Sat, 17 Oct 2026 17:33:45 +0100 https://www.greenbuildingforum.co.uk/newforum/ Lussumo Vanilla 1.0.3 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18491&Focus=311716#Comment_311716 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18491&Focus=311716#Comment_311716 Sat, 17 Oct 2026 13:25:49 +0100 Victorianeco
I am planning to use plastic pipework (I have previous experience with Hep2O) or MLCP (Multi-Layer Composite Pipe). Before finalizing the install, I would appreciate some advice or schematic recommendations regarding:

Pipe Sizing & Bore Differences: What pipe sizes should I run for main feeds versus branches? I’m aware of the internal diameter (bore) reduction with standard plastic/pipe inserts and MLCP fittings compared to copper.

Maximum Pipe Runs: Are there recommended maximum run lengths to prevent noticeable pressure drop or long dead-legs (wait times for hot water)?

Manifold Layout Strategy: Is it better to run one central manifold supplying all outlets individually (home-run system), or use local secondary manifolds/sub-branches (e.g., one trunk to a manifold upstairs, another downstairs)?

Schematics / Layout Examples: Does anyone have a basic pipework schematic or layout diagram they could share for a 2-bathroom plus kitchen domestic setup?

Any advice, diagrams, or lessons learned would be hugely appreciated!
Thanks in advance!]]> https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18491&Focus=311717#Comment_311717 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18491&Focus=311717#Comment_311717 Sat, 17 Oct 2026 14:22:24 +0100 build4less
For a two bathroom plus kitchen house on an unvented cylinder, I'd run the cold main and the hot trunk from the cylinder in 22mm, then drop to 15mm for individual outlets. With plastic or MLCP the bore through the inserts and fittings is noticeably smaller, so where a branch feeds a bath or a big shower it's worth going 22mm (or 20mm MLCP up to 26mm on the trunk) closer to the outlet than you would with copper.

On hot water wait times, the thing to watch is the volume sitting in the pipe rather than the length alone. Keep 15mm hot legs to a few metres where you can, and if the furthest bathroom is a long way from the cylinder, consider a short trunk plus a local manifold upstairs, or a pumped secondary return with a timer and insulated pipe.

Full home run manifolds are neat and give isolation per outlet, but they use a lot of pipe and longer hot runs. A hybrid usually works best: 22mm trunk to a manifold in each wet area, then short 15mm tails.

Insulate all the hot runs while the ceilings are down, clip properly, and pressure test before boarding. The unvented side itself (valves, discharge pipe, tundish) needs a G3 qualified installer and notifying to building control.]]> https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18491&Focus=311718#Comment_311718 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18491&Focus=311718#Comment_311718 Sat, 17 Oct 2026 15:04:40 +0100 Victorianeco
But somewhere on this forum someone had previously calculated better to run 10mm for WHB from a manifold etc.. Just trying to find this post]]> https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18491&Focus=311719#Comment_311719 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18491&Focus=311719#Comment_311719 Sat, 17 Oct 2026 16:22:20 +0100 build4less
That's a sensible way to do it, and the 10mm idea does have some merit for basins. Plastic 10mm holds roughly a third of the water per metre that 15mm does, so a 10mm hot leg from a nearby manifold gets hot water to a basin noticeably quicker and wastes less while you wait. A basin tap only needs a modest flow, so at mains pressure from an unvented cylinder 10mm copes fine over a short run.

Where it falls down is anything that needs a decent flow. I'd keep showers, baths and the kitchen sink on 15mm, and keep the 10mm runs short, ideally just a few metres from the manifold, otherwise you start to notice the pressure drop and a bit of pipe noise. Use proper 10mm inserts and fittings all the way rather than reducing at the tap connector, as that's where the bore gets really tight.]]> https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18491&Focus=311720#Comment_311720 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18491&Focus=311720#Comment_311720 Sat, 17 Oct 2026 16:31:55 +0100 WillInAberdeen
Then dedicated 15mm hot and cold runs from the cylinder to each shower (to avoid loss of pressure balancing if someone uses a tap somewhere else)


WHBs and kitchen sink tee off from the bath supply line in 15mm as it is in a handy location. If doing it new I would use 10mm from cylinder direct to WHBs as they don't run hot fast enough, people use cold instead and wander off before it runs through hot. In fact I might consider instant water heaters under each WHB instead and not bother with plumbing a hot feed!

We added 50mm extra PIR insulation around the cylinder to top up the insulation it came with, easier if connections plan for this. Leave space so it is easy to withdraw the immersion thermostat and test its safety cutout.

Most of our piping is reused copper from an earlier system, but trying to include a plastic section in every feed so fittings are electrically isolated and don't need earth bonding.

Our in-laws had bad incident with a mouse chewing a plastic push fit elbow so am no longer using those in inaccessible places under floorboards etc]]> https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18491&Focus=311721#Comment_311721 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18491&Focus=311721#Comment_311721 Sat, 17 Oct 2026 16:54:42 +0100 Peter_in_Hungary Posted By: VictorianecoWe are currently in the middle of installing a 210-litre hot water cylinder fed from our unvented/heat pump setup.
Just to clarify - Is the hot water cylinder a mains pressure cylinder on a heat exchanger from the heat pump or is the heat pump directly feeding the cylinder.]]> https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18491&Focus=311722#Comment_311722 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18491&Focus=311722#Comment_311722 Sat, 17 Oct 2026 17:14:15 +0100 Victorianeco