I'm having a combi boiler put in as part of a loft conversion. I was under the impression that this would involve upgrading the rising main to 22mm from the stopcock; but my plumber says you can fit it just on the 15mm rising main that's already there. Is that right? Am I going to get problems with it if I let him plumb it like that? We have pretty good pressure and about 25 l/min in the kitchen, which is right on top of the stopcock.