Gas boiler feeds UFH only - is a Mixer needed?

[John.g]

If UFH only required then nice to "get away" with just using a LLH which the OP is looking at.
Just wondering if it works well with UFH requirements changing up and down as the boiler flow will probably stay fairly steady and the UFH flow might roughly be directly proportional to the UFH demand if the pump is in constant pressure (CP) mode. If the original boiler is replaced with the same output, ie, 60kw, then can probably run with the boiler temperature approaching the UFH temperature but the downside is then the boiler minimum output to cater for very low UFH demand, very interesting.

 

[DJKent]

Thanks @ShaunCorbs and @John.g.

Am I right - when the deltaT gets lower - the boiler will slow the flow-rate to lift the deltaT (applies to rads or UFH or any load)

Q - what is the sweet-spot deltaT that a boiler will try to maintain?
Q2 - does that sweet-spot delta change, depending on Flow temperature
Q3 - does any know a manufacturer document that graphs this? Or


this question applies to all of us !
It applies to 100% of all gas boiler everywhere! Whether UFH or rads - as the building gets up to temp, the deltaT will get lower than the designed 20 or etc.

It also applies 3/4 of the time to any building: as most of the time the building is 'nearly at temperature' so deltaT is smaller. Only cold-start in the morning in winter, will the deltaT be as designed to dump the max heat into the load (20 for rads: 7 or 10 or ? for UFH).

This is confusing - deltaT has many meanings ...some Rad companies use deltaT to mean: diff ( room temperature to Flow temp) - eg dT of 50 for (room 20 C, Flow 70)

 

[John.g]

Traditionally, boilers ran with a fixed pump speed which might be simply 3 different speed settings which might be a 2M or 4M or 6M constant curve, the flow rate then depended on the pump capacity, the friction losses in the boiler Hx, the system pipework and the radiators + associated valves, etc,the boiler flowrate then changed depending on zone valves opening/closing etc and TRV's when used for temprature control. Some boilers have a 4 or more pump settings that can be user changed. The very modern boilers I think now have pumps where the dT is controlled by the boiler, presumably by altering the pump speed, @ShaunCorbs can advise.

Radiator outputs are based on a 50C basis which is the mean rad temperature minus the required room temperature (generally taken as 20C) So a rad with flow/return of 75C/65C will be a 50deg rad (75+65)/2 - 20.
A rad with flow/return of 55C/45C will be a 30deg rad with a output of (30/50)^1.3, 51.5% of its rated output at 50C.

 

[DJKent]

... regards to the boiler side you wont get a condensing boiler to run at a dt 10 or 5 it will control the pump / burn rate to manage a dt 20 hence why a llh / cct is required due to the differnet flow rates etc eg

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?

 

[John.g]

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 - which it does without LLH sensor)

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

Edit: Looks like the other way round, 100% pump speed without the LLH sensor.

 

[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.