Reasons a room 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 room thermostat (this should then be live on the common terminal of the room thermostat). The room thermostat upon calling for heat will either turn on the boiler or energise a motorised valve.
Should the common terminal read no voltage, the timer/programmer must be further investigated.
To test a room 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 room thermostat is faulty.
A method to test the room thermostat where (1=Common / 2=satisfied / 3=calling):
- Remove the wire from terminal 3 in room stat (calling)
- Ensure terminal 1 (common) is live
- Turn room stat to maximum setting so it is calling
- Terminal 3 should now be live – if not, room stat is faulty
Note: The Common, demand and satisfied terminals do vary on room 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).
The “live in” terminal from the “central heating on” terminal of the programmer in most cases.
This contact will be “made” to the common when the thermostat is calling or demanding heat.
This contact will be “made” to the common when the thermostat has reached the required temperature or is satisfied.