Boiler Fault Finder

Low water pressure switches

Low water pressure switches Reasons a low water pressure switch will fail to start: Damaged / loose wiring harness Switch contact faulty Blocked hole (sludge deposits) Calibration of switch faulty Air trapped within switch The low water pressure switch receives 230V AC from the printed circuit board via the (C) and upon adequate system pressure being applied (0.5 bar and above typically) the switch will then return 230V AC back to the printed circuit board via the (NO). The switch when new or when there is inadequate system pressure applied, will rest in the (NC) position, and only upon there being adequate system pressure again, will it switch over to the (NO) position. Not all boilers incorporate these switches to detect a low water system pressure, but instead rely just on the high limit thermostat to protect the main heat exchanger. Some switches are only a two wire connection only (there is not a NC connection). On some pressure switches, the manufacturer may need to be consulted in order to identify the relevant terminal connections for correct operation. To test a low water pressure switch (pressurised state): Electrically isolate boiler / system controls Remove wires from terminals Set multimeter on ohms scale for resistance readings, across pressure switch terminals: Measuring between C and NC – No resistance/infinity = ok Measuring between C and NC – closed circuit of less than 1 ohm = faulty Continuity between C and NO = ok Continuity between C and NC = faulty To test a low water pressure switch (unpressurised state): Electrically isolate boiler Remove wires from terminals Set multimeter on ohms scale for resistance readings, across pressure switch terminals: Measuring between C and NC – closed circuit of less than 1 ohm = ok Measuring between C and NC – No resistance/infinity = faulty Continuity between C and NC = ok Continuity between C and NO = faulty…

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