Frozen coil
Ice forming on the indoor coil, which follows a drop in airflow or a low refrigerant charge. Running a frozen system risks liquid refrigerant reaching the compressor, so the standard response is to stop cooling and let it thaw before diagnosing.
Also called iced coil, coil icing
What is a frozen coil?
A frozen coil is ice on the indoor coil, formed when its surface drops below freezing. Restricted airflow and low refrigerant charge are the two mechanisms that put it there.
Frozen coil: how it works
The coil is designed to sit below room temperature but above freezing. When less heat reaches it, either because airflow dropped or because there is not enough refrigerant to absorb what arrives, the boiling temperature inside falls. Once the surface goes below freezing, the condensate that normally runs to the drain freezes on the fins instead, and the ice then blocks the remaining airflow, which makes it worse.
Where you meet the term
- In a system that cooled well and then stopped, with visible frost on the indoor unit.
- In a water leak that appears after the system is switched off, as the ice thaws faster than the drain can carry it.
- In a diagnosis, where the coil has to be fully thawed before any measurement is meaningful.
How to check it
- Switch cooling off and run the fan only. Fan-only operation thaws the coil faster than switching everything off, and it stops the system driving the surface colder. Owner check
- Put a towel down before it melts. If ice thaws faster than the drain can carry it away, water escapes the pan during recovery. Owner check
- Clean or replace the filter, then run again and watch. If icing returns with a clean filter and clear returns, airflow was not the whole story and the refrigerant side needs measuring. Owner check
Frozen coil: what it is not
- Chipping ice off the coil damages the fins and the tubing. It thaws on its own with the fan running.
- Running the system harder does not clear ice. Cooling has to stop for the coil to come back above freezing.
- Ice on the outdoor coil in heating is a different thing. That is what the defrost cycle exists to handle.
Frozen coil questions
How long does a frozen coil take to thaw?
Running the fan alone, for one to a few hours depending on how much ice formed and how warm the room is. Nothing measured before it is completely clear will be reliable.
Can I just change the filter and carry on?
If a blocked filter caused it and airflow is fully restored, yes. If icing returns with a clean filter, clear returns, and open registers, the remaining explanations are on the refrigerant side and need a technician.
Why does the floor get wet after the unit freezes?
Ice on the coil melts faster than the drain pan and line were sized to carry, so water overruns the pan. The leak is a consequence of the freeze rather than a separate fault.
Where this definition comes from
Written from manufacturer support article material held in the HVAC Bench evidence record. Where a document limits a definition to certain models, that limit is repeated here rather than generalised across a brand.
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