Central Heating Thermostat, only live and negative, where's the switched?

JollyGreen · 24 Jun 2006 at 17:10

Right!
New thread for this one I think!

I've just been decorating (innumerable threads have shown), and I ripped out (and chucked away, DOH) my old wall thermostat as it was pretty ugly.

Fitted a new one, (easy enough one might think) and this one requires three wires where I have two!

It wants, live, neutral and switched.
I only have the first two! (red and brown)

What do I do to make it work gang?

Mel_P · 24 Jun 2006 at 17:17

Take it back and get a 2 wire one?

fatty1 · 24 Jun 2006 at 17:38

maybe link the live in the switched side??

do you have a multi meter to see how it operates??

Adam_151 · 24 Jun 2006 at 17:43

Your existing thermostat does not require a neutral, you have red as live and black (I'm assuming the black just looks brown) as switched live, it should have a bit of red sleeving on though (though if its been worked on recently then it could be brown, which is another reason it might be brown...)

Anyway, either replace the thermostat with another one that doesn't need a neutral, or bring a neutral in by swapping the T&E for 3C&E ... do not under any circumstances mis-use the earth as a neutral conductor (or anything other than earth), or use the earth as a return path

JollyGreen · 24 Jun 2006 at 18:00

Cheers guys, just went to B&Q to have a look at 'em, and as far as I can see they all require neutrals.
Where do I lay my hands on one that doesn't?
And what's it called?

Also, am I right in thinking that to test my Central Heating I can just connect the red and black wires? Or would that result in me getting electrocuted and probably a small fire?

EDIT: and Adam, on further inspection you are correct, 'tis black not brown

Mickey_D · 24 Jun 2006 at 21:27

All a two wire thermostat does is connect the two wires with a switch.

You should be okay testing the system by just connecting the two wires together.

Of course I would throw the breaker for the heating system to the OFF position, then connect the two wires, then and only then reapply power to the system. Not sure what your system is like, but my thermostats directly control the 220V, not llike some of these programmable ones that run a 24V circuit....

Adam_151 · 24 Jun 2006 at 21:38

Yes its very unlikely you'll have anything other than 230v AC (240v in reality, its only 230v offically btw) at those wires, so cut the power to the CH first, near the boiler there should be an FCU (looks like a light switch, but has a pull out fuse drawer as well ( http://www.tlc-direct.co.uk/Images/Products/size_3/AA13SCF.JPG )) switch the switch off and pull the fuse out, then find a way to check its actually dead.

Its possible that the CH will have a dedicated breaker at the consumer unit ('panel' for USA folks following this

) but as its only a small load for an some control electronics and an ignitor for the gas, its pretty common for it to be added to a socket circuit

Mel_P · 24 Jun 2006 at 22:18

See these?

http://www.gasman.fsbusiness.co.uk/images/hwm.gif
http://www.gasman.fsbusiness.co.uk/images/gpl.gif

of example wiring set ups.

Trigger · 24 Jun 2006 at 23:55

JollyGreen said:
Cheers guys, just went to B&Q to have a look at 'em, and as far as I can see they all require neutrals.
Where do I lay my hands on one that doesn't?
And what's it called?

Also, am I right in thinking that to test my Central Heating I can just connect the red and black wires? Or would that result in me getting electrocuted and probably a small fire?

EDIT: and Adam, on further inspection you are correct, 'tis black not brown

It doesn't need a neutral to work. It should really have an earth though. Don't go connecting the two cores together though, although it would only do what the thermostat does, it's probably not the best idea.

I don't mean to sound rude but you really shouldn't be doing this. It is allowed under Part P but you don't sound competant enough to be doing it- no offence meant to yourself though

JollyGreen · 25 Jun 2006 at 01:34

Lol none taken mate, I'm not really, but I'm trying to save money and only electrocuted myself once so far

Mel_P aah, interesting, HWM.gif would appear to be most like my setup!

JollyGreen · 25 Jun 2006 at 12:04

Hmm, well I ended up crossing the wires which did the trick and turned the boiler and pump on.

However it looks like the pump got turned on while the system was drained and is now.. somewhat dead (making horrible GNGNGNGNAARHGH noises).

Anyone got any tips/tutorials on replacing the pump?

Sputnik II · 25 Jun 2006 at 12:22

JollyGreen said:
However it looks like the pump got turned on while the system was drained and is now.. somewhat dead (making horrible GNGNGNGNAARHGH noises).

Anyone got any tips/tutorials on replacing the pump?

Pumps make that noise if there is air in the system. Try bleeding the air from the system at all bleed points(radiators and all high areas of the system). If you do have to replace it, Its a fairly straight forward job. Once you have switched off the mains supply to the heating system disconnect the wiring to the pump. You should have a brass 'pegler' type ball valve either side of the pump. Turn these 90 degrees with an allen key and undo the large nuts that secure the pump (you will get about a cup full of water out the pump).
Clean the surfaces of the ball valves and fit the new pump making sure that you fit the rubber washers either side and that the flow is in the same direction as the previous pump.