Gas boiler feeds UFH only - is a Mixer needed?

[DJKent]

I've searched the forums for an hour - not found anything specific on this question.

Question 1- can we remove the UFH Mixer - and just set the Boiler Flow to 45deg?

Background (and see diagram)
I'm Helping our village hall, 14 year old UFH heating - the gas boiler just packed up - getting a new 35-40Kw balanced flue eg Viessmans, vallaint

Low Loss Header - between the boiler and the rest
Pumps - the UFH has a chunky one -after the Mixer

• No pumps at the manifolds.
• no mixers at the manifolds, just actuators

Q2) Would 'no-mixer' reduce the boiler efficiency? - given it's deltaT will be ~ 7-10 deg - (not the usual 20 that people talk about with rad systems).

* the 7-10 degree value comes from a @John.g post I saw while

Condensing efficiency is higher - many people say - at lower temps - so that is not an issue.



Q3) Would the boiler simply run for 'longer bursts' with shorter gaps - without the mixer?

PS
In 5-10 years - maybe they will move to HeatPumps... so a Mixer will definitely not be needed then..

 

[John.g]

If the boiler is only required for UFH heating only, then, Q1, IMO, yes, remove mixer.
Q2: Boiler efficiency will be excellent, as the return temperature will be very low, in the region of 35/38C.

 

[ShaunCorbs]

Yes do you have the spec of the ufh as 40kw is a lot ?

 

[king of pipes]

40 kw of ufh is alot as Shaun said I did a 12 m x 6 m room and that's was 14 kw ?? Viessman, or Valliant can be range rated and set to a low run temperature . Kop

 

[ShaunCorbs]

Also another thing it’s kw at design temp eg 35 or 40dc etc so let’s say boiler was 80
Your half your load

 

[John.g]

40 kw of ufh is alot as Shaun said I did a 12 m x 6 m room and that's was 14 kw ?? Viessman, or Valliant can be range rated and set to a low run temperature . Kop

This is a Village Hall.

 

[DJKent]

If the boiler is only required for UFH heating only, then, Q1, IMO, yes, remove mixer.
Q2: Boiler efficiency will be excellent, as the return temperature will be very low, in the region of 35/38C.

thanks @John.g - that's great to hear

 

[DJKent]

Also another thing it’s kw at design temp eg 35 or 40dc etc so let’s say boiler was 80
Your half your load

Not sure I understand.
Do you mean that a boiler ran at 45 Flow - will only produce 1/2 the Kw compared to it's Datasheet rating, running at~ 70

? That would be a real pain...

 

[John.g]

The boiler will definitely produce the same, or even slightly more power, at the lower flow temperature, as it will be condensing more.

With no TMV but with a LLH, your flows etc should look like this at a 35kw UFH demand at a dT of 10C.

I wonder if a LLH is even (theoretically) required as with the boiler flow/return temps and the UFH flow/return temps exactly the same then the secondary pump is only acting as a booster pump?.

 

[DJKent]

Thanks @John.g for doing the calcs -you're a star!

The 2012 docs state
* a total flow rate of 58 l / min at manifolds (commissioning doc) - versus your diagram says 50 - so pretty close

* across 2 Zones: Manifold 1 - has 8 actuators - 25 l/min against 23kPa -says commissioning doc.
------------------------ manifold 2 - has 10 actuators - 33 l/ min
----------------------- individual actuators flow range from 2.9 lmin to 3.6 lmin

Flow temps
The original 2012 design docs say UFH Flow 55m return 45.
But we seem to have run UFH at values ranging from 45 to 50 the last year or 2 (according the 24/7 temp logging we put in - from openenergymonitor.org

I wonder if a LLH is even (theoretically) required

Good question. It's already in situ - so no work to leave it there.
- but there is a little more complexity than I've given above.

Extra circuit - 2nd circuit after LLH -with own pump: for Radiator loop.
Used to be fed at 70 deg -when the old boiler ran at that
There are 10 rads across the building but in reality virtually never used.
So we decided to run them at 45 degrees.

So the LLH header cold maybe be helpful (I'm guessing) to allow the flow rate fluctuation as the radiator pump runs ./ or not: and the UFH zones flow rate fluctuates as Zone 1and or Zone 2 actuators close?

Have you thoughts - on how we balance the two pumps for the two (UFH, Rads) circuits after we move to 45deg Flow?

radiator - full details

• only 1 room (with 1 rad) is used much.
• some rads at the end of the bigger of the 2 Halls (10% of total hall area) - the UFH heating was not built there because of marble floor: in the past they've been off mostly, in the controller anyway
• spec doc says : stelrad planar K2 - total sizing: 16kw

 

[John.g]

Well, even though you may rarely, if ever, use the rads, I suppose nice to keep the option open and if required then would certainly be more prudent IMO to retain the UFH mixing as it gives, or should give, the tightest UFH control, without the mixer then balancing the two pumps might be quite challenging as the LLH can and does act as a "mixer" in its own right depending on the pump settings.

Apart from the boiler, are the existing systems working OK??, if so, why not just renew the boiler, can do a few more calcs then to see if low boiler return temperatures can still be achieved which I think it can.

 

[DJKent]

Well, even though you may rarely, if ever, use the rads, I suppose nice to keep the option open and if required then would certainly be more prudent IMO to retain the UFH mixing as it gives, or should give, the tightest UFH control, without the mixer then balancing the two pumps might be quite challenging as the LLH can and does act as a "mixer" in its own right depending on the pump settings.

Apart from the boiler, are the existing systems working OK??, if so, why not just renew the boiler, can do a few more calcs then to see if low boiler return temperatures can still be achieved which I think it can.

Thanks again John. Yes - we'll leave the Mixer in situ as a future 'nice to have'.

Balancing the two pumps - any tips on how we do that - is it just turn everything to max and go round measuring temps at every rad and UFH actuator?

> why not just renew the boiler

Yes -that will be our Phase 1

> are the existing systems working OK??
Not really -
A) Usability
an old Siemens controller is a Usabilty nightmare for the staff - so we've googled for hours -and looks like EvoHome has the most usable Interface -a panel which can sit in the office here / supports loads of Zones / Does not need Internet to work.
B) controllability
old controller not flexible enough - we had lots of boiler hunting that we could never iron out. I think the installers back in 2012 got it wrong: wiring wise - Siemens 2-wire protocol complexity...

Phase 1 -with the new boiler - we will leave all the old controller in place: monitor all the pipe temps with our 24/7 openEnergyMonitor kit - and see how the 45 degree thing changes stuff.

Then Phase II - put in EvoHome: simple requirement really: 4 zones

• 4 wireless temp sensors (UFH 3 halls.rooms + 1 room with only radiator)
• 3 wireless Relays - to control the actuators
• 1 wireless TRV to control the 1-room rad.

Maybe Phase III -add the OpenTherm addon; and put the boiler on that instead of on/off relay

 

[John.g]

Running with the LLH only mightn't be too bad, in theory anyway. If the two pumps are set up to give no mixing in the LLH at the UFH max output, then if the UFH requirement changes then the primary circ pump will probably circulate at the same rate and the secondary will circulate close to the UFH requirement which won't have a huge effect on the boiler return, see below. Obviously the rads will change the picture somewhat but as you say, you can always reinstate the UFH mixing.
The numbers are interesting anyway re the LLH.

 

[DJKent]

Thanks John: your diagrams are brilliant. But not easy to quickly grasp for a newbie like me!

Maybe we two could take this off line?

Here's my diagram so far... we will need 58 LPM for the UFH -which is faster than when mixing down from 70:

 

[ShaunCorbs]

jon we've had this disucssion before your drawings are wrong in 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

total ufh load 58lpm (0.967 l/s) at a delta t of 7 which is optimal (anything more and you start to run into problems with even heat) is a load of 28.43kw

if you now had a 30kw condensing boiler this would need a flow rate of 21.42 lpm (0.357 l/s) to run happy with no cycling / blocking codes

as you can see this is over 1.7 times more flow than the boiler is happy / can cope with

running 50 lpm though a condensing boiler would either destroy the boiler or it would be 70kw output boiler for the flow rate to be equal

 

[ShaunCorbs]

Not sure I understand.
Do you mean that a boiler ran at 45 Flow - will only produce 1/2 the Kw compared to it's Datasheet rating, running at~ 70

? That would be a real pain...

Sorry I mean most boilers outputs are listed for a 60/40 or 70/58 output if your running at a lower temp on average 2-5kw so your load just for the ufh is 28kw you could go with a 25kw boiler if you run at 40/20 etc

 

[John.g]

jon we've had this disucssion before your drawings are wrong in 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

total ufh load 58lpm (0.967 l/s) at a delta t of 7 which is optimal (anything more and you start to run into problems with even heat) is a load of 28.43kw

if you now had a 30kw condensing boiler this would need a flow rate of 21.42 lpm (0.357 l/s) to run happy with no cycling / blocking codes

as you can see this is over 1.7 times more flow than the boiler is happy / can cope with

running 50 lpm though a condensing boiler would either destroy the boiler or it would be 70kw output boiler for the flow rate to be equal

OK Shaun, thanks, should have realized boiler size limitations.

So, in your opinion, if a 45kw replacement boiler is installed and using a LLH + manifold pump what is the maximum UFH output assuming manifold flow/return temperatures of 55C/45C with a boiler temp of say 70C/75C.

 

[ShaunCorbs]

Boiler temp to high you shouldn’t be running a condensing boiler at 70/75 max you want is 60 on a condensing other than 65dc if you have a cylinder for legionella

So 60/40 on the boiler and 40/33 on the ufh but a better option is to run the boiler at 40 and using the llh to pull / mix the return water from the ufh back into the ufh flow

 

[John.g]

So 60/40 on the boiler and 40/33 on the ufh but a better option is to run the boiler at 40 and using the llh to pull / mix the return water from the ufh back into the ufh flow

Sort of back to square one again Shaun.
No problem with the 60/40 & 40/33, problem is running the boiler at 40C with a 40C UFH. The only way I can see this working is, as I calculated originally, that the boiler dT and the UFH dT are equal, the minimum boiler flow temperature required, is, IMO, 53C to give a boiler dT of 20C with UFH dT of 7C.
Boiler/primary pump 17.92LPM @ 53C mixing with secondary pump 33.37LPM to give a mixed flow of 51.19LPM @ 40C, 33.37LPM is recirculated to the UFH & 17.92LPM is then returned to the boiler. I am assuming a 25kw UFH demand.

 

[king of pipes]

Split the load fit 2 boilers sized accordingly one for rads which are rarely used ?? and hot water , then one solely for ufh ?? give alot more flexibility Just a thought ? Kop

 

[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.

 

[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.