The coil has iced up

Why it ices at all

A dehumidifier condenses water by making a coil colder than the air's dew point. As the room cools, the dew point falls, so the coil has to run colder to keep condensing. Eventually the coil surface goes below freezing and the condensate freezes onto the fins instead of running off them.

That is a runaway. Ice on the fins blocks airflow. Less airflow means less heat reaching the coil, which makes the coil colder still, which makes more ice. Left alone it will pack solid, and at that point the machine is drawing its full power and removing nothing at all.

What auto defrost does

It breaks the runaway. When a sensor detects the coil is too cold, the unit stops the compressor and keeps the fan running, so room-temperature air melts the ice off the fins. Then it restarts. Nearly every current unit has this, and several - the GE, the AlorAir crawl space units, the Waykar models - advertise it explicitly.

So a machine that appears to be running with the fan only, in a cold room, is quite likely defrosting rather than broken. Give it twenty minutes before concluding anything.

When icing is normal

• Room below about 60F. Output is already falling and icing becomes routine. The defrost cycle will run more often, which means less time actually dehumidifying.
• Room near or below 41F. The unit spends more time defrosting than working. This is not a fault - it is a compressor being used outside its useful range, and the answer is a desiccant machine. Desiccant versus compressor.
• An unheated garage or crawl space in shoulder season. Same reasoning. Frigidaire's Wi-Fi pump model claims operation down to 41F, which is at the optimistic end of what any compressor unit will do.

When it is not, and what to check

1. 1The filter. This is the cause the great majority of the time. A clogged filter starves the coil of air, the coil runs colder than it should, and it ices even at 70F. Most are washable - pull it, rinse it, dry it, refit it. In a garage or workshop, check monthly rather than seasonally.
2. 2Blocked intake or exhaust. A unit pushed against a wall or into a corner cannot move air. Manufacturers generally specify clearance around both faces.
3. 3Fan not running, or running slowly. No airflow at all guarantees icing. If the fan is silent while the compressor is running, that is a fan or control fault.
4. 4Low refrigerant. A sealed-system leak causes the coil to run colder than designed. This is the one on the list that is not a home repair - it needs recovery equipment and certification to touch, so it is a warranty call.

The clearance nobody leaves

A dehumidifier pulls air in one face and pushes it out another, and it needs room around both. In practice most people put them in a corner, against a wall, behind boxes - which is exactly where a basement has spare floor space.

If the unit ices in a warm room and the filter is clean, move it into open floor before assuming anything about refrigerant. It costs nothing and it is a surprisingly common cause.