When the solenoid valve is energized by a timer, it opens, allowing some of the hot gas to circulate though the tubing. On the way back to the suction side, hot gas is circulated through a capillary-size section of tubing to become partially cooled before entering the compressor. When the frost is melted, the defrost bimetal, which is wired in series with the solenoid valve, cuts off power to the valve and closes off the supply of hot gas (if the frost buildup is melted before the timer completes its defrosting cycle). If this system is used in an ice machine, instead of a defrost timer, a thermostat controls the operation of the defrost solenoid. The defrost solenoid is energized in the beginning of every harvest cycle.
Usually, the two solenoid contacts are wired to a lamp that becomes energized when the unit is in the defrost cycle.
The solenoid valve is activated by a timer at a preset time, usually midnight or 1:00 AM in commercial units.
Using this very efficient type of system, makes it unnecessary to match hot gas tubing coil-for-coil with the evaporator.