HVAC Evaporator Defrost cycle
When the three-position selector switch is turned to defrost, solenoid valve A and pilot-solenoid valve D close as hot-gas valve C and evaporator-pilot valve B open. This allows hot gas to enter the evaporator. Valve D now acts as a back pressure regulator, maintaining a predetermined pressure above the freezing point. After a regulated delay, preferably toward the end of the defrost cycle, the time delay relay allows the water solenoid to open. This causes water to spray over the evaporator, melting ice that may be lodged between coils and flushing the drain pan.
When the evaporator is defrosted, the system is returned to the cooling cycle by turning the three-position selector switch. The hot-gas solenoid (valve C) and built-in pilot (valve E) close as the liquid solenoid (valve A) opens.
This system can be made completely automatic by replacing the manual selector with an electric time clock. Table 10-2 shows some of the valve sizing for the low-temperature system.