Hot water modulation on large (35kw) combi boiler.

[Blackcatgas]

Ive just installed a Viessmann 100 35kw combi boiler at the father in laws, there is currently only 2 taps connected, the kitchen mixer and a small whb, the bathroom hasnt been installed yet and is a bomb site.
The kitchen mixer is giving me 10 litres a minute of hot (there is a flow restricter fitted on it). when you turn on the mixer tap the boiler fires up and the temp on the boiler shoots up to 80deg + .The boiler then shuts down ,the water cools down etc and then it cycles on and off.The same is happening on the basin tap, I cant measure the flow rate exactly as I didnt have my flow cup or even fit a bucket under it but its much greater than the kitchen tap, est 17 litres per minute ish but cant be sure. I know its a large combi but surely the boiler should be capable of modulating down to this flow rate(10 lires per minute) without on off cycling?and why also on the basin tap too? (although I cant be sure of the real flow rate) or am I missing something?.

 

[Billy Bob Bob]

it should not do that,as you say it should modulate down you need to stick a weir gauge on and have a word with viessman technical

 

[Blackcatgas]

Problem solved!, Id fed the boiler off a pump and break tank, Id crossed the hot and cold, interestingly the Viessmann still fired up on hot working back to front, the boiler modulates down lovely on the slow tap, the temp altering no more than 2 deg.

 

[fuzzy]

your not the first