HVAC Pressure Control Switches

One safety feature for air-conditioning units with a compressor and condenser is a pressure-controlled switch. This switch is wired into the circuit to protect the system in case the system develops a leak. If a leak develops, it is possible to draw in moisture and air and damage the whole system. If the pressure builds too high, it can cause a rupture of any of the joints or weaker points in the system.

A low-voltage (24-V) relay is wired into the 240-V line that supplies the compressor motor. The relay contacts are wired into the supply line for the motor (see Fig. 14-22). The solenoid of the relay is wired in series with two pressure-operated switches. If the pressure builds too high, the high switch will open and cause the solenoid to de-energize. If this happens, it causes the contacts of the relay to open. This removes power from the compressor motor. If the low-pressure switch opens, it will do the same thing. This way the compressor is protected from both high- and low-pressure causing damage to the system.

Both manual and automatic controls are available. Automatic controls reset when the pressure is stabilized in the system. If it is not stabilized, it will again turn the system off and keep recycling, until it reaches the design pressure.

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