DIY Doctor

Pressure problem with combi boiler

Postby dns » Mon Dec 08, 2008 1:04 am

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.
Rank: Apprentice
Progress to next rank:
Posts: 18
Joined: Mon Dec 08, 2008 12:53 am


Simply Build It

Postby Steve the gas » Tue Dec 09, 2008 6:43 am


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!!
Steve the gas
Rank: Site Agent
Progress to next rank:
Posts: 780
Joined: Sun May 04, 2008 7:07 am

Postby dns » Tue Dec 09, 2008 1:10 pm

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?

Rank: Apprentice
Progress to next rank:
Posts: 18
Joined: Mon Dec 08, 2008 12:53 am

Postby htg engineer » Tue Dec 09, 2008 6:32 pm

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 engineer
Rank: Project Manager
Posts: 3256
Joined: Tue May 22, 2007 5:22 pm

Postby dns » Tue Dec 09, 2008 7:04 pm

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)
Rank: Apprentice
Progress to next rank:
Posts: 18
Joined: Mon Dec 08, 2008 12:53 am

Display posts from previous
Sort by
Order by

  • DIY How to Project Guides
  • DIY how to tutorial projects and guides - Did you know we have a DIY Projects section? Well, if no, then we certainly do! Within this area of our site have literally hundreds of how-to guides and tutorials that cover a huge range of home improvement tasks. Each page also comes with pictures and a video to make completing those jobs even easier!

  • Related Topics