OpenTherm VR33 and Vaillant ecoTEC plus 428

[bruce_miranda]

Those people are doing this on an OT compatible boiler. Yours isn't. What you are effectively doing is almost like weather compensation. The On-Off trigger is coming via the terminal 4. But since the VR33 is able to vary the requested Flow temperature, you are using a Hybrid system. I still think you should be able to move to a completely OT controlled system. Also the OTB is able to distinguish between CH demand and DHW demand, but unfortunately Evohome doesn't use that. It just assumes 10C is OFF, 90C is Full heat and only 90C is DHW.

[Vikings]

I still think you should be able to move to a completely OT controlled system.

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 always be on. When I'm heating HW then my TPI is just controlling the diverter valve to modulate the CH rather than switching the boiler itself.
I'm keen to keep it connected that way so that I have some flexibility i.e. I can swap out the Opentherm control in software and revert to TPI if I want to.
On a related note - do you happen to know what happens when using the VR33 if you stop sending Opentherm messages to it i.e. don't maintain the minimum 1 second repeat? Does it fall back to full heat or off?

[bruce_miranda]

Care to explain how all of this is connected? I'm still struggling to see why you need the Trigger on 4 when Evohome can fire the boiler as needed via OT. HW demand does not modulation via OT and Evohome. So the boiler is always on Full heat demand then. Your Y plan valve can be controlled via a CH and DHW zone valve BDR91s also connected to Evohome. So what are you achieving by wanting to operate in this Hybrid model

[Vikings]

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 loop with scheduling of target temperature etc. I have built a room thermostat that is mainly a temperature sensor that the controller can interrogate frequently. To interface to what was the existing Y-plan system I have an Arduino that can drive a couple of relays - one for CH and one for HW. The HW is heating a storage tank and I run that a couple of times a day for about 30 minutes a go. It doesn't do that much because I also have solar panels heating the tank as well. The control algorithm produces a demand value between 0 and 1 and currently converts that to a TPI pattern of on/off to the CH demand relay. The boiler only has one water feed going to the 3-position valve (i.e. CH/HW/both) and is set around 65C at the moment.
My plan is to introduce feed temperature modulation and I've built another Arduino with OpenTherm interface. That's what I'm trying to code at the moment. The control loop will then send its demand value to the Arduino which will adjust the boiler temperature and the on/off for CH can then stay on all the time.
The complication I had was how to drive it when I wanted the HW heating. I then need to set the boiler feed to max but still have modulation on the CH i.e. revert to TPI for the CH bit during that short window.
Here's a picture of the overall wiring. My control signals are the Room Stat Receiver and the HW Control. Hope this helps.
Wiring1a.jpg

[Vikings]

Sorry - slight correction to the diagram..Wiring1a.jpg

[bruce_miranda]

The moment you said Home Brewed, everything made sense to me. I have no idea why I assumed Evohome.
Why then are you bothering with OT and a VR33. Just use an eBUS interface. The VR33 is nothing but an OT to eBUS bridge and I have discovered atleast 1 bug in its software. The Vaillant can only understand eBUS any way. And if all you want to do is vary the Flow temperature when there is HW demand then that is dead easy to do with an eBUS interface, I know because I did it. But it's even easier to do with just using the VR65 wiring Centre.

[Vikings]

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 so in theory I could connect one to my Arduino but I wasn't sure if I could get it to talk to it reliably and also to be sure what messages to be using. It looked like the documentation for Opentherm was clearer - and more open. When I saw you could get a VR33 then it seemed an easier way to go. I do have the hardware and pretty much have the software clear - assuming there is only one command I need to use.
By the way, was that a typo when you said HW demand. I'm wanting to vary the flow temperature when I have CH only but keep it fixed at max when there is HW or HW+CH - but that doesn't change anything particularly.
Do you have any documentation for how to interface an Arduino to eBUS and actually get it to control the flow temperature?
What are the bugs you found in the VR33?

[bruce_miranda]

The eBUS interface and its protocol is extremely well understood by now.
https://adapter.ebusd.eu/index.en.html
https://en.wikipedia.org/wiki/EBUS_(serial_buses)

And then use the ebusd software and then it becomes pretty much fool proof.
https://github.com/john30/ebusd

The VR33 does some weird translation of the Outdoor Temperature back to OT.

[bruce_miranda]

There wasn't a typo but I can see why it wasn't clear. What I wanted to do was raise the Max Flow temperature ceiling when HW was requested e.g. to 70C. But when there was only CH demand, I wanted to run the boiler much cooler at 60C max. Within that ceiling, I knew that further step downs would be handled by the modulation.
All this is has now been done and figured out. I was previously using a combination of Domoticz and the eBus interface to achieve this. These days I just have the VR65 that manages this for me. So I am more comfortable that none of my "toys" are relied on for switching the Max Flow. And then OT+VR33 does the flow step downs within the Max ceiling set.

[Vikings]

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. However it wasn't quite fast enough for ebusd and I ended up removing it altogether and putting everything else connected in my garage into an Arduino which was a lot cheaper and easier to maintain. The other issue I had with the ebus route was identifying what actual messages I needed to send to control the flow temperature on my boiler. ebusd just gives you the data link layer but doesn't tell you what actual messages to send. The VR33 route seemed to be a clearer path and with libraries available for the Arduino to send and receive Opentherm messages.
I've searched around and the only Arduino ebus code I can find is for loading into a dedicated device that can then talk to ebusd. I guess that might be an option i.e. plug one into my boiler and then run ebusd on my server with a TCP/IP connection between them. Is that better than plugging the VR33 in and talking to my Arduino that I've already built though? From what you were saying before I just need to send the CH_SETPOINT command (ID 1) so it sounds fairly easy. What could go wrong?;-)