Discuss Unable to run bath of hot water after boiler change in the Water Underfloor Heating Advice area at PlumbersForums.net
I don’t mind trying other speeds … is there a way of getting it ‘optimum’ is this via flow & return temperatures ?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 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.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.
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.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).
Is there no mixing valve on the UFH manifolds themsevles?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.
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.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.
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