Unable to run bath of hot water after boiler change

[sargan]

Currently, the stat is set to 60C as that is the max temp that you can put into the heating circuit (UFH)
I have bought a 28mm UFH temperature Mixing valve.
I'll put that on the UFH take-off and use that to control Max temp into UFH, and then I can crank up Store temp to 70C or more.

The Nu-Heat design does not have a mixing valve (see diag) but I will add one.



Just to be fair, the Nu-Heat system has worked superbly for over 15 years ..... the Baxi Barcelona Heat Only boiler had come to the end of its life, so now trying to get optimum set-up for Viessman boiler.

 

[EvilDrPorkChop]

Currently, the stat is set to 60C as that is the max temp that you can put into the heating circuit (UFH)
I have bought a 28mm UFH temperature Mixing valve.
I'll put that on the UFH take-off and use that to control Max temp into UFH, and then I can crank up Store temp to 70C or more.

The Nu-Heat design does not have a mixing valve (see diag) but I will add one.
View attachment 80841


Just to be fair, the Nu-Heat system has worked superbly for over 15 years ..... the Baxi Barcelona Heat Only boiler had come to the end of its life, so now trying to get optimum set-up for Viessman boiler.

Is there no mixing valve on the UFH manifolds themsevles?

 

[John.g]

The store is pumped though. So it's going to be a unified temp throughout the store, minute or twoIt's not going to be like a cylinder with a coil. So I'd say the boiler flow needs setting at 80oc, and the Store stat at least 70oc. 60oc IMO is too low. Pump speed on 3.

Agree that it should be set to 80C & pump speed 3 but find it hard to get my head round the unified temperature bit when heat is being drawn off but agree the whole store will reach 80C with no heat demand. If the whole store is at 60C and assume the 30kw boiler is set to 80C and the flowrate is 20 LPM., the boiler fires up and because the dT through the boiler will be 21.5C. the boiler SP will be reached in a minute or two so the boiler will ramp down to 27.9 kw to maintain the 80C SP, the whole 300L cylinder will then heat up in 15 minutes, the cylinder bottom (stat) will then rise fairly rapidly and when it reaches 65C and the boiler will stop firing.
If there is a demand for say DHW then DHW flowswitch will start the store circ pump and when the cooled store water beneath the DHW Hx cools to 60C then the boiler fires up to to restore the boiler SP to 80C but you will then have a temperature gradient through the store?, is that the way you see it?.

 

[John.g]

If the store temperature falls to 60C without any call for heat what starts the store circ pump and does a call for Ch only start it?.

 

[sargan]

The sequence which is quite key ...
In the event of DHW draw off, the flow switch starts the boiler pump, but does not provide a Call for Heat ... so boiler does not fire.
This pump run 'stirs' the store to keep replenishing the plate heat exchanger.
Allowing the stored energy of water in the Themral store to provide heat without constant firing of the boiler.
When the store temp drops sufficiently the thermostat operates, then it provides call for heat to boiler.

My concern with Viessmann suggestion of using 'Cylinder Demand Boost unit' is that I would lose all of that functionality.


Think it needs an AND function, if flows switch operates AND thermostat is closed then switch to DHW Boost mode , once DHW demand stops (no flow switch) boiler return to normal mode.

Not sure if Boost Unit can accomodate that ....

 

[sargan]

If the store temperature falls to 60C without any call for heat what starts the store circ pump and does a call for Ch only start it?.

Had just put explanation of that in the thread .... comment #30
When stat open and flow switch active - pump runs to provide stir
If stat is closed ... pump will run and provide boiler with call for Heat signal. (orrespective of flow switch condition)

 

[John.g]

So the DHW boost mode ‘full power mode’ will presumably change the boiler SP to 80C and return it to 70C or whatever when DHW demand ceases?.
The only benefit I see in (if) that is that the store standing losses will be slightly lower than maintaining it at 80C at all times.

Have you tried the pump on increased speed and increased the store stat SP??.

 

[ShaunCorbs]

As soon as you use hot water the boiler should start up with a max store drop of 5 degrees

 

[sargan]

So the DHW boost mode ‘full power mode’ will presumably change the boiler SP to 80C and return it to 70C or whatever when DHW demand ceases?.
The only benefit I see in (if) that is that the store standing losses will be slightly lower than maintaining it at 80C at all times.

Have you tried the pump on increased speed and increased the store stat SP??.

My ‘understanding’. (No guarantee I‘m right)
is that the boiler max burn temp does not change.
What changes is that the ‘cylinder demand boost’ switches boiler to Hot Wafter mode ..
which means direct full power, no modulating.
when ‘cylinder demand.boost’ ceases boiler switches back to ordinary mode, but still set to 80 as max temp.

I have increased pump speed to ‘3’. I am unable to increase Store above 60C until I fit the temperature blending valves … candy risk damage to UFH pipes.

 

[ShaunCorbs]

Too late as tbh the ufh shouldn’t be running at 60 or even 50 you sure there isn’t a blending pump set on the ufh picture ?

 

[John.g]

Should have said the boiler SP.

 

[sargan]

Too late as tbh the ufh shouldn’t be running at 60 or even 50 you sure there isn’t a blending pump set on the ufh picture ?

Absolutely no blending valve on heating circuit .. though I may add one. The system has worked fine until the boiler change. This is the system schematic - no blending

 

[ShaunCorbs]

No but it’s at the bottom of the store so only 40/45 dc flow temp

 

[sargan]

It's physically about 200mm above the flow return pipe, and set to 60C
Store does get to 60C and then stat opens.

 

[John.g]

How is the system running since you increased the pump speed?

 

[sargan]

How is the system running since you increased the pump soiler temp also been increased to 80, so that may be making the change. Looking at F & R temp on the primaries they seem better at 70C and

 

[sargan]

Boiler temp increased to 80C
Pump now on speed III

I just looked now .. heat call to the boiler from store (it's below 60C) but Boiler has modulated down to low ... and the temperature on the primaries as they enter the Thermal store are:
43C Flow
32C Return

 

[ShaunCorbs]

If you look there’s two manifolds one for high temp and one for temp controlled with room stats so looks like yours have been piped wrong from day one

 

[ShaunCorbs]

Trying to get a dt 20 difference hence why it’s modulating there should be more of a difference than that something isn’t adding up

 

[ShaunCorbs]

Picture of the store please

 

[John.g]

Trying to get a dt 20 difference hence why it’s modulating there should be more of a difference than that something isn’t adding up

Are you saying the boiler is configured to try and get/maintain a 20C dT.
Maybe its just range rated to its minimum of 8kw, if not then it should be firing at 100% to try and get the flow temperature to 80C?
If it is running at 8kw then the flowrate must be 10.42LPM based on a dT of 11C and assuming that the 43C Flow is the boiler flow temperature (store return temperature, R) and that the 33C return is the boiler return temperature (store flow temperature, F) and obviously that the store circ pump is orientated properly ie pumping from the bottom of the store, into the boiler return, out of the boiler flow and into the store top.

Also no harm to check that the boiler flow and return have been connected in properly.

 

[ShaunCorbs]

Viessmann’s are picky they like a wide dt else they modulate

 

[John.g]

A catch 22 then, if the boiler is at a min output of say 8kw then the flowrate must be reduced to 5.7LPM & if the boiler needs to run at 30kw then the flowrate must be increased to 21.5LPM. would need a smart pump that modulates based on dT through the boiler. Surely not?.

 

[sargan]

If you look there’s two manifolds one for high temp and one for temp controlled with room stats so looks like yours have been piped wrong from day one

No it’s installed correctly … 28mm to 2 manifolds upstairs (flow & return) and 2 manifolds downstairs… no temperature control, but balanced flow. Each zone is temperature controlled by individual digital room stats.

There is no issue with heating, been working find since install.

 

[sargan]

Viessmann’s are picky they like a wide dt else they modulate

That is what seems to be borne out in practice.