Pipe runs and sizes for upgraded bathroom.

jcp

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The water system in our house has been modified several times over the years, and we now want to add a bath to the downstairs shower room. The rising main (pressure about 5 bar) runs in 15mm from stopcock to the kitchen, the utility room, the upstairs bathroom, then back downstairs to feed (in parallel) the sadia pressurised hot water tank (via 3.5 bar PRV), the downstairs wc/basin/shower room, a lastly the oil fired boiler fill loop. Hot water is mainly in 15mm, but is in 22mm from the hot water tank to the upstairs bath alone. The (3.5 bar) cold water outlet at the hot water tank inlet is not used.

I have three main questions:

1. Should I (presumably) tee off the existing 22mm hot pipe and use 22mm to feed the new downstairs bath? It is relatively unlikely that both baths will be run at the same time (it’s adults upstairs, kids downstairs!)

2. Should I take the cold feed for the bath from the 15mm main - as with the upstairs bath, and (preferably?) before the feeds to the downstairs wc, basin, and boiler?

3. Should I make new direct 15mm hot and cold feeds for the downstairs shower off the relevant pipes at the hot water tank (before the bath, wc, basin and boiler feeds - to minimise their influence on shower flow rates)?

I realise that “off the cuff” answers may not be easy, so a link to an on-line source of guidance would also be very welcome.

Thanks
 
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Should I (presumably) tee off the existing 22mm hot pipe and use 22mm to feed the new downstairs bath
You could yes

Should I take the cold feed for the bath from the 15mm main - as with the upstairs bath, and (preferably?) before the feeds to the downstairs wc, basin, and boiler?
no reason not to as long as the other taps, if mixer taps, currently work ok.

Your main bottleneck is the 15mm mains, especially if you have an unvented HW cylinder. That should really be 22mm from the mains to the cylinder and a 22mm branch from that main to then branch to 15mm to feed the other cold outlets. That would maximise flow and keep noise to a minimum. It doesn't really matter where you take the feeds for the new bath TBH. If you have 5 bar dynamic then there should be minimal impact to flow, in fact I'd possibly look at a pressure reducing valve to balance out the cold supply with the hot if that's what you are actually getting.

The balanced cold outlet ("The (3.5 bar) cold water outlet") from the combination valve that feeds the unvented is to allow balanced hot and cold feeds to be run to supply mix in the body mixer taps/showers if the mains and hot water supplies are significantly different in pressures and there is water flow issues at those taps.
 
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jcp

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Hi Madrab

I should possibly have said it is my daughter’s house, parts of which date back to the 1600s. Most of the plumbing is in more modern parts of thehouse, dating from extensions in the 1980s and 2000s, but some of the pipe sizes and routes cannot easily be checked.

Yes, I would ideally lay a new 22mm cold feed from stopcock to the HWT, but that is not really practical (too much house structure, including old external walls, between these points.

The kitchen mixer tap can be fierce (very short cold feed from the stopcock), which suggests to me the mains pressure could vary a bit, but the hot flow (via the PRV on the HWT inlet) seems ok. I don’t think there is a separate cold feed PRV anywhere, but I have considered installing one on the feed to the new shower in the downstairs bathroom. The old shower had direct feeds from around the HWT and seemed to perform ok, but there was damage from the leaky old tray, and the new shower will be in the opposite corner of the bathroom. It would be simpler to connect the new shower feeds on the end of the on bath/basin/wc feeds, but I assume dedicated feeds from the HWT would be preferable.

The upstairs bath is huge (1.8m long with a broad base too), and is fed via 15mm flexible taptails (with a reducing connector on the 22mm hot feed). There is no obvious pressure or flow balance problems, but filling this large bath does take some time. A smaller 1.7m bath is under consideration.

The new downstairs bath is 1.6x0.7m, and I doubt if both baths will be run simultaneously, but downstairs bath and shower might just be used simultaneously - so a more dedicated feed for the shower seems preferable. I could use the balanced cold feed from the HWT, but I believe this could affect the hot water flow from the tank, so a cold feed from the 15mm (before the HWT) with its own PRV seems preferable.

Sorry if this is a bit of a ramble, but I would welcome any comments

Thanks again
 
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Ideally you would take a dynamic pressure reading. Use an unrestricted tap close to the mains (outside tap or appliance tap) attach a gauge and open 2 or more outlets - hot and cold given it's an unvented HW cylinder and see what the pressure does. A flow test wouldn't hurt either, start a clock and run into a bucket over a minute, measure the contents for a L/Min figure.

That gives a baseline and therefore you know what you're working with.

No matter where you take the supply from, it is all still using the same main and same size distribution pipework, therefore if there is to be a drop in pressure or flow it will be seen no matter where on the pipework the outlet is unless the dynamic pressure is high enough to sustain the flow. If the mains is delivering 5Bar dynamic then there will be very little drop off seen at the outlet as that's plenty of pressure to sustain the dynamic flow @ multiple outlets. The only issue is if any outlet is overly sensitive to inlet pressures.

If the cold mains is significantly higher than the HW pressure then in some taps/mixers/showers (mostly mix in the body type) the higher pressure supply can overwhelm the lower pressure somewhat giving an unbalanced supply. If there's a thermo valve it can throw the calibration out. In plain mixers it can actually restrict the lower pressure supply. Hence the reason for the balanced outlet on the combination valve to the unvented HW.

That's why it is sometimes advised, especially in complicated old/new systems like your daughters, to cap the inlet pressure at the same pressure as the PRV to the Unvented, just keeps everything nice and equalised and also lowers the stresses on older pipework/fittings and outlets.
 
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jcp

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Hi and thanks again for your help Madrab.

I will test the flow rates achieved when I can next go to the house: my daughter has just tested positive for COVID, so she must self-isolate, and I will have to wait until she is confirmed as COVID free! I’ll just have to get on with fitting the new shower at my house. No rest for the wicked!
 
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