Increased Combi KW size vs running cost

ThisIsWeird

Torque79 said:

I got a flow rate cup...

cold kitchen sink tap - if I turn it open on sensibly it is about 12l per minute, however if open fully it just splashes everywhere, probably way more than 15l per minute.  

The water stopcock tap is 1 full turn clockwise from fully open, is this correct? 
thanks.  

Try this test by placing a pan under the tap, and running it full for exactly 10 seconds.

Times that by 6 to get litres per minute. It could well be way over your estimated 15lpm, in which case the option of a more powerful combi is there if you like.

If it's really only 15lpm, tho', then I personally wouldn't bother going larger than 30kW.