Indirect water cylinder

[desktop987456]

Hi,
I have a Viessmann vitodens 100-w (compact 13kw) boiler installed in OV configuration (2 pipe system) for CH only. It has a drayton (LP722) timer and a dial type Room Stat. My CH load is under 7 -8 kw.

This year I have bought a Weather compensation sensor and cable which is straightforward process for installation for central heating only.

I am in the process of adding a hot water cylinder to the same boiler and I have bought a 120 litre indirect water cylinder.
I have a 3 port (mid position valve with A & B outlets for CH & Bath) which will be fed by an external pump installed on flow out of the boiler.
I also have a wiring center which can be wired for various configurations; S,W,Y,X or PDHW.
I intend to heat the cylinder once in the morning and in evening.
Stuff I need to buy include a cylinder thermostat, a PRV/Temp valve

Questions
Can I install an open vent DHW or do you reckon that I go for unvented system with an EV?
Which configuration should I go for? I have read that Y or S plan are outdated and PDHW should be installed. I am not sure if I can have PDHW on two pipe system?
Do I need a separate pump for domestic hot water apart from CH pump or separate port actuator (the ones you require for an S Plan)
To run weather compensation, do I require a cylinder demand terminal box from Viessmann? Is it necessary to get one as it is quite expensive.
What other components do I need to install cylinder?
Any advice is welcome, I have plenty of time before next winters so I want to do my homework.
Thanks in advance. I installed OV CH system last year with a lot of guidance from this forum.

 

[Madrab]

I have a 3 port (mid position valve with A & B outlets for CH & Bath)

Hi @desktop987456 , round 2 is it D

Try and avoid using a 3 port mid pos valve (Y Plan), they are notoriously unpredictable and no longer used on new installs - use at least 2x2 port valves (S plan).
You can use you OV CH system to heat either a vented HW or unvented HW cylinder. The advantage of the unvented is it will provide HW @ mains pressure, well advised if your cold mains can handle the flow and pressure requirements.
PDHW is another option of course - that will allow the HW to be heated in the quickest time but would need to check that your boiler can be configured for 2 different flow temps, don't think Viesmann use Open Therm, rather their own controls to perform the same functions

 

[desktop987456]

Hi @desktop987456 , round 2 is it

Yes, a project for summers!

PDHW is another option of course

But my boiler cannot handle two flow tempearture. It has only one dial setting for temperature so either it has to be CH or DHW not both? Correct.
I also intend to install Weather compensation, how it will integrate for DHW in my scheme? You are suggesting:1. S Plan system using two separate actuators for DHW and CH.
With one flow temperature on boiler and no OT, can I still do hot water priority with cylinder terminal box and one flow temperature? See attached instruction on installing weather compensation. I have bought the sensor and lead last year but don't have cylinder demand terminal box. If I buy it would I be able to run PDHW with my boiler?
Thanks

 

[Madrab]

what model of 100-w is it - can you give us the GC number? I wasn't sure about the OT - (edit) Only ever done a few Viesmann's - and I was sure I remember that one of them had their own Protocol, like WB's EMS, though I do have it on good authority (@DP) that it should be OT enabled

If you are able to use controls to program different temps for the boiler to run at then PDHW is certainly an option. If the boiler is compatible with weather comp then it can have variable syste temps, that's how weather comp works. The colder it is outside, weather comp increases the system temp, conversely, the warmer it is the lower the system temps are.

The Vitodens wiring centre is their system to automate and control different system setups though I am sure that, if it's enabled for weather comp, there will be other ways to set it up. If you already have their control centre - VD-100W - there there are a number of different ways to do it but if I remember rightly you did have a 3 port (mid) left over from the last round??

 

[desktop987456]

if I remember rightly you did have a 3 port (mid) left over from the last round?

Correct on 3 port mid position valve.

boiler is compatible with weather comp then it can have variable syste temps

The instruction sheet I have sent in previous note says that DHW can run with weather compensation + cylinder demand terminal box of viessmann. I just want to make sure that it does what it says before I buy terminal demand box.

 

[desktop987456]

Message from viessmann tech support

 

[Madrab]

W plan is an old system design and I wouldn't suggest that would be the best way, a W plan would use a diverter valve rather than a MID 3 port, you can alter with wiring to essentially turn the MID into a diverter but it's not what's it's designed for and not sure how well the valve would last. That being said though a 'W plan' PDHW wouldn't be a bad idea but I wouldn't use your MID pos valve.

Easier configuring the system as an X plan, which is very similar to an S plan but needs a NO CH 2 port and it won't use the switching wires of the valve to control the boiler (though the orange Switched live from the DHW 2 port is used to power the CH NO 2port.

I don't think you need to go to all the trouble of getting the control unit etc but if that's part of the weather comp and you really want that then it may be moot. The DWH doesn't 'run' with the weather compensation per sé, weather comp alters the flow temps for the CH, determined by the outside temp, it may be just that the weather comp system comes with the control box too.

 

[desktop987456]

an you give us the GC number?

GC Number 41-819-21

That being said though a 'W plan' PDHW wouldn't be a bad idea but I wouldn't use your MID pos valve

But W plan uses a 3 port so can't I use my 3 port valve? If not then what do you suggest.

Easier configuring the system as an X plan

Just to be clear, If I go that route, I would be needing a 2x 2 port valves for CH and DHW circuit?
I was trying to not let go the items I already have but if in your opinion an X plan is better then I would buy 2 port valves and configure it that way.

 

[dilalio]

but if in your opinion an X plan is better then I would buy 2 port valves and configure it that way.

One of the 2 port has to be an NO (normally open) valve. The other is NC (normally closed).

The method of operation is that DHW demand opens the NC valve to allow primary water flow to cylinder and, at the same time, the CH NO valve closes off flow to the CH primary.

"A X plan"
https://heatingacademynorthampton.co.uk/pdhw-pt-3/#:~:text=X plan work?-,A X plan,-is a NO

 

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[Madrab]

But W plan uses a 3 port so can't I use my 3 port valve? If not then what do you suggest.

A W Plan uses a 3 port diverter valve - not a mid position valve - they are 2 different beasts. A diverter rests @ HW open - CH closed and then when it's powered it moves to open the Central Heating (CH) side and that closes the Hot Water(HW) side, it only has 3 wires. A MID pos is different. As suggested it can probably be used with some clever wiring but that's not what it's designed to do. This is probably the easiest way to set up PDHW though, especially if there is already a 3 port valve in the system and that is your W plan but it does need that diverter another valve.

As suggested - for an X plan you need 2 different 2 port valves. 1x Normally Open (NO) (CH), 1 normally closed (NC)(HW) and it works in conjunction with the stat on the cylinder.

Not remembering your system I'd suggest you have a good look at what you have and understand what would be easier to implement for your requirements.

 

[fixitflav]

But W plan uses a 3 port so can't I use my 3 port valve? If not then what do you suggest.

As it says in the Viessmann data, if you have a Y-plan (mid-position) valve, the mid-position is not used with weather compensation, so it must be wired to operate as W-plan (either/or). Without studying the valve in detail (and perhaps not even then) I don't see how to do that, but maybe somebody on here or your electrician if you have one, can help.
Is it a Honeywell valve? Another option is to replace the valve head with a W-plan V4044 head.

Changing to S or X-plan would be a lot of work, both to the pipework and the electrics.

I don't find the Viessmann data sheet too clear, as it doesn't show the room-stat. The table showing wiring of the diverter valve looks OK for W-plan - valve powered when the cylinder-stat is satisfied (assuming the live to 4 on the terminal strip is from a roomstat). For your Y-plan valve, it seems to be saying common the grey and white wires and connect to 3 on the strip.

Don't know what you asked Viessmann tech but their reply isn't much help.

 

[Johntheo5]

I think if you common the grey&white then the MPV becomes a diverter valve.
This may be of some help.

Priority Domestic Hot Water using a 3-port mid-position valve. - ellse.org

 

[dilalio]

Don't know what you asked Viessmann tech but their reply isn't much help.

Like I've said before.... Good luck getting tech advice and help from them! I'd rather herd cats!

 

[dilalio]

Changing to S or X-plan would be a lot of work, both to the pipework and the electrics.

But it is the best method.