Does Megaflo need separate water supply?

[worriedhouseholder]

I am in process of getting loft extension designed for top floor maisonette, which will include an additional bathroom. The architect and potential contractors have specified a Megaflo tank with new boiler with no mention of needing a new water supply. But three plumbers have all said that a Megaflo would need a separate water supply. Currently, my supply comes off a shared supply into the flat below and I believe it is an older narrower pipe (probably lead).

The plumbers say my current combi boiler (WB Greenstar 30CDi) should be fine to power a shower and bath in the new loft and suggest simply having an electric shower in the other bathroom. I don't know the current pressure but it is sufficient to work an electric shower at the top of the building unless a mains tap is on elsewhere. I've tried measuring the hot water flow from the boiler and it comes out at less than indicated in the spec (9l/m compared to 13l/m) but this is rough and ready as I did not measure the increase in temperature. I'm in Thames Water area.

My question is why the Megaflo would need a separate supply. I understand that it gives great flow rate to showers but if the pressure in mains and tank is sufficient then why should the flow rate into the tank be an issue? Wouldn't it just take a bit longer to refill? Clearly, if my existing combi can do a reasonable job then I need to consider that as a serious option. My friends further down the street run a shower off a combi just fine in a flat at the top of a shared supply to 4 flats but I don't think I can assume that means it will work for me.

Any comments and advice gratefully received. Thanks.

 

[Madrab]

Ideally you wouldn't want an un-vented system running off of a shared main, primarily to minimise delivery being affect by anything they may switch on.
I'd advise you to have a comprehensive site survey done, measuring static and dynamic flow rates and pressures at peak demand. Surprised no-one hasn't already suggested that. That should give you definitive information to base the decision on.

 

[Agile]

You need to understand that you can only get a flow rate out that is what is available at the inlet.

To get proper performance you need a dynamic flow rate of at least 22 li/min @ 1.0 bar.

I don't understand how anyone acting professionally can suggest anything without first measuring the capabilities of the water supply.

Almost certainly a new supply pipe in at least 25 mm and better in 32 mm.

Tony

 

[flameport]

but if the pressure in mains and tank is sufficient then why should the flow rate into the tank be an issue? Wouldn't it just take a bit longer to refill?

It doesn't refill - it is full of water 100% of the time, water in equals water out.
Therefore if the flow rate in is poor, so will be the flow out of it.