Why your heat pump steams on a 35°F Seattle morning

A heat pump that hisses, goes quiet and sends up a cloud of steam on a raw 35°F morning is almost always running a normal defrost cycle. The outdoor coil collects frost when it pulls heat from cold, damp air, and the system periodically melts that frost off. The steam is meltwater meeting cold air, not smoke and not a leak.

That cloud surprises a lot of first-time owners, especially in houses that just switched from a furnace. The Seattle Heat Pump Company plans ducted heat pump systems for Seattle-area houses, and defrost behavior is one of the things checked at commissioning, so it is worth knowing what you are looking at.

Why frost forms on the outdoor coil

In heating mode, the outdoor coil is the cold side of the system. Refrigerant flowing through it is colder than the outdoor air, which is how it absorbs heat from that air. The guide to how a ducted heat pump works walks through the full refrigerant loop. When the coil surface drops below 32°F and the air holds moisture, that moisture freezes onto the fins the same way frost forms on a car windshield.

A thin layer is harmless. A thicker one starts blocking airflow through the coil, and less air means less heat collected. Left alone, the coil would keep getting colder and icier while delivering less heat to the house. Defrost exists to stop that slide.

What happens during a defrost cycle

When the control decides the coil needs it, the heat pump briefly runs in reverse. A reversing valve switches the refrigerant flow so the outdoor coil becomes the warm side, and the outdoor fan usually stops so the heat stays on the coil instead of blowing away. Frost melts within a few minutes, water runs off the coil, and some of it flashes to vapor in the cold air. That is the steam.

You may also notice:

• A whoosh or hiss when the reversing valve shifts at the start and end of the cycle.
• The outdoor fan sitting still while the compressor keeps running.
• Water dripping or running from the base of the unit.
• Slightly cooler air from the registers for a few minutes, unless backup heat warms it.

Many ducted systems switch on part of their electric backup heat during defrost so the air coming out of the vents does not feel cold. That is normal, and it is one reason backup heat settings matter. The backup heat settings guide explains how the strips should be staged and why Seattle’s energy code keeps them off above 40°F outdoors in normal heating.

Older and simpler systems defrost on a timer, typically checking every 30, 60 or 90 minutes once the coil is cold enough. Many newer systems use demand defrost, which watches coil and air temperatures and defrosts only when frost is actually building. Demand defrost wastes less energy on clean coils, which suits Seattle’s long stretches of frost-marginal weather.

Why Seattle’s wet 30s bring more defrost than a dry 20°F night

It seems backward, but a damp 35°F morning can trigger more defrost cycles than a clear, colder night. Frost needs two things: a coil below freezing and moisture in the air. Air in the low to mid 30s near saturation, which describes a lot of Seattle’s winter, supplies plenty of both. Very cold air holds much less water, so frost builds more slowly even though the coil is colder.

Sea-Tac’s 1991 to 2020 normals show about 25 nights a year at or below freezing and roughly one night a year at 20°F or colder. The rest of the heating season is mostly long, wet stretches in the 30s and 40s, which is ideal frost weather. So a Seattle heat pump may defrost more often than one in a colder, drier climate, while still running efficiently overall. The Seattle winter performance guide covers how capacity and efficiency change across that range.

Normal frost versus ice that stays

The line between normal and not is whether the ice clears. Normal defrost leaves a wet, mostly clean coil after each cycle, then frost builds again over the next stretch of running.

Ice worth paying attention to looks different:

• The coil stays encased in solid ice after a defrost cycle finishes.
• Ice covers the top of the unit or the fan grille.
• Ice builds up from the base and starts climbing into the bottom of the coil.
• The unit never seems to defrost, or defrosts constantly with little time heating in between.

Persistent ice does not point to a single cause. Airflow across the coil can be blocked by leaves, a fence or shrubs planted too close. A defrost sensor or control board can fail. A low refrigerant charge makes the coil run colder than it should. Freezing rain can glaze a unit faster than any defrost cycle clears it. Water dripping from a gutter onto the unit, or a unit sitting too low for meltwater to drain away, can refreeze at the base. Sorting those out takes someone looking at the system, not a guess from the window.

If heavy ice persists, switch the thermostat to emergency or backup heat if your system has that setting, and do not chip at the coil; the fins and tubes bend and puncture easily. Then arrange a service visit. The heat pump maintenance guide covers the simple things owners can check, such as clearances, debris and drainage.

What good design does about it

Many defrost problems trace back to installation choices, which is why the installation process matters as much as the equipment. A pad or riser that lifts the unit clear of standing water, a location away from roof drip lines, open space around the coil, and a system sized so it does not short-cycle all reduce icing problems. Seattle averages only about 6.3 inches of snow a year, so a modest riser usually suffices. Commissioning on a new ducted heat pump installation should include a check that defrost initiates and terminates correctly, along with airflow, charge and backup heat lockout. If an older system is icing repeatedly and needs frequent repair, the heat pump replacement page explains what changes with new equipment, and variable-speed versus single-stage equipment covers why modern compressors handle shoulder weather more smoothly.