Heat pump with ice, steam or water underneath: what is normal and when to call for service
In winter, heat pump owners see it do things a boiler never does: it frosts over, puffs out a cloud of steam, leaves water underneath. In Germany “Wärmepumpe vereist” is searched 590 times a month, “Wärmepumpe dampft” 480, condensate drainage another 480; in France “pompe à chaleur givre” 170 per variant. Here we explain why it happens, where the water must go and when you should call for service.
01
Why the coil frosts up
To take heat from the air, the outdoor coil has to be colder than the air. Air carries vapour, and the colder it is the less it can hold: one kilogram of air at +2 °C and 85% humidity contains 3.7 grams of water, but in contact with a coil at −5 °C it can hold only 2.5. The rest settles on the fins as frost. Why the fin spacing matters is explained in the article on fins.
02
How much frost costs in output
A team from Aarhus University and the Danish Technological Institute calculated how much a coil loses as it frosts up with air at +2 °C: after one hour without defrosting, 9% of its output with air at 50% humidity, 34% with air at 85%. That is why the unit has to melt the frost, and when it does it matters: ours defrost when the coil is blocked, not on a timer, as we explain in the article on defrosting.
03
The steam is the defrost
When it defrosts, the unit reverses its cycle for a few minutes: the coil warms up, the frost melts and part of it evaporates into the cold air. That is the white cloud you see rising while the fan is stopped. It is not smoke: nothing burns, it is water. When the defrost ends the fan restarts and the cloud disappears.
04
Where the water goes
Defrost water drips from the tray under the unit, and on humid days there is a lot of it. It must go where it does not freeze or cause trouble: never onto a path or pavement, where in winter it turns into a sheet of ice; into a gravel soakaway below the ground’s frost depth, or into a drain where local rules allow, with the pipe protected from frost. The unit must be raised off the ground so snow does not block the coil and water can flow away. How it is installed is in the article on installation.
05
When to call for service
Frost that comes and goes, steam for a few minutes and dripping water are normal operation. Worth checking are a coil that stays completely white for hours while the house cools down, steam all the time with the fan running, a block of ice growing under the unit and fan blades touching ice or snow. What is checked every year is in the article on maintenance.
06
In real cold, less frost
It sounds odd, but the worst frost does not come with dry cold: at −10 °C one kilogram of air at 85% humidity carries 1.4 grams of water, less than half as much as at +2 °C. The hardest days are humid ones around zero, with fog or wet snow. How the unit works below zero is in the article on cold weather. If you want an opinion on your installation, ask us for information.
07
Frequently asked questions
Is it normal for a heat pump to ice up?
Yes: the outdoor coil works below zero and the moisture in the air settles on it as frost. The unit melts it with a defrost. It is not normal if the coil stays completely white for hours and the house cools down.
Why is my heat pump smoking?
It is not smoke, it is steam: during the defrost the coil warms up, the frost melts and part of it evaporates into the cold air. It lasts a few minutes, with the fan stopped.
Why is water coming out under the heat pump?
It is the melted frost from the defrost. It must drain into a gravel soakaway or a frost-protected drain, never onto a path.
Where does heat pump condensate drain?
Into a gravel soakaway below the frost depth, or into a drain where local rules allow, with the pipe protected from frost. Not onto pavements or paths, where it freezes.
When does a heat pump frost up most?
With humid air around zero, fog or wet snow. In dry cold the air carries little water and frost grows more slowly.
When should I call for service?
If the coil has been white for hours and the house is cooling down, if steam comes out even with the fan running, if a block of ice is growing under the unit or if the blades touch ice or snow.
Sources: Alvandifar, Kristófersson, Forooghi, Frost formation non-uniformity in a multi-circuit V-shape evaporator, International Journal of Refrigeration 157 (2024) · WMO, Guide to Instruments and Methods of Observation (WMO-No. 8), Magnus
See also: Heat pump not heating or using too much electricity: the real causes and what to check
All the technical tips are on the series page.
