1. As far as I'm aware CWSC = Cold Water Storage Cistern and these normally feed the Hot Water Cylinder (HWC).
Well, it stores cold water, so in that sense I'd agree, but if we call a cistern that feeds a cylinder a CWSC, one that serves cold taps a CWSC, and one that serves both purposes a CWSC it isn't clear to those on this forum that need to ask questions which we mean. My college textbooks (or perhaps it was in the WRAS blue book?) referred to a Combined Cylinder Feed and Cold Water Storage Cistern. Long winded, but at least there is no doubt what I mean when I use the term.
Of course we all call them a CWSC, but you can see that the OP might take this literally and now we've got him talking about 'upgrading the hot' and we don't know whether he means the cistern or the cylinder because we're all using terms loosely.
Also, I would note that (somewhat pedantically), the WRAS blue book insists on linking cisterns NOT one in and one out, but with separate float valves on each, balanced to kick in at the same time. Someone on this forum did once explain to me why, I think.
To reply to the OP, a negative head pump will work in positive head conditions. The basic difference is that a positive head one kicks in when it senses a gravity-induced flow, whereas the negative one senses that there is a drop in pressure due to a tap being opened.
Slightly critically, it does concern me though that you are asking these sorts of questions. While I'd rather you asked them than didn't ask them, it does strike me that you are rather out of your depth with a somewhat limited knowledge of showers, which worries me, given that this is not a DIY task but something you are taking on professionally. Are you sure you want this job? More constructively, if you're going to take on this kind of installation look up the "Mira's Guide to Pumped Showers". Once you've read and digested it, you'll be far more confident and knowledgeable.
