Wiring a room thermostat to a combi boiler

[nospam]

I have a Ravenheat Whiter Star LS 80 boiler and all bar one of my radiators have TRVs fitted. I have recently purchased a Horstmann 099A programmable room thermostat to fit to the system to give me more control but I'm having trouble understanding how to wire it.

The diagram on the Horstman RT has three terminals COM, NC and NO. The diagram is laid out as follows: Live -> COM -> NO (it says in most cases NO but can be NC?) -> boiler, Neutral -> boiler. The boiler instructions are pretty pathetic, and just show 2 wires coming out of a terminal block and going off to a room stat. No live or neutral marked on the diagram?

I'd be grateful if anyone could tell me how to connect this up. Also, what gauge of wire is best to use.

Thanks in anticipation

[John Rumm]

The stat sounds like it implements a simple changeover switch - when not calling for heat COMmon is connected to the Normally Closed contact. When it calls for heat, COM gets switched to the Normally Open contact.

If the boiler simply shows two wires then one is probably a live and the other a switched live. So you can simply wire these to COM and NO (does not matter which way round).

(use a multimeter set to its continuity range to check the behaviour of the stat contacts is as you expect)

If you are re-using a existing cable however beware!

Many older stats had a small heating resistor in them to improve performance - this required a neutral connection to the stat as well. Hence you may find you have three wires at the stat position. One of which is neutral. This wants to be terminated somewhere safely out of the way (i.e. in a bit of chockie block connector). What you don't want to end up doing is having the stat connect live to neutral when it closes or you will end up letting the magic smoke out of something! ;-)

[nospam]

Thanks for that John.

There isn't a stat already fitted so sounds like I can just connect COM and NO to the two boiler terminals. What gauge wiring would you suggest I get to connect boiler to stat.

Cheers

[Roger Mills (aka Set Square)]

It's very unlikely to be carrying any appreciable current. 1mm^2 cable will be fine.

Connect one wire to COM. Find out which of NC and NO is connected to COM when the room temperature is *below* the stat setting (i.e. heat required) and connect the other wire to that.

[nospam]

Thanks, will let you know how I get on.

[John Rumm]

And if you want to future proof the install a little use some three core and earth cable. That way you have a spare wire should you need a neutral in the future!

[John White]

Don't forget to get "Part P" permission first :-)

John John White, Electrical Contractor

[Dave Stanton]

LOL