Steam Coming Off Your Heat Pump? That Is Supposed to Happen
The defrost cycle looks alarming the first time you see it. Here is what is happening, how long it should last, and when it is actually a problem.
A heat pump periodically runs a defrost cycle to melt frost off the outdoor coil, briefly reversing into cooling mode and running the outdoor fan off. Steam rising from the unit and cool air from the vents for a few minutes is normal. A cycle lasting more than about fifteen minutes, or ice that never clears, indicates a defrost control problem.
Why frost forms in the first place
In heating mode, a heat pump extracts heat from outdoor air, which means the outdoor coil is colder than the air around it. When outdoor temperature is in the 30s or low 40s with any humidity present, moisture condenses on that cold coil and freezes.
Frost on the coil insulates it and blocks airflow, which reduces the system's ability to extract heat. Left alone it compounds until the heat pump cannot heat at all.
So the system deliberately melts it off at intervals. That is the defrost cycle, and it is a designed behavior rather than a malfunction.
What you see and hear during a defrost
- A loud whoosh or clunk as the reversing valve shifts — startling the first time, completely normal
- The outdoor fan stops while the compressor keeps running
- Steam or vapor rising off the outdoor unit, sometimes a lot of it. People call the fire department over this every winter.
- Cool air from the indoor vents, because the system is briefly in cooling mode
- Auxiliary electric heat engaging to offset that, which is correct behavior
- Another whoosh as it returns to heating
A normal defrost cycle lasts roughly five to fifteen minutes and occurs somewhere between every thirty and ninety minutes during frosty conditions, depending on the control strategy.
When it is actually a problem
| Symptom | Likely cause |
|---|---|
| Ice never clears, builds over days | Failed defrost board, defrost sensor, or reversing valve |
| Defrost runs constantly | Faulty sensor or board calling for defrost unnecessarily |
| Never defrosts, coil stays iced | Defrost control failure, or sensor not reading coil temperature |
| Ice on the top of the unit or in a solid block | Often a water source above — a gutter, a drain line, or a roof runoff issue |
| Cool air for long stretches, not minutes | Reversing valve stuck, or refrigerant charge problem |
The reversing valve, briefly
The reversing valve is the component that makes a heat pump a heat pump. It redirects refrigerant flow so the indoor coil and outdoor coil swap roles between heating and cooling.
It is also a known failure point. A valve that sticks partway leaves the system running inefficiently in a compromised state — the house heats poorly, auxiliary heat runs constantly, and the bill climbs. A valve stuck fully in cooling means the system blows cold air on a heat call.
Replacement is labor-intensive because it involves recovering refrigerant, unbrazing and rebrazing the valve, evacuating, and recharging. On a system past twelve years, the cost frequently argues for replacement rather than repair, and we will say so.
What not to do
Do not chip ice off the coil. The fins and the copper tubing beneath them puncture easily, and a punctured coil turns a service call into a refrigerant leak and possibly a replacement.
Do not pour hot water over it repeatedly as a routine. In an emergency it is survivable; as a habit it refreezes and builds up worse.
Do not cover the outdoor unit. Heat pumps run all winter, unlike air conditioners in cold climates, and covering one blocks the airflow it needs.
Do not simply switch to emergency heat and leave it. That keeps the house warm at roughly three times the operating cost while the underlying problem goes unaddressed.
Keeping defrost working properly
Keep the outdoor unit clear — two feet on all sides, nothing leaning against it, no landscaping crowding it. Make sure the unit sits high enough that melting frost drains away rather than pooling and refreezing under it.
Check that no gutter, downspout, or drain line discharges onto or above the unit. Water from above is the most common cause of the solid ice blocks we get called about.
And have the defrost controls checked at fall maintenance. Verifying that a heat pump defrosts correctly takes a technician a few minutes in October and saves a confusing, expensive January.
Frequently asked
Is it normal for a heat pump to steam in winter?
Yes. During the defrost cycle the system briefly reverses to melt frost off the outdoor coil, and the melting ice produces visible vapor. It looks alarming but is completely normal and typically lasts five to fifteen minutes.
Why does my heat pump blow cold air sometimes?
During a defrost cycle the system is briefly in cooling mode, so indoor air feels cool for a few minutes. Auxiliary heat usually offsets it. Cool air lasting much longer than that suggests a stuck reversing valve, a charge problem, or a thermostat configured incorrectly.
How often should a heat pump defrost?
In frosty conditions, roughly every thirty to ninety minutes, with each cycle lasting five to fifteen minutes. In Phoenix, defrost cycles should be relatively infrequent. A unit defrosting constantly, or one that ices and never clears, needs the defrost controls checked.