I am replacing my leaking copper cylinder (400mm x 900mm). I have low water pressure. If i replace it with a 450mm x 900mm, will there be a noticeable improvement. How large can i go without changing other items to compensate for the increase in stored water?
You need to raise the height of the header-tank in the loft-- the large one that feeds the cylinder with the water which is to be heated, and use at least 22 mm copper pipe between this tank and the new indirect cylinder. When I tackled this raising of the main header, I renewed it, with a modern plastic, flexible one, complete with thermal insulation pack.
By placing it as high as possible in the loft I have enough static head to use a blender shower, without a pump-assist. (I had used a twin-impeller pump for a year, but it siezed after a year as it was not continuously rated;-- 3 showers back-to-back was too much for it).
Using at least 22 mm copper for the hot-water output, will be a minimum, with reduction to 15 mm being very local to where it is req.