Unable to run bath of hot water after boiler change

[John.g]

Came across a few notes from a thread on here, a while ago,re a Telford 300L TS.
The gist of it was that a old fixed 17kw (gas) boiler runs almost continuously when heating the TS, the cylinder stat is in the "bottom" of the TS, set to 65C, it switches in at 60C, a hysteresis of 5C, the boiler runs with flow/return temps of 72C/60C using a grundfos Alpha2 set to max CC3 curve. By calculation, 17kw output wiyh a dT of 12C = flowrate of 20.3LPM, 1.22m3/hr which the 6M Alpha2 will deliver at a 4.5M head.
The boiler flowtemp is set to 80C to avoid the boiler cycling as it reaches 70C as soon as the stat cuts in at 60C.

Suggest you increase the boiler SP to 80C and see if the boiler ramps up. Also check setting of the circ pump as the boiler Hx may have a fairly high pressure loss and requires a high head to get a high flowrate and dT.

 

[sargan]

A bit strange how warm water return affects it, the return temperature on rads can be anything from say 35C to 50/55C and has no effect. If it is running on the lowest modulation then it seems a bit of a coincidence that its also running at its SP temperature. What are the boiler flow/return temps? If the boiler minimum output is say 5kw then a flow/return of 70/50 = flowrate of 3.6LPM, 70/60 = flowrate of 7.2LPM, very unlikely, still looks like a circulation problem but the cylinder demand boost box may rectify this?.

Where is the cylinder store temperature thermostat located?.

Stat is on bottom 1/3rd of the store.

 

[sargan]

Came across a few notes from a thread on here, a while ago,re a Telford 300L TS.
The gist of it was that a old fixed 17kw (gas) boiler runs almost continuously when heating the TS, the cylinder stat is in the "bottom" of the TS, set to 65C, it switches in at 60C, a hysteresis of 5C, the boiler runs with flow/return temps of 72C/60C using a grundfos Alpha2 set to max CC3 curve. By calculation, 17kw output wiyh a dT of 12C = flowrate of 20.3LPM, 1.22m3/hr which the 6M Alpha2 will deliver at a 4.5M head.
The boiler flowtemp is set to 80C to avoid the boiler cycling as it reaches 70C as soon as the stat cuts in at 60C.

Suggest you increase the boiler SP to 80C and see if the boiler ramps up. Also check setting of the circ pump as the boiler Hx may have a fairly high pressure loss and requires a high head to get a high flowrate and dT.

That looks like same issue .. store stat set to 60. Boiler temp set to 70C so 10 degrees hysteresis, (this is as per design guide & system manufacturer advice)
The boiler goes into low power cycling once store temp is about 45C… never full power.

As part of investigation, Viessmann found that setting boiler temp to 80C does forced boiler to go into full power burn, this is part of logic in their advice to fit ‘Cylinder demand boost module’. Which will force boiler into no -modulation mode when DHW demand.

As interim work around it is set at 80C

I asked why it worked OK with Baxi, the Viessman expert on phone advised he had previously been designer with Baxi for 15yrs, and the early Baxi condensing boilers were very simple and basic modulation … the far better control & efficiency in the current Viessmann gives much better modulation, but is an issue with the way return temp from thermal stores.

 

[John.g]

The baxi Hx may have had a much lower pressure loss than the Viessmann, looking at the pump curve below the residual head with a "6M" pump is ~ 2.8M which means the Hx pressure loss is 3.2M at 20LPM. If your pump can deliver 20LPM at this 3.3M head then assuming the minimum output of your boiler is 5kw, the boiler dT will only be 3.6C, and even with the boiler SP at only 70C then assuming a flow temp of 60C (dT of 10C) the boiler should still run at almost 14kw? theoretically a 30kw boiler with flowrate of 20LPM will run at 30kw but with a dT of 21.5C and that is why you "must" set the boiler SP to 80C?. But, just now even with the boiler SP at 70C then you should get ~ 14kw if the flowrate is ~ 20LPM.
Can you post the make/model of the circ pump and its present setting, it may only be pumping 8 to LPM just now, its vital to get a feel for its flowrate.

Ideally, IMO, the stat should be located just above the flow outlet to the pump, that way the store should never go below 60C as long as the boiler can keep up with the demand.

 

[EvilDrPorkChop]

Was always my understanding you ran a thermal store at 80oc+ anyway. Why are you running it at 60oc? When you've got a HW and heating demand, surely the store isn't hot enough to cope with all the demand.

 

[John.g]

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).

 

[sargan]

The baxi Hx may have had a much lower pressure loss than the Viessmann, looking at the pump curve below the residual head with a "6M" pump is ~ 2.8M which means the Hx pressure loss is 3.2M at 20LPM. If your pump can deliver 20LPM at this 3.3M head then assuming the minimum output of your boiler is 5kw, the boiler dT will only be 3.6C, and even with the boiler SP at only 70C then assuming a flow temp of 60C (dT of 10C) the boiler should still run at almost 14kw? theoretically a 30kw boiler with flowrate of 20LPM will run at 30kw but with a dT of 21.5C and that is why you "must" set the boiler SP to 80C?. But, just now even with the boiler SP at 70C then you should get ~ 14kw if the flowrate is ~ 20LPM.
Can you post the make/model of the circ pump and its present setting, it may only be pumping 8 to LPM just now, its vital to get a feel for its flowrate.

Ideally, IMO, the stat should be located just above the flow outlet to the pump, that way the store should never go below 60C as long as the boiler can keep up with the demand.


View attachment 80835

The pump came as part of the system kit … it’s a 3 speed Grundfos UPS 15-50, and is set to speed 1.
The thermostat set in bottom 1/3 PFD of store, about 200mm above the return pipe.
the store has an internal baffle allow differentiation between bottom (heating) and top (DHW) and part of reason it relies on pump to create a ’stir’ when DHW is being drawn off.

 

[ShaunCorbs]

Wack that to speed 2 and try again if that’s the boiler pump

 

[sargan]

Wack that to speed 2 and try again if that’s the boiler pump

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.

 

[ShaunCorbs]

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