Steam Off the Heat Pump in Winter? That Is Defrost
On a cold, damp North Texas morning the outdoor unit of a heat pump can frost over, then hiss and steam. That is a normal cycle, but there is a version of it that is not.
September 7, 2026 · 5 min read
Heat pump owners often make their first winter call to us because of something that is not a fault. The outdoor unit is covered in frost, then it suddenly goes quiet, a cloud of steam rises off it, and the air inside turns cool for a few minutes. Then everything goes back to normal. That is the defrost cycle, and it is the system doing exactly what it was built to do.
Why a heat pump ices up
In heating mode a heat pump pulls warmth out of the outdoor air, which means the outdoor coil runs colder than the air around it. When the outdoor temperature is somewhere in the thirties and forties and the air is damp, which describes a lot of North Texas winter mornings, moisture condenses on that cold coil and freezes. A thin layer of frost is expected. A thick layer blocks airflow through the coil and the system stops extracting heat.
What the defrost cycle does
The control board watches coil temperature and run time and, when it decides the coil is too iced, it briefly reverses the system into cooling mode. Hot refrigerant runs through the outdoor coil and melts the frost off. The outdoor fan stops so the heat stays on the coil, which is why the unit goes quiet, and the melting frost turns to steam, which is the cloud. Indoors, the air handler is now effectively in cooling mode, so the system fires the auxiliary heat strips to keep the supply air from blowing cold. After a few minutes the coil is clear, the system reverses back, and heating resumes.
A normal defrost lasts a few minutes and happens a few times an hour in the worst conditions, less often when it is dry or when it is genuinely cold, because very cold air holds little moisture.
When it is a problem
- The outdoor unit is encased in ice rather than frosted. Solid ice on the fan, the top of the cabinet or the coil that never clears means defrost is not happening or is not finishing.
- Defrost runs constantly, every few minutes, with cool air inside most of the time. That points at a coil sensor, a control board or a system that is low on refrigerant.
- Ice forms at the bottom of the unit only. That is usually melt water refreezing because the unit is sitting too low or the drain path is blocked, and it can damage the fan.
- The auxiliary heat runs for long stretches and the electric bill jumps. That can mean the heat pump has quietly stopped doing its share and the strips are carrying the house.
- Water pooling and freezing on a walkway beneath the unit.
What causes defrost to fail
The common culprits are a defrost sensor that has drifted or come loose from the coil, a reversing valve that is sticking, a control board fault, and low refrigerant charge. A dirty outdoor coil makes everything worse because it restricts airflow and runs colder. Gutters that drip onto the unit and sprinklers that spray it in the evening also cause ice that no defrost cycle can keep up with.
What you can do
Keep the unit clear. Leaves, mulch and fence panels within a couple of feet restrict airflow. Make sure the unit sits on a level pad a few inches above grade so melt water drains away. Redirect any downspout that empties near it. Do not chip ice off with anything sharp; the coil fins are soft aluminium. If the unit is solid ice, switch the system to emergency heat so the house stays warm, and call (469) 338-5533.
Family-owned in Rockwall, licensed and insured. TX HVAC License TACLB2360E.
Frequently asked
Is it normal for a heat pump to blow cool air in winter?
Briefly, during defrost, yes. Heat pump supply air is also generally cooler than furnace air even in normal operation, because it warms the house gradually rather than in bursts. Cool air for more than a few minutes at a time is worth a look.
Should I pour hot water on the iced unit?
Warm water can help clear a lightly iced coil in an emergency, but it will refreeze if the underlying problem remains, and it should never go on the electrical components. It is a stopgap, not a fix.
Why does my heat pump steam when it is not even that cold?
Damp, mild weather in the thirties and forties produces the most frost, because the air carries plenty of moisture and the coil is below freezing. It is the classic North Texas winter morning.
Can I turn off defrost to save energy?
No. Defrost is a control-board function, not a setting. A coil that cannot defrost stops heating, and the electric strips take over at several times the cost.