water pressure/combi boiler/pressure reducing valve.........

27 Oct 2007
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United Kingdom
The problem:

New Combi Boiler installed (British Gas - managed to flood ceiling and kitchen! not the reason for the thread)

Installing a new Thermo Mixer shower and was hoping that as both Hot and Cold are now mains pressure that they would be similar, however as the cold mains fed water goes through the combi boiler and heats up it drops the water pressure enourmously. The Hot water pressure was fine but the cold water to the taps (that were gravity fed) was ridiculously powerful - to the point that 1/4 open a cold tap and it was so powerful that it would hit the base of the sink and then rebound out showering water all over the floor.
BG guy's suggestion was to half close the main stop cock - did that and then the Cold water pressure was fine but the hot water was a dribble once the mains fed gold goes through the combi and is heated up.

The only solution that I can think of (not a plumber) is to keep the boiler mains fed at full pressure and then fit a pressure reducing valve after the feed to the boiler but before the cold taps that it feeds so that the boiler is fed with full pressure and then drops during the heating process but is still a decent pressure and the cold taps on mains fed via the pressure reducing valve are lowered via this to equalise the pressures.

Is this a sensible suggestion and something quite 'normal' or not?

Advice please........
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keep the boiler mains fed at full pressure and then fit a pressure reducing valve after the feed to the boiler but before the cold taps that it feeds
You wot guv?

The boiler reduces the FLOW, not the pressure, a different thing.

You need a pressure reducing valve on your cold water main. 3 bar should be plenty and stop your problems. You might want to Tee off to the garden tap before the PRV, if that's practical.

Half closing the mains stop cock doesn't reduce the pressure, but again the flow. Pressure is something you basocally measure with no flow.
Thanks ChrisR for the quick reply, however I'm more confused now.

I understand that pressure and flow are different, but kinda don't. I get that if you put a big FLOW of water into a small pipe the PRESSURE will be higher than if it was in a larger pipe but don't get how you can alter one without the other.
Like the 'half closing the stop cock' idea (not mine - the British Gas plumbers) I thought that would slow the FLOW down and therefore the PRESSURE would be less?

Anyhow, the problem, based on what you have said is that the FLOW from the cold taps is very high and the Hot is much less once it has gone through the combi heating element. I want to get the FLOW / PRESSURE (whichever it is) to be similar so that if I was to put the hot tap on and the cold tap the same amount of water will come out of them during the same time period. And as I said before I am installing a shower mixer and ideally (I understand) the PRESSURE/FLOW should be similar from both Hot & Cold feeds and at the moment the are far from equal.

Hope that helps.

I bet you love Non-Plumbers like me don't you!
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Yuh I can see what I said was confuddling. It's all easier to explain in terms of Volts Amps and Ohms, but...

Your basic problem is that your mains pressure is very high - it would be useful if you know what it was, which needs a £12 gauge.
Hence the enormous flow from the cold taps. The boiler has a clever thing in it which provides a constant flow, which it has to in order to heat the water up enough. To do that it has to provide a large resistance.
It would provide exactly the same hot flow if the mains pressure were much lower, (ie without the flow pre-knobbled by half closing the stop cock).
If the pressure at the bath cold tap were lower, you'd need less resistance (tap further open) to get good flow. That would more closely match the hot.
You COULD put a prv elsewhere than at the source, but if you have 8 bar mains it goives other things like washing machine hoses and flexible tap connectors a hard time - they burst.

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