Steam, ice and sound: normal heat pump behaviour
A heat pump defrost cycle is a short, automatic period when the unit melts frost off its outdoor coil, and it is normal in cold, damp weather. Some steam, a change in sound and a brief pause in warm air from the indoor unit are all expected.
Why does a heat pump need to defrost?
In heating mode the outdoor coil is colder than the surrounding air. When the air is humid, moisture condenses and freezes on the coil, and a layer of frost reduces airflow and efficiency. The unit therefore reverses the refrigerant flow for a few minutes so the coil warms and the frost melts. Most defrost cycles last several minutes and occur at intervals set by the controller.
Frost is most likely around freezing, say -5 to 5 C, with high humidity. In very dry, deep cold the air carries little moisture, so defrosts are less frequent.
What will you see and hear?
- A cloud of vapour from the outdoor unit that looks like steam.
- Water draining from the base, which can refreeze if it lands on a walkway.
- A whoosh as the reversing valve changes, followed by a different fan and compressor sound.
- Cooler air from the indoor unit for a few minutes, since heat is being redirected to the outdoor coil.
Ductless units may pause the indoor fan to avoid blowing cold air. Ducted systems often turn on backup heat during defrost, which is why the controller setup matters, a point covered in the guide to dual-fuel systems.
When is it a problem?
Defrost is not normal if the unit is encased in ice, if the coil never clears, or if the cycles run back to back for long periods. Causes include a blocked or dirty coil, poor airflow, a refrigerant fault or a failed sensor. Do not chip ice off a coil, because the fins are easy to damage. Switch the unit off and call a technician. Routine checks that reduce the risk are listed in the heat pump maintenance checklist.
How loud is a heat pump, and where should it go?
Outdoor units produce a steady hum from the fan and compressor, quieter on low speed and louder at full capacity. Manufacturers publish sound ratings in decibels, so compare them. Placement matters as much as the rating. Avoid putting the unit under a bedroom window or against a wall that reflects sound toward a neighbour, and check local bylaws for setback or noise limits.
Plan for snow, too. Mount the unit on a stand above the typical snow depth, keep a clear air path on all sides and keep drip water off paths. Roof run-off and drifting snow should not bury the coil. The sizing and rating questions in cold climate heat pump selection also affect how hard the unit runs.
Do ground-source units defrost?
No, a ground-source heat pump does not have an outdoor coil exposed to the weather, so there is no frost cycle. The borehole or loop field supplies heat from the ground, where temperatures are steady. The Drake Landing system took this idea much further, storing summer heat in 144 boreholes, as described on the borehole storage page. The trade-offs with air-source units appear in air source versus ground source.
What can you do to reduce noise and ice problems?
Choose a model with a published low sound rating, and ask whether it has a night or quiet mode that limits fan and compressor speed. Mount it on vibration-damping pads so the hum does not travel through the structure. Keep it away from property lines and from windows of rooms where people sleep.
For ice, aim downspouts and roof run-off away from the unit and from walkways. A shield or small roof above the unit can keep heavy snow and icicles off the coil, as long as it does not block airflow. If you are still deciding what to buy, the cost guide lists the site items that tend to add to a quote.
Quick answers
Is steam from my heat pump normal?
Yes. During defrost the warm coil melts frost, and the moisture can rise as vapour. It looks dramatic but is harmless.
How often does a heat pump defrost?
It varies with temperature and humidity. In damp weather near freezing it may defrost every hour or so, and in dry deep cold much less often.
Can I stop a heat pump from freezing up?
You cannot stop frost forming, but you can keep the coil clear of snow, leave space around the unit and have it serviced so the defrost controls work as designed.