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Thread: OpenTherm VR33 and Vaillant ecoTEC plus 428

  1. #31
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    If you have already invested in a VR33 and an OT interface then might as well use them.
    And you are right, ebusd needs a host.

  2. #32
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    Thanks. Yes just need to work up the nerve to plug them in.

  3. #33
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    Plugging the VR33 with its own OT jumper in place, while you still use terminal 4 to fire up the boiler will basically give you a Normal On Off system, but with a target flow of 90C. That target will never be reached because your boiler CH knob will clamp the max to whatever you have it set.

  4. #34
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    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 any fault-finding.
    Am I right in thinking that if my device stops sending repeated commands at the required rate then the VR33 will drop to its 'off' state? i.e. target temperature minimum and therefore turn off flow?

  5. #35
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    No idea what the fail safe of the VR33 is. All I can say is jumper out is 10C and jumper in is 90C (I think). I have only had the VR33 connected to an approved OT device.

  6. #36
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    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 connected by Arduino to it so far. No reply detected. I'll need to do a re-test on my interface circuit to make sure it's working as it was meant to. The circuit I'm using is the 'master interface' half from the one shown here https://create.arduino.cc/projecthub...arduino-7818f5
    I suspect OpenTherm signals are going to be quite tricky to debug given the way they use combinations of voltage and current.
    I presume if feed temperature commands are getting through then I would see a change in the D.5 value on the boiler or perhaps D.9?

  7. #37
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    D.9 is the all important value to watch. The beauty is you can have this value displayed on the boiler and watch it changing.

  8. #38
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    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 with my interface board. The rise time was far too long and it wasn't reaching a high enough voltage. It had been working when I had done basic level checks at DC but not at operating speed. When I copied the circuit design I had substituted a different optocoupler and it turns out to have a very slow switch off time - doh!
    There will be a short delay while I produce a revised version of the board with the correct optocouplers. I'm quite optimistic that things will go a lot better once I've done that.

  9. #39
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    Are you really building this to save money or have fun. Because the cost of an OTG these days has dropped dramatically since more people started porting the original PIC design

  10. #40
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    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 the Ethernet board as well already the stack is getting big enough. Overall it saves me a little - although probably not much now I've had to do it twice!

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