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Combi boiler - pressure issue affecting both hot and cold

Postby dns » Fri Jan 30, 2009 11:25 am

My girlfriend recently had her conventional boiler and cylinder replaced with a Main combi boiler.

When a single tap is opened, be it hot or cold, we get good pressure. However, when hot is running and cold is also opened, the hot water flow drops to a trickle. Particularly noticable when running hot water into the bath and flushing the toilet.

I have taken a look at the new pipework and it appears that the feed to the boiler has been taken from a 15mm pipe which used to feed the cold water taps alone. I think the problem lies with this narrow pipe now having to feed both the cold taps and hot taps via the combi.

Previously there was not a pressure problem, presumably because the supply to the cylinder in the airing cupboard came from the much larger pipe which come into the house from the street.

What would be the best way of curing this problem? I am considering upgrading the cold water supply from a 15mm to a 1" (or metric equivalent) pipe. Would this do the trick?
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Simply Build It

Postby htg engineer » Sat Jan 31, 2009 3:08 pm

That's a problem you have with combi boilers, cold from the mains tap is 15mm, waterways through boiler and DHW outlet is 15mm - you can only get so much water down a 15mm pipe - not enough to feed 2 taps at full bore.

Many people have to put up with it and learn to live with it - i'm one of them.

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