How to Adjust a Johnson Controls Low Pressure Switch
Johnson Controls low pressure switches are used for low-pressure cut-out control, pump-down control, and capacity control on commercial air-conditioning and refrigeration projects. Some low-pressure switches are adjustable, allowing you to set the desired amount of pressure from a compressor manually. This is done by adjusting screws on the switch that control the cut-in and cut-out pressure levels. The term "cut-in" refers to the lowest desired pressure level that signals the compressor to turn on. The term "cut-out" is the maximum desired pressure level or the level at which the compressor should shut off.
Use the low side range screw --- usually found on the left side, towards the back of the switch controls --- to set low side cut-in. For All-Range controls, turn the screw clockwise to raise the cut-in set point. For Micro-Set controls, turning the screw clockwise lowers the cut-in set point.
Adjust the differential screw, located on the front left side of the switch controls. Turning the screw clockwise raises the cut-in set point on All-Range controls. With Micro-Set controls, turning the differential screw clockwise increases the differential setting.
Set high side cut-out set point by adjusting the high side range screw on the front right side of the switch controls. Turn the screw clockwise to raise the cut-out set point. Note that the high side differential setting is fixed and cannot be adjusted.
If your Johnson Controls low pressure switch is a P70S or P170S model, set the reset operation for high side automatic reset or manual reset lockout with the reset button on the front of the switch. After the sensed pressure has reached the desired level, press and release the reset button to restore operation of the controlled equipment.
Cycle the controlled equipment at least three times at normal operating conditions. Monitor the equipment with a reliable pressure gauge to verify that the pressure switch is correctly set and all equipment is operating as expected.
- Never exceed the manufacturer's pressure rating of the equipment being controlled by the pressure switch.
- Do not adjust pointers past the highest or lowest indicator marks on the switch's pressure scale. Doing so could damage the screw threads, causing inaccurate operation.
Heather Kimbrell began writing professionally in 2009. Most of her work is featured online, where she publishes health and technology articles. She is a certified medical assistant and a part-time PC technician. Before pursuing a medical-assisting career, she received information technology training from Cisco Networking Academy. Kimbrell earned an associate degree in medical assisting from Fortis College of Montgomery, Ala.
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