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Discuss can anyone clear up my pressure/flow issues on an unvented cylinder? in the Plumbing Forum | Plumbing Advice area at PlumbersForums.net

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Hi all, this relates to a previous problem ive posted here before. Without going into too much detail, I have an unvented cyclinder boiler (ideal istor) that up until now had been delivering measly flow rates of 8LPM max and if I ever attempted to run 2 showers at once one of them would become unusable. I had my mains pipe changed to 25mm around a year back and still no difference.

Anyway, just recently I determined that where the pipe enters the property it is instantly reduced to 15mm and has a PRV attached with the cold and hot pipes after the PRV. The boiler/cylinder is fed with 15mm but the hot pipework leaving it is in 22mm. The cold throughout the house is 15mm.

Having noticed this I've had the 15mm piping upgraded to 22mm with a new PRV installed still set at the recommneded 2bar. As a result the flow rates with the hot are much better - around 15LPM and I can run 2 showers are once with minimal drop off.

The only issue I have now is that the 15mm piping for the cold delivers a much slower flow rate at all outlets in comparison with the hot. I've simply put this down to the fact the hot is 22mm but then cant figure why the hot becomes 15mm on a short run to the showers/taps etc. So, to me every outlet is 15mm although the hot travels a good distance at 22mm.

Would the 22mm run for the hot (reduced to 15mm at the outlets) give better flow rates than a 15mm run throughout for the cold?

I'd love to know why I have different flow rates yet the cold is off the PRV just like the hot pipe? If anyone can clear this up I'd be very grateful.
 
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