no click, no difference if one tap or two.
no difference if you turn one or two taps on, no click. the boiler is 2 years old
If it is less than 2 years old it will still be under the manufacturers warranty
Let me dig out a diagram
You have an impeller sensor on your boiler (like a flow switch), if this is not triggered then the status code on the boiler will not change to S10.
If you are not getting S10 then the boiler does not know there is hot water being drawn
Perhaps a call to Vaillant first to see where you stand with the warranty - I find the more grief you give them (expensive boiler only lasting 2 years, etc) the more chance of a fix for free.
Contact Vaillant Service Solutions (0870 6060 777)