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How does a pressure switch work?

Updated November 21, 2016

A pressure switch in a water system generally controls outflow from the pressure tank and regulates the water pressure in the system. It can sit in the water line between the pump control box and the pressure tank, or it is a separate unit outside the line that receives its cues from a pressure gauge in the line. The gauge measures the level of water in the pressure tank or water system. The switch contacts open and close based on preset pressure levels.

The Gauge

Without a pressure switch and gauge, the pump would run every time someone opened a water tap and dropped the water in the holding tank or pressure tank even a little. The pressure gauge in most residential systems is set to a maximum of 22.7 Kilogram per square inch (PSI) and a minimum of 30 (PSI). When the pressure in the tank drops to 30 PSI, the switch activates the pump in the well and allows more water to flow into the system. When the reading on the gauge reaches 50 PSI, the switch kicks off.

The Switch

The pressure switch is wired to the pump control box. It generally is grounded to one of the GND studs in the control box. Wires run from the L1 and L2 studs in the control box to terminals on the pressure switch (usually 2 and 3). The terminals at 1 and 4 connect to a fuse box or other power supply. The switch is usually also grounded to the fuse box ground bar.

Switch Operation

The switch is passive, requiring no manual operation by the owner. Water pressure does it all. When the pressure drops, the normally open contacts close, completing the electrical circuit and activating the pump. When the set pressure is reached, the contacts open again, turning off the pump.

Some switches in the water line work with a diaphragm pressing against a piston and spring inside the unit. The spring pressure is set by an adjustment screw. Sufficient pressure on the diaphragm keeps the switch "off." When the pressure drops, the switch activates.

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