hope these replies to all your question will help you guys in helping me sort this dilema out.
spoke to the Hansgrohe chap and he just said that i should contact the heating design engineer for this solution. but all the experience i have had in the past with plumbers is that their ok for any mainstream bog standard installations and anything slightly complicated they tend to shake their heads and just disappear!...... pls someone help
Note that you mentioned
TWO trades there - although there is considerable overlap.
The Hansgrohe chap is completely correct - if you aren't in a position to design it yourself (your questions and attitude demonstrate you are not) then you will need to consult a
heating design engineer. A plumber is generally the guy that fits stuff - but he should also be capable of doing designs for most "common" types of installations. A heating design engineer will do all the calculations to ensure that you get something that will work.
I write as a disinterested party - my day job is nothing to do with plumbing or heating, I only do a bit of plumbing for DIY purposes. There are some well seasoned professionals who have been trying to help you - but all you've done is criticise them for expecting you to have some idea of what you are asking them to design for you (for free I might point out). They aren't asking about flow rate and pressure to be awkward, they are asking because these things are critical to a design. "Brilliant" is not a measure. Get it wrong and you could find your "amazing" flow is just inadequate and your showers "disappointing"
Now, if you have no idea how to find out the information they are asking for, then just say so and I'm sure someone will point you to instructions.
Now, speaking personally, I think you are way OTT thinking about those water guzzling shower heads - and especially four of them. It's not affecting us up here in the North West, but I gather you guys in the south are finding water a tad short. I'll give you a hint - it's probably not going to be getting better with the constant increase in number of people/homes. Of course, if every new/refurbished house fits showers that will allow them to get through 1200l/day in water (20l/min x 15 min = 300l, x 4 showers = 1200l) then that's going to make the problem a lot worse.
Back to your issue though - 20l/min means a minimum of about 50kW by my reckoning (that's 20/60 l/s x 35˚ x 4.2, where 35˚ is potentially the temperature rise needed in winter, and 4.2 is the specific heat capacity of water. If you wanted to heat them directly then that would need a supply capable of 200kW to run 4 showers - so forget about instantaneous heating (eg combi boilers) unless you have an enormous gas supply. It also means you need a considerable amount of stored hot water - because unless you put a huge heating capacity in, then you'll still be mostly using stored heat. You can't necessarily rely on the heat added by the boilers while you are drawing off - it depends on the overall design as to whether the heat will keep you with usable heat at the top of the cylinder or leave you with a cylinder full of lukewarm water. The figures given earlier suggested a 2:1 ratio between hot and cold, so your 1200l/day becomes a requirement for about 800l of stored hot water and anything up to 1200l of stored cold water depending on your available mains flow rate
Of course, if you reduce any of the figures (20l/min flow rate, 15 minutes/shower, 4 showers going on at once or in short space of time) then you can reduce that.
You might also like to ponder what your proposed system might cost to run. 1/2m^3 of water, and 50kWHr of heat every day.
