My girlfriend has just had her boiler replaced, as the old one was very old and inefficient.
She opted for a combi model as she owns a very small house that is usually only occupied by one person.
There seems to be a plentyful supply of hot water when individual hot taps are run, but as soon a cold tap is also opened, the flow reduces to a trickle.
I have investigated the problem myself (I am a fairly experienced DIYer, but nothing more) and it appears that the new boiler takes it's supply from the same 15mm pipe which feeds the cold taps in the rest of the house.
As the old system used a conventional cylinder to provide hot water which was fed from the rising main, we never had a pressure problem.
Am I correct in thinking that I can solve this problem by running a bigger pipe from the main feed into the house, to the boiler/cold water feed?
Unfortunatly the plumber who installed the system was totally incompetent (we are currently looking into taking him to court for various reasons relating to the install) so we cannot get him back for advice!
I hope I have explained this in an understandable manner. Any help would be very much appreciated.
Apologies for not including more detail, wasn't sure how much to write!
The stop cock is fully opened and the cold water pressure on it's own is as good as it ever was.
The hot water pressure is also good when run on it's own.
The problem only comes when doing something like running a bath, when both taps are on together.
I think it's because both the cold water and feed to the boiler are coming off the same small diameter pipe (albeit mains pressure). Do you think this is likely? If so, would replacing it with a much larger pipe sort this?
The problems we're having aren't major, but the shower pressure isn't very good and baths take aaages to run. I'm also slightly worried about someone running the cold water downstairs and killing the upstairs supply and scalding someone (we're also looking at a thermostatically controlled mixer).
The pipe coming into the house from outside is much bigger (an old 1" pipe I think). I'll get a plumber to quote for continuing this feed to where the boiler is (thankfully not far away and the space under the floor is easily accesible)
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