes, and then the outdoor fan starts again.

Does defrost happen in Las Vegas?

Yes, though less often than in colder places. NOAA's 1991–2020 normals for Las Vegas put the average January low at 40.5°F and the average January high at 58.5°F. Frost forms on the coil based on coil temperature and moisture, not just the air temperature, so early cold mornings in December and January are when you're most likely to see steam. The rest of the day, many heat pumps here run with no defrost at all.

Signs defrost isn't working right

Call a technician if you see any of these:

• Thick ice covering the outdoor coil that's still there after an hour or more of running, or ice building up into a block around the base.
• Ice on the top of the unit or on the fan blades.
• Defrost that seems to never end, with the outdoor fan stopped for long stretches and the house getting colder.
• Constant cool air from the vents, not just for a few minutes. Our heat pump not heating guide covers the bigger no-heat checks.
• A grinding or scraping noise from the outdoor fan, which may mean ice is hitting the blades.

Common causes include a failed defrost control board or sensor, a stuck reversing valve, a low refrigerant charge, an outdoor fan motor problem, or poor airflow across the coil.

What you can safely do

• Keep the unit clear. Trim plants back and keep at least a couple of feet open around the sides so air can flow. Move anything stored on top.
• Check the drainage. Melted frost needs somewhere to go. Make sure the ground under the unit isn't building up water that can re-ice.
• Check gutters and roof runoff. Water dripping from a roof onto the unit can turn into ice on a cold morning.
• Check the indoor filter. Low indoor airflow makes a heat pump work harder in both modes.
• Leave the thermostat alone during defrost. Turning the system off and on mid-cycle doesn't help and can cut the cycle short.

What not to do

• Don't chip or scrape ice off the coil with a tool. The aluminum fins and copper tubing damage easily, and a pierced tube means a refrigerant leak.
• Don't pour boiling water on the coil.
• Don't cover the outdoor unit in winter while the heat pump is in use.
• Don't switch to Emergency Heat just because you saw steam. Defrost is a normal part of running in heat mode.

Heat pump or furnace?

If you're still deciding which system fits your home, our heat pump vs. furnace guide compares them for the Las Vegas climate. If your system is a gas furnace and the air turned cold, see heater blowing cold air instead.

When to call

If the outdoor coil stays iced, defrost doesn't seem to happen, or the house won't warm, it's time for a technician to check the defrost controls, sensor, reversing valve and refrigerant charge. See our Las Vegas furnace and heating repair service or our heat pump service.

FAQ

Why is my heat pump steaming in winter?

It's usually defrost mode. The heat pump briefly warms its outdoor coil to melt frost, and the melting frost makes water vapor. It's normal and usually lasts a few minutes.

Is steam from my heat pump smoke?

Defrost steam is water vapor and has no smell. If you see smoke with a burning or electrical smell, turn the system off at the thermostat and call a technician.

Why does my heat pump blow cool air for a few minutes?

During defrost the heat pump runs briefly in reverse, so the indoor air can feel cooler. Many systems turn on backup heat strips to offset it. It should go back to warm air when defrost ends.

How much frost on a heat pump is normal?

A light layer of frost that clears after a defrost cycle is normal. Thick ice that stays on the coil, covers the top or blocks the fan is not normal and needs a technician.

Should I chip the ice off my heat pump?

No. Scraping can bend fins or puncture refrigerant tubing. Turn the system to Emergency Heat if you have it, or off, and call a technician.

What hours can I reach Colton?

Colton Air Conditioning is open Monday to Friday, 8am to 5pm. You can call or book online during those hours.

Colton Air Conditioning

Colton Air Conditioning, 6545 S. Fort Apache Rd Ste 135 #467, Las Vegas, NV 89148. Nevada contractor license 0092120. Hours: Monday to Friday, 8am to 5pm. Call (702) 738-6976 or book online during those hours.

Want routine maintenance on a schedule? See our Comfort Club maintenance plans: Gold, Platinum and Diamond tiers are all listed there.