Now's probably a good time to mention I was a domestic electrian with British Gas for two years, I didn't like to mention it before because... well... BG, boilers... what's my problem??!!

However they didn't train us up on C/H wiring. Believe it or not I, as an "Electrical Services Division" spark could carry out any job on the wiring of the house up to and including the boiler FSCU but would be in mega trouble if I went any further, and a Cenrtal Heating Spark could do all of the stuff after the FSCU but not before it! (I was trained "in house" by BG on an "intensive course" so have no real experience outside that environment).

I have a fair idea what's needed wiring-wise but its the types of gadgetry out there that I'm clueless about, so I'm glad I asked on here now!

As for the room 'stat, it is surface mounted and we have spoken about chasing it in (we hate trunking) but it would have to stay put as 1) I can't see where else it could go (very old house and all the rooms have vastly different temp ranges and 2) I'm not lifting the floor upstairs again!

The only thing I'm wondering about is in the link to the prog stat you showed, there seems to be three line conductors and a neutral; to my stat there is only a 3 core and CPC, would I need to run a new cable for the neutral?

Thanks DA!

We've got something similar. We've got an old-fashioned timeclock on the thermostat, with on-off switches for each 20 minutes. Except rather than 'on' and 'off' the two positions are 'day' and 'night'. It allows you to maintain a 'background' temperature (our hous, in theory, never drops below 12 degrees.) so the system doesn't have to work as hard to bring it up to the warmer temperature you want in the morning and evening. Because it's got a timeclock, you ca have as many 'ons' as you like. The other quite nice thing about it is that it's wireless. The receiver fits on the front of the boiler, where a programmer would normally go, and the thermostat/programmer is powered by an AA battery, and mounted to the wall with two screws, so you can experiment with where you put it.

New toy for work, ironic when most of the snow has melted, yes that is a snow plough on the front! And yes I know I work at a supermarket! Random!

Forgot the pic -

Hi Louise.

I thought for a moment your were going to cite Part P and all that electrical malarkey... Phew

(I had to - out of curiosity - look up what 'CPC' actually stood for; I knew it was the 'earth' wire, but now know it's conductive protectio...offek, I forget...)

Anyways, both your old - and potentially new - thermostats need 3 conductors and an earth, so, nothing extra needed.

These wires are:
(a) a LIVE feed (which only becomes 'live' when your timer goes 'on'), (USUALLY RED)
(b) a SWITCHED LIVE OUTPUT which is the switched 'return' from your thermostat, and that's what now supplies the 'live' feed to turn your boiler on when your thermostat calls for heat. (USUALLY YELLOW)
(c) a neutral (USUALLY BLUE)
(d) an earth (USUALLY SLEEVED GREEN/YELLOW)

I don't know off-hand which terminal numbers they go to on your current stat, but it should be easy to find out.

The Heatmiser has terminals which, I think, are labelled L, N, A1 and A2? (Or, A1 could be 'COM' and A2 'NO'). The L & N are self-explanatory, and the A1 and A2 terminals are the actual 'switch' part. So, you connect the live (RED) to 'L' and take a link to A1 to supply the switch part. Your BLUE neutral goes to 'N'. Then your YELLOW (switched live output) goes to A2.

Your CPC is terminated in the metal back box - no need to connect to the actual unit; it's double-insulated.


All the room thermostat is is a 'switch'. Therefore all it actually needs in theory is a LIVE feed and a SWITCHED LIVE OUTPUT return - two wires. However, your old dial thermostat is a clumsy electro-mechanical device which is sluggish in operation as it uses a bi-metallic strip to detect heat - not exactly sensitive to room temp changes. Therefore, they've added a small heater inside to hurry it on! This is a resistor known as an 'accelerator resistor' (I think...) and it's turned on whenever the 'stat turns on. For this resistor to work, it needs a 'neutral' at its other end, and that is why your old stat needs a neutral! (The resistor sits under the bi-metallic strip and warms it up making it react more quickly to rising room temps. Crude, eh?!)

In your new all-singing prog stat, the neutral has a different job - to provide power to run the electronics and light up the display!

Once you prise your old stat off the wall to have a look, can you report back with which colour wire goes to where, please, and I'll confirm if it's all as I think. This is important as I don't want you just taking my word for all this - and you are the one doing the dangerous bits... There is a remote possibility that your old stat is working in 'volt free' format, and if you connect the new one up the way I've said above, your boiler will be zapped...

But I don't think so.

Rebel, stick a double-chevron on that and you can post it in the 'cool pics' forum

Mind you, it's 'cool' in more ways than one...

picture fail hang on!

It just seem's here is some kind of 4-core and cpc cable?

Where?

http://www.amazon.co.uk/Heatmiser-PRT-P ... pd_cp_kh_1

second thumbnail... roll over it and it says 5core cable required in this case...?

Ah, I see...

Not a problem - you only need 3+E. The LIVE going to the 'L' terminal ALSO links to A1. Job done! He used two separate wires for this, and I don't know why although he mentions he has a 3-way motorised valve to control, so I suspect he needs the permanent LIVE to 'L' and then it's a kind of 'switched live' (from his valve) going to A1. Usually the thermostat comes before the valve, but hey...

Anyways, you are ok - honest! (and your wiring is too... fnurr fnurrr)

When I was looking at the Heatmiser fitting instructions, there was no mention of a 'surface box', only that it would require a sunken back box. So, if your current 'stat doesn't have a sunken back box, you may need to go this route. It will certainly look hugely better this way, although there's no reason you couldn't use a single plastic surface box as used for surface-mounted single power sockets and lights, etc.