As suggested, a MID pos valve could be used with some clever wiring but it's not as designed so unsure as to how the actuator will cope with it long term as the internal circuitry wasn't really designed to work like that.
A Y plan (before S plan was really a thing) gave the advantage whereby both the HW and CH could be run at the same time - this was before variable system temps and PDHW was a viable approach. The old W was very inefficient because the old cylinders would take for ever to heat up and the house would go cold if the there was a high HW usage.
I'm not convinced that was much of an advantage. If it's on HW and then CH is called, with the rads cold, the HW cylinder could cool somewhat, before things settled down. I never had a problem with W-plan, even with 3 young kids and an old cylinder. Weather comp (with PDHW) has been around for a while now, and I've not seen any comment that separate CH and HW is a problem. As you say, with modern cylinders it's less of an issue.
I have taken the picture from sticker on boiler. I keep reading/hearing from everyone on the forum that this heat only variant of Viessmann is not OT compatible but the newer model with digital controls are. Tech support are not of much use except that I give them credit that they respond to my requests!
it's in the manual for GC: 41-819-21 - pages 40/41
Physically check your boiler for the actual connection if you can, same block (X21) as the weather comp. Wouldn't be the first time the manual on that GC number says it is and then you look at the PCB and they're not there though the PCB is marked for them.
On wiring harness from X21; port 3 and 4 are marked as WC.
Port 1 and 2 doesn't say anything for OT which made me think that it is not compliant but on the manual it does.
When communicating with Viessmann it's better to use the boiler's serial number than the GC! As said, their models and iterations are all over the place.
OK - so your model of the boiler doesn't have the software to enable the OT control, which seems absolutely mental given the exact same boiler in combi and system guise does (tho really not surprised). I also cant understand either why it isn't OT enabled in software but if you buy their weather comp and control unit package then variable output has to be used as it's the only way it can enable PDWH?!? That just doesn't make sense. I never understand why manufacturers do that.
So if we go back to that cylinder demand module, how does that allow PDHW, as that would need to be able to increase the system temp when in HW mode. I think Viesmann have implemented it in such a way where their box needs to be bought @ £200 odd just to control what must already be enabled anyway. Wonder what those connection on the X20 bus do.
Looking at the wiring diag they are also wiring the mid pos valve where the white and grey are both powered together and the orange is the SL and only enabled when the CH calls, the HW call is through the cylinder stat as has was previously described and that then runs through their 'cylinder control box'. That has to be the system temp control that ramps the boiler to max when calling for HW (hence the PDHW). Looking at those connections on the X20 bus, following the weather comp wiring diag, it connecting to 47 in the boiler circuit, which is labelled as the temperature limiter, so that has to be how it works and that turns the boiler up to max.
A couple of leads are tucked from X20 in my boiler (I have not connected HW to the boiler yet) which I think goes into cylinder demand box. The box cost around 110 from viessmann direct and on ebay is available for 50.
Viesmann Direct (.co.uk) is £210 for the kit you linked to - Outdoor sensor kit Z007 933 - not £110?? Just the box on it's own 7574319 is yes but I thought you were implementing the weather comp?
Careful getting it from fleebay, never know what you are getting sometimes.
Viesmann also support OT out of the box, they even said so for the combi and the system variants of your boiler - absolute madness that it isn't on yours.
Cylinder box compatible with my boiler is 7296 968 I didn't buy the box as I wasn't sure what it does. Now that I know, I will look at options to buy it. Thanks for advice on ebay purchases.
Do I require HT limiter? I think it is a safety or 1st line of defence in case cyl stat fails, correct?
All you need to do is follow the wiring information in the box. the wires will connect to the corresponding numbers in the wiring centre connector block. Just note if the High Limit stat (HL) isn't being used then the link is between 7 + 12 though if it's an unvented then it will usually come with a HL and normal stat in series.
Obviously the valve will be connected correctly into the system so the B outlet leg goes to the HW cylinder and the A outlet leg goes to CH and the AB bottom leg is the inlet.
And just back on the OT subject - you alsoi get this for the X21 connector -
Yes Sir, already bought it.
And if I put cylinder demand box with it and corresponding wiring, I would be installing it in W plan configuration since the boiler doesn't support OT and hence no X plan, correct?
Not sure about it but it doesn't come up with expansion tank or limit stat. Its a 120 litre tank with coil in it for flow and return connection.
I need to buy a cyl stat (NTC type). all plumbing is with 3/4 male inlet type connectors.
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