Unable to run bath of hot water after boiler change

[John.g]

The faster the better IMO, I would suggest speed 3. Speed1 is only a 3.7M pump.

 

[sargan]

Problem is you need 0.35 lps (21 lpm) flow through the boiler so it can ramp / max

Viessmann’s ramp at the start and then slowly drop to match the flow / ntc temp for them to keep ramping you need to keep the boiler cool / get the heat away

I don’t mind trying other speeds … is there a way of getting it ‘optimum’ is this via flow & return temperatures ?

 

[ShaunCorbs]

You will know as it will stay on ideally you want a difference of 20d between f and r on full ramp / 3/4 burn depending on the store size as boiler might be oversized etc

 

[John.g]

The on site Viessmann guy said that would make it worse, needed slower flow to give it chance to transfer heat, but I’m not sure if fast or slow flow is better.

I think your boiler has a minimum output of 8kw so will need a minimum flowrate of 11.5LPM to avoid exceeding 70C from stat cutin of 60C, more importantly, all boilers fire up at ~ 65% output and then modulate down which means a high flowrate or a big dT required, because your dT of 10C (70-60) and probably lowish flowrate then its quite possible that the boiler will repeatedly cycle and not get away until the boiler return temperature (store bottom) drops well below this.
Its also possible (if unlikely) that the circ pump is faulty.

Optimum setting?, flowrate as high as possible but not > 20/25LPM. Boiler efficiency/condensing depends on getting the return temperature as low as possible but you are limited to 60C (just now) whereas a convenrional CH system can easily get down to 35/40C for alot of the time and use priority DHW to recharge the HW cylinder as fast as possible and then revert to CH.

 

[EvilDrPorkChop]

Because of the dT through the boiler and you are heating from the top down then you have to set the store stat well below 80C?, as above, as long as the demand is below the boiler max output of 30kw then once the stat falls to 60C you will be filling the store at ~ 80C and up to 85C when the stat opens at 65C. (this assumes a flowrate of 20LPM).

The store is pumped though. So it's going to be a unified temp throughout the store, It'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.

 

[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?