Hi.
I have been recently called to a cellar cooler system, (below ground,) in a hotel with a Searle evaporator coupled to a Prestcold 1 HP semi, that has been installed for years but has suddenly started frosting up. The thermostat appears to operate correctly and is cycling around 12 degrees. There is no defrost cycle and the evap fan stops every cycle. The unit is fully charged, the fins are totally clean. The fan is running at the correct speed. The superheat is correct and the expansion valve is feeding the coil evenly. No one has changed the stat setting or increased the size of the room, but for some reason the coil seems to ice up solid including the suction line. There is a suction line heat exchanger which I don't think has collapsed internally. This system is on R12 and is evaporating at about -1 degrees when the room is at 12 degrees.
Has anyone any ideas?
I have had this happen to me before at other sites and cannot find anything wrong.
I end up fitting a defrost clock and it solves the problem. Any suggestions would be greatly appreciated.