Boiler Fault Finder

Cylinder thermostats

Cylinder thermostats Reasons a cylinder thermostat will fail to start: Poor terminal connections Switch contact faulty Faulty timer/programmer When the timer/programmer is on, it sends a signal to the cylinder thermostat (this should then be live on the common terminal of the cylinder thermostat). The cylinder thermostat upon calling for heat turns on the boiler and/or energises a motorised valve. Should the common terminal read no voltage, the timer/programmer must be further investigated. Common terminal: The “live in” terminal from the “hot water on” terminal of the programmer in most cases. Demand terminal: This contact will be “made” to the common when the thermostat is calling or demanding heat. Satisfied terminal: This contact will be “made” to the common when the thermostat has reached the required temperature or is satisfied. To test a cylinder thermostat: 240 V LIVE TEST   Set multimeter to correct voltage scale and check at connections (Checking for voltage on the terminals when at maximum and minimum dial settings, will ascertain the common wire, as it remains live during both these settings). Turning the timer/programmer off with no voltage reading at the traced common terminal, will prove and complete this test. Should only one pair of wires be live when at maximum and minimum settings, then the cylinder thermostat is faulty. Closed position (calling for heat) = 240V between common terminal (3) and calling terminal (1). Stat is faulty if terminal (1) does not become live when calling Open position (satisfied) = 240V between common terminal (3) and satisfied terminal (2). Stat is faulty if terminal (2) does not become live when satisfied Some models maybe marked C, NC and NO where NC is calling and NO is satisfied. Other models may be pre-wired where red is C, black is calling (NC) and yellow is satisfied (NO). Note: The Common, demand and satisfied terminals do vary on some cylinder thermostats, depending on make and model, where some will be terminal 3 for C, terminal 1 for demand and terminal 2 for satisfied. Others however, will be terminal 1 for C, terminal 3 for demand and terminal 2 for satisfied. (Generally there will be a small switching diagram inside the outer casing of room thermostats for guidance purposes). Terminals 3 (C) and 2 (NO) make on temperature rise. Terminals 3 (C) and 1 (NC) make on temperature falling….

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