Viessmann Vitodens 100-W Compact boiler and pump overrun

[MJN]

The subsequent drawing was done using Paint Shop Pro if that helps. Whilst it's getting old now I find it something of a familiar tool with which to make modifications to line drawings.

Mathew

 

[Norcon]

Just from a set of diagrams I had from my last install.

Did it work ok. How does 2 in the cyl stat become energised?

 

[MJN]

Just from a set of diagrams I had from my last install.

Apologies Norcon, my poor grammar was somewhat ambiguous. What I actually meant was that the original diagrams were from those collected during my last install, however it ended up being a simple S-plan layout in the end so the w-plan diagram (or modified version) was never used.

Did it work ok. How does 2 in the cyl stat become energised?

With reference to my proposed modifications to the standard W-plan wiring you are right; terminal 2 of the cylinder stat does not play a role. This was my slip-up - I'd solved one problem but still left a capability gap (in particular, CH would not function if the HW timer was calling yet the cylinder stat wasn't).

Following my discussions with Viessmann, terminal 2 is brought back into play and, with the changeover contacts of the HW timer still being used, every state is accomodated as required.

I didn't upload the final circuit because there was only me that seemed to be showing any interest in them! For completeness however, it can be viewed here.

My only caveat for this diagram would be that if I were to now use this modified w-plan layout I would wire it as per Viessmann's terminal block positions (from their weather sensor instructions). The method and end result is exactly the same, but the benefit of following their numbering is that it at least represents a common baseline from which to discuss technical support issues with them!

Mathew

 

[matt1e]

I've been in discussion with Viessmann technical support
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).

you don't need to have an explicit programmer with a hw off signal,
same as you dont have to fit the valve the same way as per a honeywell W-Plan just design it so it suits your application

this should suit more or less


fit the valve so that it powers open to the cylinder and wire as pic above
I have altered the viessmann drawing and put the links in for you
this will still be HW priority btw but you won't have any problems due to the cylinder stat not satisfying due to compensated flow temp etc
I've added the extra limit stat for ufh systems


Matt

 

[MJN]

Where were you 5 pages back?!

I think that would indeed work. Tracing through the logical combination paths seems to function as required. It also has greater compatibility with more timers and requires fewer connecteions so unless I've missed something it seems to trump all the other designs discussed so far (including Viessmann's own given their reliance on the changeover like mine).

If pump overrun was required, I know we don't require it on this model but if it did, then it also has what is presumably a desirable property in that excess heat is dumped into the heating circuit rather than the cylinder given the 'default power off to CH' configuration of the diverter valve?

Mathew

 

[mysteryman]

No-one claimed the Viessmann plan was a standard W Plan. It is a specific diagram fot putting weather comp on a Compact heat-only-open-vent boiler as simply as possible. The point being that it runs the boiler flat out for DHW, and at variable temp for CH. It will not be in compensated mode when heating the cyl. I do see that putting the valve in the wrong way round [motor to DHW; spring to CH] has some advantages, but it also has disadvantages.

A Viessmann technical advisor said "it only saves 2%."

 

[MJN]

No-one claimed the Viessmann plan was a standard W Plan. It is a specific diagram fot putting weather comp on a Compact heat-only-open-vent boiler as simply as possible.

It was Eaton's persistant suggestion to use the standard (Honeywell) W-plan wiring and all would be fine:

[...] it's a very basic wiring function, check out the Honeywell diagrams, it's really not complicated, I've done quite a few

For gods sake, I do this for a living and have many times, either listen or do one, get a w plan and follow it, it's so bloody simple it's boring now, it works, they all do, your complicating things, how f*****g hard is it to tell people nowadays, if you cant understand get someone to do it for you who can

I'll admit I've not looked at any plan you've put up as I don't need to as the one I use ( Honeywell) works, I suggest you get a copy

I see now that he was alone in that advice, and indeed that it was flawed, however at the time I had to take all contributions at face value because I don't know who knows their stuff and who doesn't and whilst Eaton's aggressive manner was somewhat off-putting I could not ignore the fact that he seemingly does this for a living.

It was only your revelation that the outdoor sensor came with wiring guidance (which is not available on their website with the rest of the documentation) and that it contained a wiring diagram that works. I knew it wouldn't have been the standard Honeywell for the reasons I tried to explain.

As for Viessmann's wiring layout being 'as simple as possible', I'd say Matt1e beats it on that score...

I think there must be a misunderstanding about the savings with weather comp.

Perhaps.

I can't believe that a Viessmann technical advisor would say "it only saves 2%."

There's little more I can say on that front because I admit we didn't analyse the assertion in any detail. For what it's worth he said it 'increased boiler efficiency rating by 2%' rather than 'it only saves 2%'. Pedantry perhaps, but I wouldn't want to paraphrase and inadvertently misrepresent what was meant.

Mathew

 

[Kev F]

Also made a point about viessmans wiring but no one listens, it's rather simple either way, when hw is needed it just ramps until stat is done then let it modulate low for heat, it is the most efficient way, as for your price on sensors etc, they are way over priced, I'd pay £60 tops.

Whole point of w plan is hot water priority, as I said before, put hw timer on 30 mons early and it works very well.

Oh and if you turn a 3 port I'd suggest a auto bypass

 

[Kev F]

No-one claimed the Viessmann plan was a standard W Plan. It is a specific diagram fot putting weather comp on a Compact heat-only-open-vent boiler as simply as possible.

It was Eaton's persistant suggestion to use the standard (Honeywell) W-plan wiring and all would be fine:

[...] it's a very basic wiring function, check out the Honeywell diagrams, it's really not complicated, I've done quite a few

For gods sake, I do this for a living and have many times, either listen or do one, get a w plan and follow it, it's so bloody simple it's boring now, it works, they all do, your complicating things, how f*****g hard is it to tell people nowadays, if you cant understand get someone to do it for you who can

I'll admit I've not looked at any plan you've put up as I don't need to as the one I use ( Honeywell) works, I suggest you get a copy

I see now that he was alone in that advice, and indeed that it was flawed, however at the time I had to take all contributions at face value because I don't know who knows their stuff and who doesn't and whilst Eaton's aggressive manner was somewhat off-putting I could not ignore the fact that he seemingly does this for a living.

It was only your revelation that the outdoor sensor came with wiring guidance (which is not available on their website with the rest of the documentation) and that it contained a wiring diagram that works. I knew it wouldn't have been the standard Honeywell for the reasons I tried to explain.

As for Viessmann's wiring layout being 'as simple as possible', I'd say Matt1e beats it on that score...

I think there must be a misunderstanding about the savings with weather comp.

Perhaps.

I can't believe that a Viessmann technical advisor would say "it only saves 2%."

There's little more I can say on that front because I admit we didn't analyse the assertion in any detail. For what it's worth he said it 'increased boiler efficiency by 2%' rather than 'it only save 2%'. Pedantry perhaps, but I wouldn't want to paraphrase and inadvertently misrepresent what was meant.

Mathew

Matts drawing is the Viessmann drawing

 

[Kev F]

Also 2% saving against running it without Wc is correct

 

[matt1e]

I do see that putting the valve in the wrong way round [motor to DHW; spring to CH] has some advantages, but it also has disadvantages.

and the disadvanages are?
feel free to list them so a solution can be thought out

Matt

 

[UpgradeME]

However, I think there must be a misunderstanding about the savings with weather comp. I can't believe that a Viessmann technical advisor would say "it only saves 2%."

Perhaps he was infering that it's 2% better than the OP's previous TPI controller (I don't know by how much but arn't TPI's more efficient than a standard on/off version)? (see below)

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.

 

[matt1e]

Matts drawing is the Viessmann drawing

not quite, it's the viessmann drawing modifed to suit this one the op posted that I had altered
View media item 44013but since the op posted the Viessmann drawings, and is thinking about getting one' I thought it would be more useful to him to alter theirs

Matt

 

[Kev F]

Easier still fit system boiler 4 piped, cy sensor and rf prog stat inc wc

 

[matt1e]

Oh and if you turn a 3 port I'd suggest a auto bypass

Reason being in this scenario?