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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...

I will keep the thread alive!

My boiler doesn't support OT as far as I know so X plan is not for me?

exactly! Even if it was, I couldn't afford it. and as John says, I can use it as a 3 wire system so my inclination is towards a W plan system. Not sure on this one yet. See attached spec sheet which I have short listed.

Thank you for sharing this. A very good read for understanding various systems.

So mid Position valve that I have is for Y plan system and have a different actuator. Am I right in thinking (as you have said in your post as well) that valve body for mid position and diverter valve is the same? Its just the actuator that needs to change from ZA5 (5 wires) to ZA3 (3 wire)...

No its not and hence the post. The boiler is working in CH mode only (with your help off course). I have bought the cylinder, will hire someone to do the pipework. I have attached a photo of pipeloop with mid position valve which I intend to utilize with cylinder installation. The mid position...

GC Number 41-819-21 But W plan uses a 3 port so can't I use my 3 port valve? If not then what do you suggest. 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...

Message from viessmann tech support

Correct on 3 port mid position valve. 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.

I have a two pipe heat only boiler (viessmann) so a 3 port valve is required to run PDHW, but that would be a Y plan or PDHW? What is viessmann cylinder demand box used for (see attached instructions) and I am confused about which configuration to run. As OP, I am in similar position and trying...

Yes, a project for summers! 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...

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...

Pros in the forum would be interested in - existing Pipework; - rads sizes; double or single panel - existing or prior HW arrangement in the property - which boiler did you have in the property and were there any issues - are you upgrading the heating system or just the boiler etc.

Yes. This is what I have put; a 13kw heat only boiler (Viessmann vitodens 100-w). I have a two pipe system. I didn't change any pipework, its an open vent system. All single glazed windows. I had to change a couple of radiators upstairs since old ones were single panel and were inadequate for...

I had two 24+18kw old boilers serving two separate zones in my house. All single glazed windows, poor insulation no cavity walls. I plugged all draughts and holes. I carried out HL calculation and it turned out I need 12kw for 4 beds, a huge lounge (600sq.ft), 3 baths. Installed 13 kw Viessmann...

Fill in these details and I can calculate it for you.

I am a home user and not a pro and would like to clarify as a user that I did check with Viessmann UK and they confirmed and it is written in MI as well that OV boiler can be used in a pressurised or seal system too.

Couldn't agree more. I have a double storey 4 bed + 4 baths + Lounge Upstairs (650sq.ft with a fancoil unit) and a Lounge downstairs. We heat our bedrooms (1200mm rads) , baths and Lounge (1200mm rads) mostly during day time. The heatloss of my property (which I did it myself in Excel) turned...

Have you tried resetting the timer? Is pump connected to pump port on pcb or in wiring center? I have the same boiler and programmer. My pump is connected to one of the port on boiler PCB so it is getting power from boiler and it works fine in 'timed' or 'on' mode.