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Unvented HW System Pipe Sizing

Discussion in 'Plumbing and Central Heating' started by JamieB3009, 9 May 2021.

  1. JamieB3009


    24 Mar 2020
    Thanks Received:
    United Kingdom

    I have recently had my HW system converted to an unvented system and would like some advice regarding pipe sizes as i have an opportunity to upgrade any pipework before my new bathrooms are fitted in a couple of weeks.

    The existing HW system was a gravity fed system with header tank in the loft and HW cylinder in the bathroom airing cupboard. The outlet 'main' from the HW cylinder was 22mm initially (Bathroom Sink and Bath taps tapped off in 15mm), and then reduced to 15mm after the take off for the bath to feed the en-suite and then run downstairs to feed the kitchen sink, utility sink and downstairs toilet sink. See the green circle on the drawing to highlight this. All cold pipework was in 15mm. The system was as below.


    To free up space in the bathroom, I had the gravity fed system replaced with a pressurised unvented system and had the hot water cylinder relocated to the garage. When this was installed, a new 22mm cold from the main stop tap (in the kitchen) directly to the new cylinder was installed, with the existing 15mm feeds to all cold taps being retained as it was. The outlet of the new cylinder is then run in 22mm pipe from the garage back into the house, where it then ties into the existing Hot water pipework between the en-suite and the bathroom where the pipework is 15mm. This was the only realistic place access could be gained. The system is now as below;


    My question really is what size is best for the pipework on the output side of the unvented system? Is the above acceptable or should the 15mm pipework to the outlets be upgraded to 22mm. Hot water performance is miles better than before this mod was done and the pressure is great (approx. 3 bar). With both the hot and cold taps on the bath open fully, the temperature of the water is warm but not too hot. My concern on the hot side is that the pipe from the cylinder is now 22mm, which then ties into a 15mm tee where it splits, but there is still a section of 22mm further down on the upstairs leg where the feed from the old cylinder came from (bathroom). Should this be changed? Can the cold remain at 15 or should these all be upgraded to 22mm?

    I have a limited opportunity to access the upstairs pipework primarily before bathroom works start at the end of the month.

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