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Thread: Vaillant ecofit pure 435 + evohome + opentherm

  1. #1
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    Default Vaillant ecofit pure 435 + evohome + opentherm

    I thought I'd share my findings from setting this up. Overall it works very well!

    Bought the vr33 opentherm board from this Dutch seller: https://www.cvonderdelenhartog.nl/Va...-0020017895/en

    Bought the Honeywell opentherm controller R8810

    Opened boiler up and plugged the vr33 into the X32 socket(not x31 as shown in vr33 manual) using cable provided with vr33. Did scare me a bit that the boilers PCB isn't shown in the vr33 manual, but this worked. Then you connect the vr33 to the r8810 with a pair of wires (not supplied).

    Removed the wire going into the RT socket from the current boiler controller (a BDR 91) and put a loop in to live mains. Without this the boiler won't activate (it just thinks the heating is off)

    Overall the skills needed were similar to that of wiring a plug. Ideally get a pro in, but I found they were not so keen to plug some unknown card into a boiler and potentially have to deal with the consequences.

    Turned on boiler, bound opentherm controller to evohome as per instructions in controller manual
    Did a lot of watching the system to see how it behaves!

    I needed more boiler bypass. The boiler errored out with F23 after a while due to this. This wasn't a problem previously with the BDR91 controlling the boiler, but it seemed to be with opentherm. it's keener on firing up the boiler with no or little flow through the radiators available. I'm not sure if this is because the timing of the opentherm commands are different or just because it tries to fire the boiler at a low level with the rad valves mostly closed (suspect a bit of both?)

    The system summary in the evohome controller needs to be taken with a pinch of salt - I think it just doesn't update very often. So you easily see it saying everything at 0% but the boiler is still firing (and rad valves are partially open). So don't assume your system is broken if you see this!

    There does seem to be a bug where if the system has overshot on all rooms, so turns off completely (OT temp of 10) and then turns on again after a room cools down, it starts with the OT temp set to Max (90) and then reduces it after a minute or two. This is despite what is surely a low heating demand. Other people have reported this: https://www.automatedhome.co.uk/vbul...ng-my-hair-out!

    These two issues combine to mean you can witness 0% demand in the system summary and 90 degrees request on opentherm (d09 on the boiler) when the house is warm. But the result of this is not bad, because the 90 degrees occurs only after the boiler has been off, and the vaillant always starts slow from cold, and by the time the boiler is ready to go full tilt the opentherm temp has dropped.

    When hot water is demanded, it sets the system to Max temperature (90 degrees). Because of this turning in HW when the house is warm can cause the rooms to overshoot (the valves do not close more to compensate for the increased water temp). To mitigate this you want to ensure as much flow as possible goes through the hot water cylinder when the hw valve is open, so the radiators don't get so hot whilst the hw circuit is on (I had a valve that controlled the the hw flow).

    So once I understood these things I realised its actually working really well. On a cold day it runs steadily with a water temp of about 55 degrees which is surely really efficient. It does seem to learn how to match the heat demand, I think it was a bit all over the place to start with but settled down over time. On a less cold day I now see a steady demand for perhaps 30-40 degrees. The boiler can't do this on minimum burn and has to switch on and off, but each time it switches on its with a set temp of 40 and so there is no cycle of roaring full blast then backing off etc.

    Personally I like the fact it goes full tilt (90 degree demand) if any room is 1.5 degrees under temperature. This means if you want a room heated up (assuming you have the boiler temp control set to 75 on the boiler itself) it does it really fast, then it settles back down to efficient mode. Also the hot water gets done very quickly.

    However it might be nice if you could control the opentherm temp it uses for hw though, I could perhaps imagine wanting to turn it down.

  2. #2
    Automated Home Jr Member
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    I know that in my Vaillant setup d78 can be used to set the DHW max flow temperature. 75C works fine for me.

    TBH I wouldn't know whether the VR33/R8810 controls you are using would simply override any manual changes you might make to this setting.

  3. #3
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    I think that depends on what kind of hot water system you have. with mine the boiler doesn't get to know whether it's making hot water or doing the heating - the hw valve is controlled independently of the boiler and the boiler always thinks it is doing regular heating (hw symbol never flashes)

  4. #4
    Automated Home Jr Member
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    Sorry, I should have realised that would be the case.

    I now have Vaillant controls which do activate the dedicated DHW mode of the boiler, giving a bit more control over the flow temp and max power output when heating the water cylinder.

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