Wiring advice.

[Humph]

Need some advice on wiring a room stat.Stat is already fitted but was not connected to new boiler.Inside stat has 3 wires,Red-1 Blue-2 Yellow-3.
Problem i have is boiler only has a A and B connection on it.
I had a freind connect it for me but i'm not sure its working correctly.
Can anyone tell me which 2 wires should be used and does it matter what wire goes to A and B.

 

[Taylortwocities]

Need more info.
Most stats need 3 connections Permanent live, neutral and call for heat (like when it gets cold).

What sort of boiler do you have (make model?)
and what sort of central heating system is it?
Does yr heating system have valves to control heating & hot water, or is it a gravity (non pumped) system?

Is there a timeswitch and a wiring centre where all teh wires go to?

A lot of new boilers have just two wires to switch it on but these go to a wiring centre where everything gets connected.

 

[Humph]

Boiler is a Alpha CB24X Combi.
Front panel on boiler has 2 stats,1 for water,1 for heating. There is a timer switch also.
I have to unscrew front panel for it to drop down and there are 2 connection blocks.1 to power boiler and 1 to power room stat.
The room stat is a honeywell,model unknown.
Heating does come on when temp drops but it doesn't seem to know when to switch off. Also once heating comes on the boiler stat tends to take over and switch boiler on and off repeatedly

 

[Steve3948]

Need some advice on wiring a room stat.Stat is already fitted but was not connected to new boiler.Inside stat has 3 wires,Red-1 Blue-2 Yellow-3.
Problem i have is boiler only has a A and B connection on it.
I had a freind connect it for me but i'm not sure its working correctly.
Can anyone tell me which 2 wires should be used and does it matter what wire goes to A and B.

Boiler Connections.
2 = Live Out to Common on Thermostat (use red)
1 = Switched Live return from Thermostat (use yellow0
Neutral (use blue) connect to Neutral connection on combi.

Make sure your red goes to the common in the stat, with the yellow to the switched live return and the neutral to the neutral connection.

 

[Steve3948]

Remember to remove the link between 1 & 2 on Combi connections, before connecting thermostat.

 

[Steve3948]

See http://www.harrian.com/Prod_info/info/alpha/cb24_28.pdf

 

[Humph]

Thanks for your replys Steve,just checked all wiring from stat.
cable connected to 1 in thermostat runs to 2 on boiler.Cable connected to 3 on stat runs to 1 on boiler and connection 2 on stat is not connected to anything on boiler.
Problem is i dont understand mini diagram inside stat cover.

I think the stat model is a honeywell T40.
I hope this helps.

 

[Steve3948]

The live on the T40 is in terminal 1, the neutral to terminal 2 and the switched live to terminal 3.

 

[Steve3948]

Neutral on 2 from stat needs connecting to the neutral terminal in boiler, to operate the shunt in the stat, if not it will take hours to operate.

 

[Humph]

So if i read this correctly.Terminal 3 on stat is switched live and should go to position 1 on boiler.Terminal 1 on stat is live and goes to position 2 on boiler,which is as it is .Terminal 2 on stat is neutral and i have got to connect this to the neutral on feed to boiler??

What difference will this make to how boiler works???

Thanks for your help.

 

[Steve3948]

Without the Neutral shunt, the contacts will take for ever to make and break on the stat.

You need the shunt to make the stat function correctly.

 

[Space cat]

Inside the stat there is a very small heater and it's connected between live-out and neutral. When the temperature falls and the stat switch closes, it also switches on that heater. Result: the stat opens again. The heater cools and the stat closes - and so on ad infinitum. "What's the point?", I hear you ask.

The switch has hysteresis. That is to say that as the temperature changes, either up or down, nothing happens until it reaches some critical point and the switch contacts jump cleanly open - or shut. It has to be done this way. If the contacts drifted slowly apart they would soon burn up.

Bcause of this hysteresis the switch-off temperature is higher than the switch-on temperature. That's not something you really want in a thermostat. That little heater, often called an accelerator heater, makes the switch cycle on and off. If the temperature is very low the switch will be closed. As it rises the switch opens briefly but spends most of its time closed. As it rises further the switch spends more and more time open. It's what electronics engineers call pulse width modulation.

If you don't connect the neutral wire the heater can't function. Your room temperature will have to swing between the two switching points of the stat. By now you can probably work out what would happen if you got live-in and live-out mixed up.

 

[Humph]

Thanks for your reply.

So from what i understand my stat is connected correctly,live in-live out.
But i need to connect the neutral asap our else it will damage stat and heating will not function correctly.

 

[Space cat]

Leaving the neutral out won't damage the stat but it will stop the heating from functioning correctly thus:

Heating does come on when temp drops but it doesn't seem to know when to switch off.

PS: Once you have neutral connected you can do a LOT of damage if you mix up the wires at the stat. Think what will happen if you accidentally connect one of the switch terminals to neutral. It's happened before:

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[Humph]

Ok, i'm 99% confident that the 2 wires already connected are the live and live switch and that they are connected right way around.
I will connect the spare wire to the neutral on boiler and see what happens...

Might leave it untill tommorrow though