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I have just moved into a victorian cottage that has a shared water main. The water supplier advises that at the stopcock the pressure is nearly 2bar but the flow rate we are getting is only 5l/min. I need to change the boiler and replace with a combi (space issues around water storage), clearly this flow is unacceptable, but has anyone got any suggestions as to the way forward?

There are a couple of points worth a mention here.

Firstly, unless the supply to the property has been upgraded in the past, you may find the pipes are lead which of course ought to be replaced anyway.

Secondly, 2 bar is not a very high pressure. The average is over 3 bar and has been seen as high as 10 bar. Combi's need a minimum of 1 bar to work.

It would seem logical to consider replacing the supply to the property with a new 25mm (or even 32mm if no extra charge involved). Most water companies will install a new supply from the main to your boundary FOC especially where it replaces a shared supply.

There is only one way without diging up your water main and replacing it with a larger service and that is some form of pump, pumping direct from the main water supply is against water board regs so you have to introduce a break tank of a suitable size to suit the house and then pump from this. Grundfos do a packaged system called a home boost pump, but as with all pumping solutions it's not cheap and will cost in the region of £800. The advantage it's got is that it's preplumbed and prewired, so installation time is reduced to a minimum. There are other methods but they all involve a tank, they all involve a pump. The thing about digging up your water main is that if you put a larger pipe in, a 2 bar pressure is not good so you may increase your flow rate but decrease your pressure and you are back to the same solution, a break tank and a pump.

My brand new 28kw Potterton HE+ combi delivers 14.5 l/min hot water, the new all-systems shower is great, but the bath flow is the only noticable disappointment ;-- basins and sinks and toilet cistern are OK.
Nine radiators are well within its capability.
The water pressure quoted by the plumbers test is 1.95 bar with 5 l/min flow running during the test.
If you DO have lead pipes supplying your house, they will attract a replacement grant thro' your local Council, on health grounds, but often grants are in packages, eg thermal insulation/draught-proofing.