Hi Dan.
Can you confirm, please: for these 4 flats you are "installing a 40kw boiler, 15 rads, 4 Heating zones plus a hot water zone. Each flat has a kitchen sink, toilet, basin and electric shower" - so that's two hot and four cold outlets in total in each flat?
And the boiler and unvented hot cylinder are going in a plant room?
I'm not a plumber, but have been looking in to boosting cold mains supplies recently, so may have some points that might help.
Your plant room will have a single hot cylinder to provide hot water to all 4 flats, essentially to two taps in each one? What size hot cylinder is this?
I think the solution to your problem could be as easy as a pumped accumulator as mentioned by yourself above, tho' probably a much larger size - 300 litres plus. There are a number of models to choose from like the Stuart-Turner you mentioned or the Challis CB+450 (
Challis Booster ) or Grundfos (more costly) or the BoostaMain (
very much more costly...) equivalents.
I've spoken to the boss of Challis and was impressed by the spec. The outlet from this system can be as large as 28mm, so I understand that can deliver the stored supply at a crazy rate of around 100 lpm at 3 bar should it be required (which, of course, it won't), but this would surely supply the hot cylinder and colds to the 4 flats nicely? (And the big plus of the Challis model, I've been told, is that the accumulator's stored water outlet goes
directly out to the building's supply, and doesn't pass back through the pump like on some designs.)
I think it can even store and supply water at up to 4.5 bar tho' 3 bar would be the most usual setting. In the unlikely event that you find that 450 litres isn't enough capacity (some git leaves their tap running...) then an extra accumulator can be added which would operate off the
same pump, so little extra cost. If it ever did run out, then it'll still draw the cold mains at the max 12lpm rate to keep a supply going. And the price of the CB+450 is, I think, only around the £1k mark?
The really nice thing about these pumped accumulators is the way the (3bar) pressure is
stored so that it is then released to the outlets in exactly the same natural manner as a normal mains supply. Ie: the stored 3bar delivers the water
smoothly from the moment you creak open a tap; you don't have an initial pitiful trickle followed by a pump kicking in as you would with pumped flows.
If you work out the rough max flow requirements for each flat and then contact Challis and others and put your issue to them, you should hopefully have a good idea whether it's the answer. If it is, then it should be cheap and reliable.
Oh, and since these systems are air-sealed, I have been told that they are effectively like just a 'very large supply pipe', so the water passing through is still potable and WRAS approved - so they can supply ALL the taps with drinking water with no need to have a separate mains supply to the kitchen cold