Wiring worcester 28I to Honeywell cm900 wireless room stat

[jamesevans]

Hope someone can help
I have a combi (it is worcester 28i junior)
Theres no time clock and room thermostat - so no control over the heating. the water side is fine.
Nothing at all here for htg though
I would like to install a HONEYWELL CM900 wireless room stat / receiver.
The instructions in the boiler manual say that to wire the boiler with a room stat, etc that you just need 3 cores from the boiler terminal to the stat terminals, which I understand.
But on the CM900 wiring diagram, am I right in thinking that the boiler to the receiver requires just 2 wires, and that the receiver itself ALSO requires a permanent live and neutral wire (supply to it)?
The reason I bought the wireless is to save running a cable down to the hallway and boxing in, etc.

Anyone wired one of these before and can help, please?

 

[D_Hailsham]

The only part which has to be wired in is the receiver (box with two lights and a button) this can be placed anywhere you like as long as it can be connected directly to the boiler. The only think to worry about is the wireless communication between this box and the control unit.

The way to check this is to attach a long lead with a 3 pin plug on one end to the L and N terminals of the receiver, plug it in and check various locations for communication from the hall, living room or wherever you intend to keep the control unit. Instructions are in the installation manual.

As for wiring up, you just need a three core cable (not two core plus earth). Connect as follows:

Brown core: Boiler Ls to Receiver L
Blue core: Boiler Ns to Receiver N
Third core: Boiler Lr to Receiver B

Now insert a wire link between Receiver's L and A terminals.

 

[jamesevans]

Thanks for your reply.
I will wire as you have stated, but I am not sure why it is this way (it is confusing me)!
I understand how a programmable room thermostat or a programmer and a standard room thermostat can be wired to the worcester 28i junior, but with the wireless room stat's receiver, i cannot get my head around it!!
what is the link for? - is it because the receiver requires a permanent live also??

 

[mogget]

... the wireless room stat's receiver, i cannot get my head around it!!

what is the link for? - is it because the receiver requires a permanent live also??

The link saves having to run another wire to the boiler. You can imagine the receiver having two sets of terminals - L&N supply, and two switch terminals.

The Worcester uses 240v switching, when you put 240v on the Lr terminal the boiler will fire for heating. So you can combine the live supply and one of the switch terminals.

 

[jamesevans]

ok thanks for explaining that.

so basically, unlike a normal standard programmable room stat (which requires 3 core from the boiler to the prog room stat), the wireless receiver requires an extra live supply to it to the switch, thus the link from the L to the A terminal??

 

[mogget]

This should help

Untitled

• mogget
• 9 Jun 2009
• 2

 

[jamesevans]

Thanks for explaining mogget, really appreciate it.

How comes a wireless receiver for some reason needs another live to it (to make the switch) whereas if i ran a 3 core to a prog room stat i wouldn't need that extra live??... (ie the link)??

 

[mogget]

It's because the wireless receiver runs off mains voltage, but programmable stats usually run off batteries.

edit - see post below

 

[mogget]

OK, I answered the wrong question there! It's because the wireless receiver is for 'zero-volt' switching, for example you could run the receiver off 240v but the switch contacts could be in a 24v circuit. So to get the switched live you need to run live to the switch!

 

[jamesevans]

OK, I answered the wrong question there! It's because the wireless receiver is for 'zero-volt' switching, for example you could run the receiver off 240v but the switch contacts could be in a 24v circuit. So to get the switched live you need to run live to the switch!

ok thanks for explaining that. i really appreciate you helping me understand how the receiver/stat works.

you're gonna flip at me but can i ask another question (which'll probably sound daft to you) :

if the receiver is volt free, then why do you need the additional 240v to the switch?!..... (sorry if i sound like a complete **** here btw, am just trying to get my head completely round it).

 

[mrgas]

James,
Forget the volt free phrase.
With your boiler Worcestergreenstar junior, the stat switches 230v.
So the reciever need permenant Live and a neutral.

The switching (roomstat bit) needs mains from the boiler and mains going back to the boiler if the roomstat says it need the heating on.

Other boilers use 24v or volt free switching and this honeyewll unit can cope with all these options hence the volt free bit.

Hope this helps and doesnt confuse.

 

[jamesevans]

Hi mrgas

i do get it (sort of)!

in the past i have only wired up normal room stats and a prog room stat, hence my confusion here

Basically, with my boiler, for a normal PROG ROOM STAT: i would need 3 core from the boiler to the prog room stat (or 2 if no neutral was required)?

But with the receiver - if my boiler was using 24v then i would just need 3 wires, but as my boiler uses 230/240v switching, i NEED 4 wires?

have i got it yet....?

 

[mrgas]

Afraid not !
Try again.
Forget normal roomstats.
If all is 230v you only need 3 wires and a link.
If the switching used ANYTHING other than 230v then you would need 4 cores.
230v Live
Neutral
Switch common (24v or other)
Switch return (24v or other)

Any help?

 

[mogget]

have i got it yet....? icon_cry.gif

Not quite..

Ls is permanent 240v supply to heating controls
Ns is neutral
Lr is switched live from the heating controls.

so if you had an ordinary mains stat (programmable or not) you would need three wires

Permanent live to stat,
neutral,
and switched live from stat to boiler to make it fire up on demand.

now since the receiver has 4 terminals, and your boiler has 3, it has to be adapted to work, so we'll start off with

Permanent live from Ls to Live on receiver,
Neutral from Ns to Neutral on receiver,
And switched live from Switch terminal B on receiver to Lr on boiler, this provides the signal for the boiler to fire up.

But we have a problem - the 240v for the switched live has to come from somewhere, and the other side of the switch isn't connected to anything!

So we connect a link between switch terminal A and the permanent live. Now when the thermostat demands heat, the receiver switches A and B together, electricity flows from the Live terminal, through the link, through the switch, and into the boiler Lr terminal. The boiler senses this and fires up the heating.

 

[jamesevans]

I get it now!! Thanks so much for helping me here guys, I really am greatful!

I guess I should've bought a wireless stat/receiver which was all of 240v then i'd only have needed 3 wires!

Anyway I get it now and will get the wiring done tonight

It's good to know why you are doing what you are doing.....



What last point, not about my wireless receiver/stat - but....
some room stats (digital display ones mainly/prog ones) do not need a neutral.... why is this.... how do they complete the circuit (is it via the battery)?? - does the battery also act as the anticipator (hence no neutral wire).?