PEX pipe sizing (Geberit Mepla) for domestic installation

Hi,

[Apologies if this has been requested before - I see quite a few pipe sizing questions but not much on plastic variants]

I am about to install new hot and cold pipework for a new floor on a house in Switzerland. This will involve:

- Running H & C supply up two floors from basement to 1st manifold - roughly 4m rise.
- 3m runs from this to main Bathroom Bath/Shower/Basin/WC.
- Another trunk line from 1st manifold to 2nd manifold (4m) and ensuite items (Shower/Basin/WC).

I prefer manifolds over trunk/branch to avoid burying any junctions in walls or floors.

This is a very simplistic schematic - sorry for the lack of 3D or or other details but I didn't have time to do something more sophisticated.

We have a vented HW tank in basement, with both H &C are fairly balanced pressure of at least 4 bar.
I've settled on using Geberit Mepla PEX/alu/PEX pipework as it seems to be increasingly the standard here. As the new extension is entirely in wood, not waving a blowtorch about is also quite appealing.



So (finally) to my question!

Mepla comes in three smaller 'domestic' sizes with outside (inside) diameters in mm of 16(11.5) , 20 (15), 26 (20) : http://catalog.geberit.com/public/product.aspx?cat=GB_GB-en_1&ch=212&p=38107

Starting from the 'rule of thumb' for 22mm/15mm copper pipes I've seen quoted/in books and matching inside diameters, these would be the closest match:

- 26mm Mepla for supply lines, bath & shower
- 20mm Mepla for basin & WC

However given the layout, the fact there are very few/no junctions on pipes to create pressure drops, and based on other installations I've seen I suspect the pipe sizes are excessive. I'm leaning more towards using 20mm & 16mm instead. Current shower on ground floor has a 20mm plastic pipe for some of it's run and has very acceptable pressure & flow.

Can anyone give me some advice / thoughts on this, or direct me to a good resource?

Thanks very much

Richard

http://www.1728.org/flowrate.htm

Pipe size/flow/velocity calculator.

Thanks that is useful as a starting point. It certainly suggests that 15mm internal diameter is adequate for getting a decent 12l/min shower.

However I understand that a lot of other factors (length of run, number of joints, curves etc) will also impact this.

My inclination is to oversize the pipes - as the additional cost is marginal in overall project. However that has downsize of longer wait times to bring hot through and more heat loss after.

I'd be interested to hear what people who've done PEX domestic installations have used in a reasonable sized (4 bedroom) family home.

Richard

The problem is, the system you are thinking of using isn't standard in the UK domestic market. Mapress/crimped connections would be used in places where a naked flame isn't allowed, but a permanent union is needed, ie hospitals etc.
I would be tempted to oversize my pipes too, and insulate them thoroughly. There are a lot of services there, even allowing for diversity of use. You could install a secondary circulation system (DHW ring main) for hot water, if you want it at the outlet as soon as you open it.
PEX is a good idea. It's much more resistant to bursts when pipes freeze, if this is also an issue.
You say hot and cold are balanced at 4 bar? Is the DHW unvented? Or a thermal store with header tank?

Thanks for the helpful replies. Some thoughts/follow up questions:

- Hot and cold are at least 4 bar - they could be higher but I don't have a easy way to check at present. It's a 200L unvented DHW tank, which is heated by an oil fired boiler.

- DHW ring would be nice, but if I can keep the runs short enough, and insulated them then probably can live without it? My maximum deg leg is about 15m.

- When you say "isn't standard in UK" I assume you mean Mepla specifically? I understand that copper is still by far the most common choice for domestic installations, but also that PEX or other plastic pipes have started to get more popular? So any experience with other types/sizes would probably apply. I will probably source pipes in UK, so I could go for another brand if there was a good reason to do so.

- I was planning to use pre-insulated (6mm layer) pipe for hot supply, but for cold leave it uninsulated (links for both types below). As the pipes run in heated, very well insulated building which is always occupied, and none are for drinking, insulated cold didn't seem important. Do you think otherwise?

http://catalog.geberit.com/public/product.aspx?cat=GB_GB-en_1&ch=212&p=98018
http://catalog.geberit.com/public/product.aspx?cat=GB_GB-en_1&ch=212&p=38854

Thanks!

Richard
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PEX is becoming more popular, but with compression or locking pushfit unions. Not crimped ones, unless as I said, for commercial installations where required or specified.
As for the insulation of pipes, it depends where they're installed, what the minimum ambient temperature is likely to be and the proximity of other hot pipes.

Hey Rickety, I am planning to do a project almost exactly the same as you and am looking at the Geberit Mepla system. I hope you don’t mind if I ask a few questions or add a few bits of info:

1) Have you made any decisions on pipe sizes? I was thinking of 26mm and 16mm but am wondering whether 26mm & 20mm would be the better option.

2) Are you buying the pressing tool or hiring it? Our local Brandon’s rents out the manual one for a week at £80 with 16, 20 & 26mm tongs. There appears to be other makes of tool like REMS which sell manual tools. Cost for the manual tool is £95 and the tongs are about £90 each size. I will only need two sizes of tongs and also I plan to have as few joins as possible so don’t really want to spend the money on the tools. But I know I’ll probably end up hiring the tool twice and wish I’d bought the REMS tool.

3) Who have you bought the manifolds from? I have looked around and have been recommended Emmeti. They have a good choice of manifolds and connectors that match the melpa pipe sizes and seem to be cheaper than others.

4) Have you found any supplier of the mepla system cheaper than others?

5) Also what software did you use to do the pic you poster earlier?

Many thanks in advance for any extra info.

Cheers
Scott

Hi Scott,

Happy to help. I bought a load of Mepla pipe and parts, but haven't started fitting - will probably do this over next couple of weeks so will have some more experience then. One comment worth making now is that the pipe feels surprisingly rigid. As I bought 25m/50m coils I'm not sure how difficult it will prove to be to straighten and feed it through various holes etc. Will know better soon.

I got preinsulated (6mm) for hot runs which saves on having to sleeve it all manually, but the insulation does feel a bit fragile - you wouldn't want to drag it through a rough hole.

On your specific questions:

1) Have you made any decisions on pipe sizes? I was thinking of 26mm and 16mm but am wondering whether 26mm & 20mm would be the better option.

Click to expand...

I went for 20mm and 26mm: 26mm for the trunk runs to distributors, a feeds to showers and baths. 20mm for runs to basins and WC. Probably these are oversized even with thick walls of the triple layer pipe but preferable to constricting flow (we have fair good water pressure)

2) Are you buying the pressing tool or hiring it? Our local Brandon’s rents out the manual one for a week at £80 with 16, 20 & 26mm tongs. There appears to be other makes of tool like REMS which sell manual tools. Cost for the manual tool is £95 and the tongs are about £90 each size. I will only need two sizes of tongs and also I plan to have as few joins as possible so don’t really want to spend the money on the tools. But I know I’ll probably end up hiring the tool twice and wish I’d bought the REMS tool.

Click to expand...

I bought the two manual 20 & 26mm tools from Geberit: http://catalog.geberit.com/public/product.aspx?cat=GB_GB-en_1&ch=237&p=57867
They seem pretty decent, as they should be for 130 GBP each. I hope to recover some of this selling later. As I'm in Switzerland renting isn't really an option. If it was you probably won't need a week rental (as I understand it). You can fit all the parts, install all pipes etc and do the pressing at end in one pass - so a day might suffice?

3) Who have you bought the manifolds from? I have looked around and have been recommended Emmeti. They have a good choice of manifolds and connectors that match the melpa pipe sizes and seem to be cheaper than others.

Click to expand...

I got various 2/3 port Geberit distributors: http://catalog.geberit.com/public/chapter.aspx?cat=GB_GB-en_1&ch=212
and various adaptors to fit. Note these distributors have 3/4" taper Rp thread fittings on the input/through connections, and 3/4" G flat washer connections on ports. So you need to chose appropriate adaptors to match - nearly caught me out.

There are no valves on these - but I will probably put a valve inline, either on the input port for whole dist or each output, depending on cost.

I'm not clear you can fit Mepla on to other makes of connectors?

4) Have you found any supplier of the mepla system cheaper than others?
I used Ashworth/Grahams: http://www.ashworth.eu.com/
They were very helpful and gave me a decent discount (35%) on list price. I didn't look elsewhere as didn't have time.

Click to expand...

5) Also what software did you use to do the pic you poster earlier?

Click to expand...

OmniGraffle on Mac: http://www.omnigroup.com/products/omnigraffle/

It's a pretty generic drawing program, similar to Visio on Windows. I grabbed some random icons from web which I'm happy to send you if useful.

Hope some that is useful. Good luck!

Rick

Geberit Mepla, excellent system used extensively in hospitals in Scotland as copper is not allowed. The 16 and 20mm od sizes bend easly to save fitting and pressure loss.
My main concern about your choice to run 26mm od for 15m is the volume of water this run will contain. Which apears to be 4.7 l.
running it in 20mm od would near half this.
Calcs here.

http://ncalculators.com/area-volume/pipe-volume-calculator.htm

So if the flow restrictors on the ensuite WHB are set at 4 or 6 lpm (can’t remember which) you could be looking at a 1 minute reaction time for DHW.
Not only that but after you have washed your hands, no matter how well insulated the pipework, this 4.7 l at 60deg c will probably cool to ambient before the next use, losing energy you have paid for.

If you are indeed running at 4 bar I would download the Geberit literature and do the math. It may save you money on fittings and energy.

Also consider how you get from 20mm od onto your brassware. I would expect that 16 od should be fine for these short runs.

I dont work for Geberit and i am only expressing a concern. Geberit may be able to help if you provide the drawing above complete with pipe lengths etc.

Please don’t take my advice before checking for yourself.

Thanks for the thoughts, however I've already bought the 26/20 lengths so I'm committed now. I'll look again at my runs as I can probably shorten them a bit more to reduce the delays and loss.

Time will tell if it was oversized - I'll let you know when it's up and running what the delays are like.

Cheers

Rick

You can always put in booster pumps. . .?

Here's another link
http://www.engineeringtoolbox.com/piping-tubing-systems-t_6.html
and they have a section on the heat loss you can expect with insulated pipes.

Hi Rick, Thanks for your reply and all the info. Now the summer is here I have the central heating off and will be getting the parts in the next few weeks. Hope your installation is going well. If you have any further tips after you have finished they would be much appretited.
Cheers
scott

I wanted to know if anyone had experience of connecting the 16x2.25mm Mepla pipe to radiators.

I think I've got a couple of options:

1) Bring the Mepla pipe up through the floorboards and connect to the radiator valve via part number 601.555.00.5 (1/2in female). Both the valve and the connector are 1/2 BSP I believe, so I am assuming I can just screw them together. But the radiator valve is meant for a compression fitting so not sure whether I need another coupler. Also I was wondering if the plastic pipe might get accidentally damaged or punctured above the floorboards.

2) End the Mepla pipe under the floor boards and attach to a 90 deg bend adaptor part no. 601.252.00.5, then attach this a to 1/2inch BSP to 15m coupler, and finally run a piece of copper from that to the radiator valve.

The Drayton valves have a 16 x 2mm MLCP adaptor but the Mepla pipe is 16 x 2.25mm so don't think it is worth the risk.

Any help or advice would be appreciated,

Cheers
scott

This might do it.

http://catalog.geberit.com/public/chapter.aspx?cat=GB_GB-en_1&ch=213