Gas boiler feeds UFH only - is a Mixer needed?

[DJKent]

Do you interpret this to mean that the pump does modulate normally, when not using LLH sensor.

YES.
thanks John -I edit'ed the post now - to remove that ambiguity.

 

[ShaunCorbs]

Hi Shaun
I just phoned Vessmann tech support.
They said that their boilers do NOT sense the Flow temp at all - no sensor there.

So they don't try to change DeltaT - they only control to get desired FlowTemp.

Pump modulation - with and without LLH
Also - if you use their LLH with a temp sensor in it - they will modulate for that temp instead of Flow. And the pump will sit at 100% and not modulate - (compared to without LLH sensor - when it does modulate)

Does that make sense.... ? or did I misunderstand the guy?

What version do you have ? 100 200 as the 200 defo does else how does the boiler know what temperature it’s at eg on the front panel did you phone the Telford office ?

Delta t is set at 20 burner will modulate to keep this as close as it can also thought you said they don’t monitor flow temperature?

You can set the pump curve in the installer menu same as vaillant

Yes the llh sensor plugs into an external module as you need extra pump controls

 

[DJKent]

What version do you have ? 100 200 as the 200 defo does else how does the boiler know what temperature it’s at eg on the front panel

The front panel controls are for Flow temp - not Return. Only Flow temp is what users/installers can see and adjust.

did you phone the Telford office

I phoned 01952 675000 - asked for tech - after some minutes got a chap who advised as above.

Delta t is set at 20 burner will modulate to keep this as close as it can

Where did you read that?

No mention of that in their detailed "Design guidelines" in ViBook
* https://static.viessmann.com/resour...df?#pagemode=bookmarks&zoom=page-fit&view=Fit

The guy on the phone said the opposite - it does NOT try to control delta - in fact it cant do that - as it doesn't even sense the Return temp !

 

[John.g]

A typo Dj but you did say that (they said) "boilers do NOT sense the Flow temp at all - no sensor there."

I have read through a lot of boiler emanuals but have never seen reference to dT control, ironically Vaillant do have a form of return temperature control but it never seems to be used or even tried out,

 

[John.g]

A typo Dj but you did say that (they said) "boilers do NOT sense the Flow temp at all - no sensor there."

I have read through a lot of boiler emanuals but have never seen reference to dT control, ironically Vaillant do have a form of return temperature control but it never seems to be used or even tried out,

Someone sent this to me today, re a Vokera Unica Boiler I think.

Its a Vokera Unica Max S.

 

[DJKent]

Yes - that doc does say that
* https://qr.vokera.co.uk/qr?action=download&key=Generic+Document|Doc-0213599|1&name=DOCUMENTS/8a/27/8a27lp9bftbwb_xezjgbjt4mzynboc1ruclfk93dmsr._h_

The boiler is weird tho - on heat-demand it runs at a fixed 75% heat for 15 minutes - and only after 15 minutes does it adjust flame to suit the demand..... 15 minutes seems a long time?

 

[John.g]

Thats the same wording in my relations Vokera Vision 20S but that certainly modulates within seconds of firing up, I would read this to mean that it limits the boiler to 75% of its max output for 15 minutes after every fire up and then allows it to reach 100% output, if required.

 

[DJKent]

it's weird anyway - to have a 15 minute limitation on every fire up?

 

[John.g]

It does on the one hand, one might expect it to only operate if the boiler temperature is very low after a prolonged outage, but, on the other hand, full output will not be required if the boiler is cycling which will only happen if the demand is continuously below the boiler's minimum output.

The restriction doesn't apply to the DHW side of a combi.