Unable to run bath of hot water after boiler change

[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.

Adding Thermostatic Mixing Valve

I have NuHeat UFH system, and this has no Thermostatic Blending valve on the heating circuit. (Heat Master II cylinder has pick up coil) Nu-Heat System installed - This means I can't run store above 60C due to risk of overheating UFH pipes. I want to add a Thermostatic Mixing valve so have...

www.plumbersforums.net

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.