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I've confirmed it's working correctly now. Sending the STATUS message with ch_enable set to 1 is what it needed. The demand control into terminal 4 is working fine alongside the OpenTherm temperature...

I've just spotted an error in my code. I was sending the SLAVE_CONFIG message instead of STATUS. I'll give it another try later and hopefully that might fix the issue.

Do you think I need to do anything with terminals 7-8-9 on my boiler? At the moment there is nothing connected but I wondered if I need to loop any of these to make it fire up.

Just to be clear that's the 'terminal 4' demand signal.

I've been trying this out but I'm having some problems.
Whilst the loop is applied to the VR33 OT connections then the boiler works fine in response to the 240V demand signal as it always did.
When...

No I'm using it to generate OT messages into the boiler (VR33 interface) e.g. set feed temperature and also to query data e.g. feed and return temperature. So it's acting instead of a thermostat.

I'm just using the circuit shown here https://ihormelnyk.com/Content/Pages/opentherm_adapter/opentherm_adapter_schematic_o.png and also the same here...

I've built up my own revised board now with the 'correct' optocouplers and it now works perfectly!! I can set the boiler temperature and read back some data e.g. feed and return temperature.
Just...

Unfortunately it turns out he is in Ukraine and unable to supply these at present for obvious reasons. He was kind enough to refund my payment right away though.
I then tried ordering a similar...

By the time I allow for postage etc then it works out very similar. That is a good suggestion though as that board is pretty small and so I could stick it underneath and bypass the OT part of my...

It's a bit of both really. I also have a few other things on the same board as that particular Arduino is running several other sensors outside my house. I know I could stack the boards up but with...

That's very helpful once again. Thanks.
I did see a very occasional small change in the set point but not matching the setting I was sending. I put a scope on the OT lines and I spotted the issue...

I installed the VR33 and with the connection looped all seems to be working fine. The diagnostics on the boiler also shows that it's detected an 'ebus controller'.
I've not had any joy when I...

That's all good. I'm planning to install with an easily accessible switch so I can swap between my OpenTherm interface and a loop across the VR33. That'll make it quick to swap out if I need to do...

Thanks. Yes just need to work up the nerve to plug them in.

I understand and that is a useful suggestion. Some time back I had embarked down this path and even built an interface board for the Raspberry Pi that I had installed in my garage at the time....

I had been looking at building an eBus interface some time ago but I was struggling to find enough information on how to actually communicate with that. I have circuit designs for the eBus interface...

Sorry - slight correction to the diagram..1917

There may be a bit of confusion here. I don't have an Evohome system or any parts thereof. I've designed my own controller that runs on my home server (Linux) and this operates a simple PI control...

Yes I agree that I don't really need to control terminal 4. Thinking about it, as I'm driving the CH and HW inputs (using relays) to my Y-plan box, then in reality the demand to the boiler will...

Thanks that helps. I'd seen examples of code where people were sending 'set status' commands with 'enable CH' flags set etc. I'll assume my signal on terminal 4 will be enough to fire it and then...

Yes I know. I did have a separate thread on this. My system is connected using Y-plan and the boiler feeds CH and HW based on the valve position. Most of the time it will be constant demand (i.e....

Are you able to clarify which OpenTherm commands are needed to control the Vaillant heating? I can see that message ID 1 i.e. set CH water temperature is the key one I must use. However, I've seen...

That sounds workable for me. Thanks for the help.

That sounds perfect. Just to confirm in this arrangement - if I set the target temperature low enough the boiler will still turn off the pump (as you mentioned in an earlier post)?