viessman/opentherm/evohome/pandora poor DHW

[Dan Robinson]

It's not a fault with the boiler. It a fault with the system design / implementation.

 

[sandyman44]

I did seek advice, from my installer, an evohome specialist.

There are specialists, and there are "Specialists". I think it safe to say that Simon and I are the later.

ok, apologies if offence given, I will put it differently. I decided I wanted evohome+a new boiler, I found 3 local installers on the honeywell site who were evo certified, talked to all of them, went with the one I felt happiest with. they specced the viessman, and the OT. I still have an amicable relationship with them but I would like to steer them in the right direction to resolve it, rather not have to call someone else in (implication being I'd be trying to recover cost of that from the first guys). if we can fix it with relatively small changes (i.e. not a new cylinder or boiler) then it stays amicable.

it is being controlled as if S plan+++++ (zone valve to the heat bank, 18 zone valves one to each rad). If we ditch OT and use a BDR91's as boiler relay, will the viessman now be able to run at full chat, or is it's primary control targetting still based on the return temperature?

 

[sandyman44]

It's not a fault with the boiler. It a fault with the system design / implementation.

no I know the boilers not faulty. but its the only way to get them out (my installer is aware, he is going to be there same day).

how exactly do you think the system should be laid out? use BDR91's only? interlock so that CH shuts off when DHW demand? or something else?

 

[Dan Robinson]

I think we have given you the answers you need already. To be Frank though, as Simon implied, if the installers came up with the spec, they should be solving the issue.

What's the point of getting the manufacturer out under warranty? It is a controls issue, they will test the boiler as if the controls are not there just to prove the point. They will know somewhere between diddly squat and sweet fa about Evohome, although they may well repeat the advice already given here.

Get the hot water doodahh for the boiler (well, IIRC it came with the boiler but the installers threw it away). Then connect the system as per the drawings in the Honeywell books.

 

[sandyman44]

I think we have given you the answers you need already. To be Frank though, as Simon implied, if the installers came up with the spec, they should be solving the issue.

What's the point of getting the manufacturer out under warranty? It is a controls issue, they will test the boiler as if the controls are not there just to prove the point. They will know somewhere between diddly squat and sweet fa about Evohome, although they may well repeat the advice already given here.

Get the hot water doodahh for the boiler (well, IIRC it came with the boiler but the installers threw it away). Then connect the system as per the drawings in the Honeywell books.

I have just RTFM the boiler installers manual. Its not exactly clear what a hot water doodah that you mention, however, page 15

block G = external 230v controls sw-live/Live/N
block C = terminal box cylinder demand accessory see separate instructions. Is this what you mean?

I am demanding that the installers resolve it (at no cost to me) but I am seeking 3rd party advice (such as this forum) to try to figure out what the root cause is myself. as I have already been given one bum answer (make it y-plan) directly from the honeywell product manager.

 

[Dan Robinson]

I know the person you mean, and I know what he is aiming at, you can't blame Honeywell for your installer's failures. I was playing with this kind of set up 5 years ago and it was never part of the Evohome environment, so you can't blame them.

Like I said, the answers are here already. If you want your hand held a little further, you're into the realms of dedicated specialist advice and IMHO your wallet needs to be willing. Or at the least your contractor's.

 

[Dan Robinson]

make it y-plan

Did he actually say y-plan?

 

[sandyman44]

make it y-plan

Did he actually say y-plan?

he didn't say the words y-plan. but what he described is effectively that: re-instate the master zone valve on the rad circuit - so it goes back to being S-plan in plumbing terms. However then wire the ouput (C I think from memory) of the evohome DHW kit so that when DHW demands, it forces the master CH zone valve shut. So its then logically behaving like a Y-plan in terms of where the water can go. Its either the CH zone valve is open, or the DHW one, but never both. I get why he's proposed it - it means that the boiler is only ever dealing with one flavour of load. however I have tested this behaviour manually myself - by shutting all rads down and running the HW - and it still takes forever for the boiler to come up to temp.
don't you ever sleep

 

[Dan Robinson]

That isnnae a Y plan.

 

[Dan Robinson]

The boiler can only output one temperature. At a time, but it needs to know the difference between Heating and hot water to adjust its control logic.

 

[simond]

PL= Permanent live.

In other words, try removing all OT stuff and running the boiler into the Pandora only, with the zone valve wedged open.

We've fitted a number of Pandoras (whilst DPS were extant) and it is a great product. It is just a cylinder as far as the boiler is concerned.

WB1B is a pain sometimes because the Viessmann logic does try to modulate the burner too much IMHO.

Adding OT to one was a crazy idea; many of us in the trade understand the benefits of OT on paper, but like Dan says, the reality is often not quite as compelling.

There are people on here encouraging others to use OT, but they are in the main not heating installers and have no experience, just opinions and Google.

 

[sandyman44]

The boiler can only output one temperature. At a time, but it needs to know the difference between Heating and hot water to adjust its control logic.

yep, makes perfect sense. I think my installers expected OT information from the evo controller to enable to boiler do that adjustment automagically, but its clearly not working. So, ditching the evo OT for a BDR91 and wiring the hot water kit's output into the cylinder demand terminals on the boiler is the next best bet, am I correct?

 

[sandyman44]

That isnnae a Y plan.

I know its not a y-plan as strictly defined based on where and what type of valves, how they are controlled, etc. But it gives a logical OR, i.e CH OR DHW, which is exactly what Y-plan does, therefore to me, that proposal was pretty much analogous to running the system as Y-plan.

 

[sandyman44]

PL= Permanent live.

In other words, try removing all OT stuff and running the boiler into the Pandora only, with the zone valve wedged open.

We've fitted a number of Pandoras (whilst DPS were extant) and it is a great product. It is just a cylinder as far as the boiler is concerned.

WB1B is a pain sometimes because the Viessmann logic does try to modulate the burner too much IMHO.

Adding OT to one was a crazy idea; many of us in the trade understand the benefits of OT on paper, but like Dan says, the reality is often not quite as compelling.

There are people on here encouraging others to use OT, but they are in the main not heating installers and have no experience, just opinions and Google.

thanks. as part of this test, do I also need to send a live to the relevant terminal on the cylinder demand terminal box? am I correct to assume that is there to tell the modulation logic to stay out of the way, and thus makes the difference between the boiler running in CH-mode, cool and efficient, and DHW mode, hot and full-power? I haven't had the lid off yet, will do so , however I very much expect this terminal box is not connected.

 

[Dan Robinson]

That isnnae a Y plan.

I know its not a y-plan as strictly defined based on where and what type of valves, how they are controlled, etc. But it gives a logical OR, i.e CH OR DHW, which is exactly what Y-plan does, therefore to me, that proposal was pretty much analogous to running the system as Y-plan.

That really isn't a Y plan