Dehumidifier repair

[Bodgit]

I've been having a problem with a 5 year old Amcor dehumidifier. When the room is cold, the motor start relay keeps firing - the colder the room the more frequently it fires. I've taken the cover off and looked at the PCB which doesn't show any signs of any problems. There is a socket with a training lead connected to some sort of thermistor or something positioned directly in front of the air intake. If I unplug this, the motor start relay clicks like crazy, so it's my first suspect.

Does anyone know what this component is and if my diagnosis is likely to be correct?

Thanks, Dave.

[Andrew Gabriel]

If the unit has a humidistat, it may be that. It could be a thermostat to shut the unit down if the air intake exceeds about 30C to protect it from overheating, or drops too low for the unit to be able to do any dehumidifying. If the thermistor is actually pushed in to the evaporator pipework and fins, it's probably a freezing sensor to trigger a defrost cycle if the evaporator gets down to freezing (and/or speed up the fan to prevent freezing if it's a variable speed drive). If it wasn't for your comments about unplugging this thing affecting the sympotoms, I would also suggest the over-current trip on the compressor, which could be due to the compressor drawing excess current (check with a plug-in power meter) or could be due to the overcurrent contacts having got dirty and self-heating the trip. I would still consider that a remote possibility.

A defrost cycle in most cheap dehumidifiers is just a switch-off until the evaporator gets back up nearer room temperature. In more expensive refrigeration systems, the roles of the evaporator and condenser are temporarily swapped and run without the fan for a short time to heat the evaporator back up, and this involves relays and/or solendoids operating, but I doubt that level of sophistication would be found in a portable dehumidifier.