replacing a faulty room thermostat

dmx005 · 06-09-2008, 11:03 AM

Hi all,

would be grateful for some advice. Our room thermostat is faulty and I have replaced it with a new one. There are 3 wires, red, yellow & blue although the blue is cut short and wasnt connected to anything in the old thermostat.

There are 3 terminals on the thermostat and Ive connected the yellow to 'on', the red to whats only numbered as '2' and no cable in the terminal marked 'off'.

When I put the power back on after connecting, the boiler starts but the thermostat doesnt switch it off.. Ive tried other combinations with the cables but it seems to be either always on or not at all.

Should I be using the blue (neutral I guess) in the terminal marked 'off'?

any advice much appreciated.

hufartd · 06-09-2008, 11:10 AM

No.
The nuetral is there for a pre heating resistor that some themostats use to even out large swings in temp.

dmx005 · 06-09-2008, 11:12 AM

Quote:

Originally Posted by hufartd

No.
The nuetral is there for a pre heating resistor that some themostats use to even out large swings in temp.

thanks.. so any ideas why the boiler is always on regardless on whether the thermostat dial is switched on of off..? There are not many combinations - Ive only got 2 cables and 3 terminals to play with!

diagram & my wiring attached..

neilball · 06-09-2008, 2:15 PM

The wiring diagram shows that terminal 1 should be your live feed from the boiler/boiler control circuit and that terminal 2 is the switched live back to whatever is to be controlled. Depending on your current wiring arrangments the stat will be wired in series with your central heating timeclock/programmer and then feeds back to the boiler directly (if it's a combi) or to a 2-port or 3-port zone valve (standard S-plan or Y-plan arrangement common in most houses). In the s-plan/y-plan arrangements the boiler is activated by a switch in the valve actuator so there could be 2 problems if the heating is not switching off - the new stat could be faulty or the end switch of the actuator has failed and is constantly supplying a feed to the boiler regardless of whether the valve is open or not.

*Charlie2* · 08-09-2008, 9:42 PM

If you turn the dial to the lowest temp and slowly increase it do you hear / feel a click ? the switch breaking ? could be that the stat isn't switching properly ? do you have a meter ?? so you can test the switching of the stat ?

KW1816 · 09-09-2008, 10:42 AM

The thing to remember is that the thermostat is simply a switch. To find out where the fault lies try this:
Remember to turn off the power before making or breaking connections - potential 240V on these wires

Disconnect the thermostat and leave the 2 wires apart. Turn on the heating (making sure that the hot water is turned off). Note what happens.
Turn the power off - connect the 2 wires together (using abit of choc block or similiar) and turn it back on. Note what happens.

If the same thing happened in both states then the fault is not with the thermostat

If it did turn off and then on in both cases then you now know whether the boiler expects a normally open (NO) or a normally closed (NC) connection. Work out which connection is which on the thermostat and Bob's your auntie

Make any sense ??