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Help with changing thermostat

Postby erp » Fri Mar 12, 2010 9:30 pm


I currently have a faulty 1 year old honeywell circular thermostat on my system (it permanently thinks it is 10 degrees in our house). I would like to replace it with a digital programmable one.

The problem I have come across is that the current thermostat has 4 wires connected to it (one earth, one blue linked to one grey/black, and one brown), whilst the new one only requires two. (live and neutral). All wiring in my house is only one year old.

Can I use the new thermostat, and if so, what do I do with the two spare wires? Wrap them up in tape?

Thank you very much for any help!
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Postby sparx » Mon Mar 15, 2010 7:49 pm

Hi, you are wrong in your assumption, stop!
your new stat cannot operate with a live and Neutral!
Some battery powered stats will use live and SWITCHED Live not a Neutral.
Ignoring the earth for a moment, the 3 wires connected to old stat should be :Brown=live from programmer/clock.
the remaining 2 wires being black and grey, by suggested convention should have grey sleeved blue as neutral and black sleeved brown as switched line (call) for boiler/valve if fitted.
The neutral is needed to operate a small internal heater inside stat. called an accelerator which cuts down the 'dead band' or hysteresis loop. put another way it should make the stat respond to temp changes of 0.5deg,
without Neutral takes more than1.5deg to react.
from your discription I would query the original connection rather than blame the stat.
NOT everyone uses same colour coding so take care or get help from ELECTRICIAN not plumber.....
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Postby MrEnergySaver » Sat Mar 20, 2010 12:09 am

I agree. The old thermostat will have had and earth, a live in from the programmer, a switched live out to the valve and a neutral. This is because it is a mechanical thermostat and requires the anticipator for accuracy.
The digital themostats only have two terminals, live in and switched live out.
If you have a Honeywell thermostat, all the wiring details are on their website (google "")
You can download their wiring book, or just the page you need. You have to register first (a bit of a pain, but they don't send you anything). You can also get the installation instructions for most of their current stuff.

Come to think of it, in the package, there is usually a wiring diagram for you to follow anyway.
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