a commercial refrigeration system for example has converted the refrigerant from a liquid to a gas this gas is compressed by the compressor 15 after which it passes downstream through the pipe 20 into the condenser 16 the condenser 16 liquifies the pressurized gas by removing heat from the gas and the liquified refrigerant leaves the condenser 16 through the pipe segment 21 for the expansion valve 25 in the expansion valve 25 a reduction in pressure of the liquified refrigerant takes place the liquified refrigerant at a reduced pressure passes downstream into the evaporator 17 where it evaporates absorbing heat from its surroundings during a refrigeration operation of the aforedescribed nature it is not unusual for the evaporator coils to accumulate frost as the system operates this frost builds up especially rapidly where a system is operating in a high humidity environment as the frost builds up the refrigeration effect of the evaporator coils is reduced normally the hot gas defrost pipe segment 30 is closed by the hot gas defrost valve 31 while the pipe segment 21 remains open through valve 26 when a defrost cycle is called for the valve 26 is closed and the hot gas defrost valve 31 is opened the pressure regulator valve 27 is held wide open during normal refrigeration operation but is de energized during defrost and becomes a regulator of pressure upstream of the evaporator 17 with the valve 26 closing off the pipe segment 21 and the hot gas defrost valve 31 opening the pipe segment 30 high pressure refrigerant gas is delivered to the evaporator 17 while this high pressure gas is being supplied.