Indirect water cylinder

[Madrab]

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?

Yes seems to be that way but they are implementing the high temp PDHW another way through the connection on x20 which is obviously a fudge to get it to max fire - I just can't get past the whole things about the X21 connector mentioning OT - the manual mentioning OT - even an error when OT fails to connect but your specific boiler not having OT, seems that's purely down to the fact that there's none or very little configuration/software options available.

So you need to wire the 3 port MID as a diverter plum in as normal then implement the cylinder demand control box to ( I presume) bypass the temp limiter on X20 when in HW mode.

What type/make/model of HW cylinder is it? If it doesn't have an expansion vessel or relief valves, control group, etc then it sounds like a standard Open Vent HW cylinder - these are not usually fed by the cold mains and normally need a cold water storage cistern (CWSC) to gravity feed them, although that may all be different in the US.

 

[desktop987456]

I just can't get past the whole things about the X21 connector mentioning OT - the manual mentioning OT

I recall when I was buying the boiler, I was looking at borchures for vitodens 100 and one of them even mentioned (in brochure from Viessmann) that a heat only has pump inside boiler! I sent that exact brochure to their sales team and they came back that brochures have all kind of generic information and heat only doesn't have an internal pump even though the brochure when listings its features had one of the bullet point that internal pump is fitted! I learned my lesson then.

then it sounds like a standard Open Vent HW cylinder

Yes, it has not been delivered yet but it seems like OV and will be gravity fed by overhead tank.

 

[desktop987456]

I will share the details of cylinder as it doesn't come with jewllery that goes with installation and I need to buy them separately.

 

[Madrab]

OK - nothing really needed as far as the OV HW cylinder fittings is concerned - it will have 3/4" (22mm in UK money) tappings for the coil flow (B from the MID valve)and return and the cold inlet - out the top will be the 3/4" HW outlet and that needs to tee with vent up and over the CWSC for venting and the HW supply downwards.

 

[fixitflav]

Yes seems to be that way but they are implementing the high temp PDHW another way through the connection on x20 which is obviously a fudge to get it to max fire

Not sure why you say it's a fudge, something has to do it! When HW is calling, terminal 1 on the Cylinder demand box gets live, via the 7 to 12 link. The boiler then goes by the normal control-stat, ignoring the WC sensor.
I'm not into OT, but if the kit on the Viessmann wiring diagram does its job, it should work OK.

 

[Madrab]

Not sure why you say it's a fudge, something has to do it! When HW is calling, terminal 1 on the Cylinder demand box gets live, via the 7 to 12 link. The boiler then goes by the normal control-stat, ignoring the WC sensor.
I'm not into OT, but if the kit on the Viessmann wiring diagram does its job, it should work OK.

It's a fudge because Viesmann have already implement OT on that boiler via the WC/OT on X21, the only thing (edit) they haven't done is loaded the software onto that specific model, the exact same model of that boiler in system or combi guise has the required software and implemented OT.

One definition of 'fudge' is defined as - 'to devise or implement as a substitute or fake'. To implement PDHW on this boiler, Viesmann are using a control box and cables (@£100 odd) to bypass the temp limiter control on X20, not by just ignoring the WC, to perform a function that is probably already there but can't be accessed because (it seems) the software allowing that hasn't been loaded. Of course it works, I'm not saying it won't but they already had everything there and all that extra hardware and cost shouldn't be needed. To me that's the definition of a fudge.

 

[fixitflav]

It's a fudge because Viesmann have already implement OT on that boiler via the WC/OT on X21, the only thing (edit) they haven't done is loaded the software onto that specific model, the exact same model of that boiler in system or combi guise has the required software and implemented OT.

One definition of 'fudge' is defined as - 'to devise or implement as a substitute or fake'. To implement PDHW on this boiler, Viesmann are using a control box and cables (@£100 odd) to bypass the temp limiter control on X20, not by just ignoring the WC, to perform a function that is probably already there but can't be accessed because (it seems) the software allowing that hasn't been loaded. Of course it works, I'm not saying it won't but they already had everything there and all that extra hardware and cost shouldn't be needed. To me that's the definition of a fudge.

OK, just curious.
As you say, Viessmann approach seems a bit odd. As far as I can see, all the boiler needs is something to tell it when HW is calling, and use the appropriate flow temperature. In principle, that could be built-in, without needing the cylinder demand box. Does it do anything I wonder, or is it just a £100 connection box? On the wiring diagram, 1 doesn't connect to anything, but that can't be right.

 

[Madrab]

From what I can gather, the control box seems to convert the 240V signal from the programmer/wiring centre SL call for HW into a low voltage signal to connect to the Viesmann PCB @ X20.

I don't have much confidence in the actual wiring diagrams tho as in the boiler MI it shows the 2 output wires from the control box are connected together, which I don't think they wouldn't be - (never say never tho)

That's why using OT would be so much simpler with one connection on X21 for OT/WC and that's it. Output temps/WC controlled by that and the flow to the HW cylinder controlled through the wiring centre and the 2 port valves/3 port diverter or MID valve.

 

[fixitflav]

I will share the details of cylinder as it doesn't come with jewllery that goes with installation and I need to buy them separately.

Are you planning to install a timer or manually switch? If a timer, see the last line of #24

 

[desktop987456]

One more point - the Viessmann data shows a timer with all 6 terminals accessible. Some timers have the both commons connected internally at mains voltage, and inaccessible.

Yes, already have drayton 2 channel programmer i.e 722. So does mine has both commons connected?
I am also thinking of putting a dedicated boiler for HW cylinder? What do you suggest; good idea or waste of resources.

 

[fixitflav]

Yes, already have drayton 2 channel programmer i.e 722. So does mine has both commons connected?

I don't think that one is suitable. On the Viessmann diagram the CH common is fed from the HW off contact (the CH off contact is not used). Can't do that with yours, assuming I've got the right one.
Edit - on Viessmann, CH is only available if HW is Off on the timer, or On on the timer, but cylinder stat satisfied. That's how it does PDHW.

I am also thinking of putting a dedicated boiler for HW cylinder? What do you suggest; good idea or waste of resources.

Seems like overkill to me!

 

[desktop987456]

Seems like overkill to me!

Ok. Thanks.
I have the same timer as pdf in your post.

 

[fixitflav]

I have the same timer as pdf in your post.

IMO that isn't suitable. I don't know offhand who makes the right type, maybe Viessmann can point you to one.

 

[Madrab]

I actually think that programmer may be fine - once the hot water is satisfied, then the cylinder stat switches over and sends the SL to T6, that then connects to the CH common, the reason for the double connection - from HW S (off) to the CH - will be for when the HW isn't being called for by the programmer, it then sends the HW off SL to the CH.

it does all seem very convoluted using this box but it is like the Y Plan, only difference is the Grey and White are joined so there is a live on orange to the boiler's SL only when the CH is calling and the valve is in CH mode.
The HW on from the programmer to the HW control box is now being sent via the cylinder stat and a HL stat via the 7>12 bridge + a N. That then presumably 'tricks' the temp limiter on X20 and switches the boiler on and into high fire mode.

Talk about a long road for a shortcut tho.

 

[desktop987456]

Talk about a long road for a shortcut tho

Hmmm. I have the boiler with CH last year. Bougt WC sensor this year to save on gas and now have to buy Viessmann cylinder box to make HW work. For 100 bucks I can buy a used boiler and dedicate it to HW cylinder separately!

it is like the Y Plan, only difference is the Grey and White are joined

From what I have read on forums and documentation. I don't have any practical experience but I am on my learning curve.