Worcester Bosch 40Cdi - wiring new thermostat

pt44 said:

So If I was connecting a two wire stat - I would use?

Ls & LR?

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yes as I've said (twice now )

pt44 said:

That's what I don't understand. In theory my underfloor heating control should put out two wires - they demand the boiler to turn on. But, they are listed as a Neutral and a s/l (I presume switched live). So, I connected them to Ns (neutral) and LR.

I probably tried Ls at some point and got a nice bang Didn't do it again.

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they are listed as a switched live and neutral supply to the boiler
your boiler already has a neutral, therefore it only requires a switched live, which why you don't req the ufh N out to boiler Ns connection

I would have wired the switched pair to the controller and just used the volt free contacts, I also would have fed the controller supply from Ls and Ns but dont worry about it
I can see how you are getting confused though

Matt

So - to summarise

You are saying my s/l wire (labelled s/l on UFH), should go to Ls

and the Neutral wire (labelled neutral from the UFH) should go to LR on the boiler

or the other way round?

pt44 said:

So - to summarise

You are saying my s/l wire (labelled s/l on UFH), should go to Ls

and the Neutral wire (labelled neutral from the UFH) should go to LR on the boiler

or the other way round?

Click to expand...

No Niether!

s/l on UFH), should go to Lr.............that is all you need
just leave the neutral where it is or completely disconnect it from both ends

So I'm only connecting one wire?

That seems odd. But at the same time it does make sense. So its like the UFH is sending a pulse along the wire, to turn fire up the boiler. When it stops, there is no pulse and so the boiler shuts down.

When I do that (will give it a try tomorrow in the daytime) does that mean the UFH will be able to fire the boiler - and the wireless will still be able to fire it too? Or will this method disable the wireless stat? Ignore my zone valves issue, that's something I can sort later once I've got this part sorted.

So, if I wanted to disable the UFH side - and simply have a new wired stat. That would be different wiring? I presume a new stat (on its own - no ugh) would need two wires to tell the boiler to fire up? Or does it all depend on the type of stat - i.e. some have 2 some have 3?

Firstly thank you for all the help so far - it really is much appreciated.

I've removed the Neutral wire from the UFH to Boiler connection. I now have only 1 wire connected from s/l on the UFH to LR on the boiler.

Result - the system still works.

However, it works exactly as it did before. The UFH has to be ON - AND the wireless stat has to be ON (i.e. both demanding heat) for the boiler to fire up. Neither stat has total control. So, in reality I've not solved anything. Sorry.

I don't get it. I really don't. The UFH is basically a wired stat (if we simplify it). So, when it demands heat, the switch goes on and tells the boiler to fire up. But it doesn't in my case. The wireless stat is still connected somehow - it doesn't bypass the wireless. How can that be? I cannot believe that its not possible to wire a 40Cdi with an wired stat - surely it must be possible?

I've Solved the problem

I spoke to Worcester Bosch again - but this time asked a different question. If I want to install a hard wired thermostat programmer, do I need a new plug in board?

The boiler currently has a plug in board on the front, that got installed when I installed the DT10RF wireless optimiser. As I suspected all along - this plug in board is the problem. Because it's hard wired (via its plugin) it totally overrides all other commands sent to the terminals inside the boiler. That's why the wireless stat has to be fired up to get those terminals to react to my UFH controller.

Solution - simply remove the plug in board.

I was under the impression that the boards also controlled other functions like the Water Pre-Heat etc. But the tech guy assured me this is actually a built in function of the boiler, the board doesn't affect this. It's only function is to act as the receiver for the Wireless Optimiser. Hurrah - now I can finally move on to the next stage - New Stat. Going for a Heatmiser, that I can control from my iPhone when I'm away.