Unable to run bath of hot water after boiler change

[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.

 

[John.g]

Practically speaking, that sounds truly incredible, most gas boilers will only go into recycle or perhaps modulate down if the dT exceeds say 30C.
Surprising (or maybe not) that the Viessmann guys didn't reveal this.
I would ask them straight out the facts of the above.

 

[ShaunCorbs]

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.

But no thermostatic overheat protection / temp blending

 

[sargan]

But no thermostatic overheat protection / temp blending

Correct it was not part of the Nu-Heat system, they did design & supplied the kit, it relied on the Thermal Store stat controlling temp.
I do intend adding a Thermostatic mixing valve, I have posted a Q on it today …. welcome any comments if you take a read.

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But no thermostatic overheat protection / temp blending

 

[ShaunCorbs]

What’s the floor construction/ covering ?

 

[sargan]

Ground floor 75mm reinforced on 1000mm insulation, screed over pipes, tiled finish mainly. Engineered wood floor on 2 rooms.
1st floor pipes clipped direct to underside of Wheyrock deck, then over covered with reflective foil & insulation. Floor covering tiled floors or laminate flooring.

Floor construction and finishes were all factored into Nu-Heat design.

 

[ShaunCorbs]

Can we see the manifold ?

 

[sargan]

Here is 1st floor manifold

 

[ShaunCorbs]

And the tiled floor is exactly the same as that ?

 

[sargan]

And the tiled floor is exactly the same as that ?

That is manifold fo4 all rooms on 1st floor.
Alupex F&R from manifold to each room, dual loops of apex pipes as the actual heating pipes.