Bath tap too fast for the boiler?

[Alan Wheeldon]

I have a Viessmann boiler 100 35Kw. When I turn the bath tap on full the water comes out cold. If I turn it down the water comes out hot. The tap flow rate is 12 litres per minute. Why can't my Viessmann boiler cope with that? Does it need a new part? Thanks

 

[ShaunCorbs]

Fit a flow limiting device to the boiler flow rate

 

[John.g]

Any 35kw boiler should give a temp of 50C at a flow rate of 12LPM from mains water presently ~ 7C. Don't know what your bath tap flow rate is but you can either install a flow limiting device as above or just throttle the bath tap to give you the required temperature, in the summer the mains temp may reach 15C so the flow rates will be a fair bit higher at the same temperatures, examples below.
From mains temp: 7C
Flow/Temp: 18LPM/35C,15LPM/40C,13LPM/45C,12LPM/50C,10LPM/55C,9LPM/60C.
From mains temp: 15C
Flow/Temp: 25LPM/35C,20LPM/40C,17LPM/45C,14LPM/50C,13LPM/55C,11LPM/60C.