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