thermostat wiring problem

[davidlaw]

was hoping for some help from all you experts out there.

i've replaced the old thermostat on my central heating, with a very simple analog dial unit. all i did was disconnect the old unit and wire in the three wires (red, blue, yellow) to the new unit - no other changes to wiring.

when the room is cold and the heating is firing, it works fine, but whenever the room reaches temperature it blows the fuse for the whole boiler circuit. is this likely to be a faulty unit, or am i missing something in the way i have wired it?

thanks

 

[holmslaw]

..

 

[Taylortwocities]

What model was the old thermostat?
What wires went on which terminals on the old stat?
What model is the new thermostat?

My guess is you have the neutral on the wrong terminal.

Edit: re Holslaw's post

Disconnect and make safe the blue wire.

The blue wire will have been for a small heating element on your old stat.

Try it like that first and see if it works but you may need to connect the blue properly as the final stage. It may need to be there so your stat works accurately.

Be careful with Holmslaw's advice. They do not need central heating where he lives so he's probably guessing.

 

[BS3036]

I suspect that the old thermostat had a neutral connection to preheat the thermostat, and the new one just has changeover contacts, so when reaching temperature is just shorting N to L. So probably one of the wires (the one which was neutral on the old thermostat) should not be connected, but put into a terminal block instead.

EDIT: Doh. Everyone finishing work at once.

 

[davidlaw]

ok, thanks.
i've currently connected all three wires. i'll try removing the neutral, and if that doesn't work i'll try swapping the neutral and switched-live (maybe?).
if that fails i'll be back in the morning with more details!!

 

[davidlaw]

Disconnecting the neutral worked a treat. Thanks all for your help

 

[Taylortwocities]

Don't forget my advice

Try it like that first and see if it works but you may need to connect the blue properly as the final stage. It may need to be there so your stat works accurately.

If you tell us what the new stat is (make and model) we can advise if the blue wire is needed, and wher it should go.