Anyway you'll always want the hw stat satisfied as its hw priority 
Viessmann Vitodens 100-W Compact boiler and pump overrun
- Thread starter MJN
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gaswizzard
My post was not directed at you Eaton.
Can you post up a link to that diagram? , MJN seems to be getting confused when looking a the wireing diagram he posted.
Can you post up a link to that diagram? , MJN seems to be getting confused when looking a the wireing diagram he posted.
I didint say that, I said its a drawback of the w plan that you cant have ch without the dhw stat being satisfied.
My post was not directed at you Eaton.
Can you post up a link to that diagram? , MJN seems to be getting confused when looking a the wireing diagram he posted.
Thought so, it's in docs I think. I know they said it works like that on diverter valve but on 3 port it would be as you said.
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Cue a simple relay, and possibly a better boiler
How do viessman cope with wc sensors suddenly vanishing?
How do viessman cope with wc sensors suddenly vanishing?
M
mysteryman
It will assume it is freezing outside, control accordingly, and give a fault indication.
If you want CH only, select CH only. Do people in a house ever really want that anyway?
If you want CH only, select CH only. Do people in a house ever really want that anyway?
I only as, as I have been doing variations of this for a while where, for the short time that both ch and hw are needed at the same time, the wc circuit is ignored.
The boiler needs to NOT have a hissy fit just because the wc sensor has been disconnected all of a sudden.
Chances are too that when you are using lots of hot water more then one person is starkers and an artificial boost to the ads is preferred anyway.
Tried and tested - tried and tested
The boiler needs to NOT have a hissy fit just because the wc sensor has been disconnected all of a sudden.
Chances are too that when you are using lots of hot water more then one person is starkers and an artificial boost to the ads is preferred anyway.
Tried and tested - tried and tested
I can categorically state that that is wrong. Look at the wiring (this is straight from Honeywell):
With the above setup if the cylinder stat is calling (C and 1 connected in the L641A) then the diverter valve will not be energised hence flow will remain with the cylinder until the stat is satisfied.
If the boiler is operating in weather compensated mode this might never happen because its flow temperature may well be less than the cylinder stat target.
Mathew
I'm sure me bringing this thread back to the top of the pile will inevitably result in a few groans from those subscribed to the topic, however for what it's worth it's actually to provide some closure on the subject!
I've been in discussion with Viessmann technical support who I must say have been extremely helpful and seemingly more than happy to get stuck into discussions of the options available, and the intricacies thereof. I got in touch with them with some trepidation fearing that they may only be willing to give the time of day to RGI's and the like, but they couldn't have been more helpful.
As many of you have been saying, they do indeed recommend the use of the W-plan configuration, however it is not exactly as per the standard Honeywell design. This is where my concerns were coming from because, as I attempted to demonstrate, the default W-plan wiring layout is inadequate given the suspected result of the nuance of weather compensated boiler operation in regard to its HW priority provision.
Viessmann confirmed that with a standard W-plan implementation there could indeed be a catch-22 situation in the scenario where all of the following are true:
1. There is a demand for CH (by both CH timer and room stat*) (*or results from the outside sensor if used without a room stat)
2. There is no demand for HW by the HW timer
3. The temperature of the cylinder is below the target set by the cylinder stat
4. The weather compensation algorithm determines that a less-than-maximum flow temperature is required
In this state the system would effectively be 'trapped' in a catch-22 situation whereby the boiler is running in weather compensated mode and the diverter valve would never switch over to the CH circuit because the boiler's flow might be of a lower temperature than required to satisfy the cylinder stat.
As per my suggested modifications to the standard W-plan (which at least one person admitted to not even having looked at - it's no wonder our discussion didn't really get anywhere!) it is imperative that a HW timer with changeover contacts is used such that there is an explicit 'HW is not required' output available. This allows the CH demand to be satisfied without attempting to satisfy the cylinder stat first (which it might never do as discussed).
As an alternative to modifying the standard W-plan wiring, they did say that you could use a combined CH/HW timer of the variety that does not allow CH demand to be called without HW (i.e. three states: CH off HW off, CH off HW on, CH on HW on) but he said sticking with my CM927 would make far more sense and so I should just swap my timer (Sangamo PSW) for one with NC/NO changeover contacts (e.g. a Honeywell ST9100C).
To ease common understanding we based our discussions and correspondence on the wiring diagrams provided with their outdoor sensor kit (as Mysteryman recommended) and so I thought it might be useful for anyone else not fully conversant with this particular kit and variation from the conventional W-plan wiring that I make them available here:
Weather Compensation Outdoor Sensor Kit Instructions
Cylinder Demand Terminal Box Instructions
After all this I hesitate to admit that I am actually now starting to have second thoughts about having the weather compensation kit fitted at all because I am very doubtful of its ability to prove cost effective. At a cost of ~£180 for the sensor and cylinder interface box and promises of only 2% greater efficiency it is going to take 20 years to recoup the outlay given that my annual gas bill currently resides at only £450! To his credit, the chap from Viessmann did say that the benefits are more to do with comfort rather than cost-savings (despite the advertising blurb bias being the other way) and that cost savings would be greater realised for much larger properties/consumers. He also said that had I not already been using a TPI controller that can at least approximate a modulated demand and the comfort and control benefits that this brings that it would still have been of noticable benefit.
Thanks everyone for the discussion - I may have gone full circle but it was a beneficial journey for me and if nothing else it has cemented my intent to go with the 100W Compact.
Cheers all,
Mathew
I've been in discussion with Viessmann technical support who I must say have been extremely helpful and seemingly more than happy to get stuck into discussions of the options available, and the intricacies thereof. I got in touch with them with some trepidation fearing that they may only be willing to give the time of day to RGI's and the like, but they couldn't have been more helpful.
As many of you have been saying, they do indeed recommend the use of the W-plan configuration, however it is not exactly as per the standard Honeywell design. This is where my concerns were coming from because, as I attempted to demonstrate, the default W-plan wiring layout is inadequate given the suspected result of the nuance of weather compensated boiler operation in regard to its HW priority provision.
Viessmann confirmed that with a standard W-plan implementation there could indeed be a catch-22 situation in the scenario where all of the following are true:
1. There is a demand for CH (by both CH timer and room stat*) (*or results from the outside sensor if used without a room stat)
2. There is no demand for HW by the HW timer
3. The temperature of the cylinder is below the target set by the cylinder stat
4. The weather compensation algorithm determines that a less-than-maximum flow temperature is required
In this state the system would effectively be 'trapped' in a catch-22 situation whereby the boiler is running in weather compensated mode and the diverter valve would never switch over to the CH circuit because the boiler's flow might be of a lower temperature than required to satisfy the cylinder stat.
As per my suggested modifications to the standard W-plan (which at least one person admitted to not even having looked at - it's no wonder our discussion didn't really get anywhere!
) it is imperative that a HW timer with changeover contacts is used such that there is an explicit 'HW is not required' output available. This allows the CH demand to be satisfied without attempting to satisfy the cylinder stat first (which it might never do as discussed).As an alternative to modifying the standard W-plan wiring, they did say that you could use a combined CH/HW timer of the variety that does not allow CH demand to be called without HW (i.e. three states: CH off HW off, CH off HW on, CH on HW on) but he said sticking with my CM927 would make far more sense and so I should just swap my timer (Sangamo PSW) for one with NC/NO changeover contacts (e.g. a Honeywell ST9100C).
To ease common understanding we based our discussions and correspondence on the wiring diagrams provided with their outdoor sensor kit (as Mysteryman recommended) and so I thought it might be useful for anyone else not fully conversant with this particular kit and variation from the conventional W-plan wiring that I make them available here:
Weather Compensation Outdoor Sensor Kit Instructions
Cylinder Demand Terminal Box Instructions
After all this I hesitate to admit that I am actually now starting to have second thoughts about having the weather compensation kit fitted at all because I am very doubtful of its ability to prove cost effective. At a cost of ~£180 for the sensor and cylinder interface box and promises of only 2% greater efficiency it is going to take 20 years to recoup the outlay given that my annual gas bill currently resides at only £450! To his credit, the chap from Viessmann did say that the benefits are more to do with comfort rather than cost-savings (despite the advertising blurb bias being the other way) and that cost savings would be greater realised for much larger properties/consumers. He also said that had I not already been using a TPI controller that can at least approximate a modulated demand and the comfort and control benefits that this brings that it would still have been of noticable benefit.
Thanks everyone for the discussion - I may have gone full circle but it was a beneficial journey for me and if nothing else it has cemented my intent to go with the 100W Compact.
Cheers all,
Mathew
I'm only passing on what Viessmann told me, and that being that it will increase efficiency by 2%. I cannot imagine an ulterior motive for why he'd misstate this fact, other than being wrong of course but he seemed fairly knowledgable to me!
Regarding the diagram, yes it does - but my point all along (and now verified by tech support) is that it is not a standard W-plan layout given the use of both poles of the the DHW changeover switch. Let's not drag this back up though - I think we're all in violent agreement now! (Edit: Apologies, I think perhaps you were just indeed saying follow the diagram and all will be well... which I concede is what you were saying all along... it's just a shame the diagram wasn't available from the off!)
Mathew
Regarding the diagram, yes it does - but my point all along (and now verified by tech support) is that it is not a standard W-plan layout given the use of both poles of the the DHW changeover switch. Let's not drag this back up though - I think we're all in violent agreement now! (Edit: Apologies, I think perhaps you were just indeed saying follow the diagram and all will be well... which I concede is what you were saying all along... it's just a shame the diagram wasn't available from the off!)
Mathew
It was just the standard Honeywell Sundial W-Plan wiring layout, redrawn and colourised for clarity. I then modified it for the proposed layout to include the HW timer changeover contacts.
Mathew
Mathew
Was referring to this...
http://www.newtonnet.co.uk/permanent/w-plan-wiring.gif
Is this your own work on photoshop?
http://www.newtonnet.co.uk/permanent/w-plan-wiring.gif
Is this your own work on photoshop?
Just from a set of diagrams I had from my last install. I'm not sure where it originated. It might well have been Honeywell as I contacted them a few years back with regards to some queries I had about their Chronotherm range.
Mathew
Mathew
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