DIY Doctor UK DIY Forums

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.

Hi,

Is the stop cock opened right up? Is the valve on the cold feed to the boiler properley open? Whats the cold water pressure like?

As you can tell need more info!!

Hi Steve,

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?

Thanks!

Quite common with combi boilers, as the mains cold supply from the stop tap to all taps will be 15mm, the cold supply to the boiler will be 15mm and all the hot taps 15mm.

You can only get so much water down a 15mm pipe so opening 2 taps at the same time of course the flow rate will reduce.

What type of problems is this causing you ?

Yes incresing the pipe size from the stop tap will help as long as the pipe entering the house is greater than 15mm.

htg

Brilliant, thanks for the help!

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)