Plumbing Question

Hi All,
I'm helping out with the re-plumbing of a house, the house has been gutted and significantly extended. So rather than adding a new bathroom and just joining the pipe work onto the existing pipes, I'm in the situation of there's the domestic supply pipe we need a bathroom here and here etc.
My first question is really about the sizing of the pipe, the house in question will have 3 bathrooms ( I've been told it has low mains pressure);
Up-stairs bathroom 1 bath, 1 w.c, 1 basin, 1 mixer shower. Downstairs cloak room with 1 basin, 1 w.c Downstairs shower room with 1 basin, 1 w.c, 1 mixer shower
The owner has been advised by the builder that a 15mm supply will be adequate, now I'm no plumber but when you also include the sink, washing machine and dish washer into the equation and not forgetting the supply to the combi boiler. I don't think this is right, gut feeling is 22mm (boiler has 22mm connection anyway) reducing down to 15mm at the furthest point seems to be more sensible. Anyone any thoughts on this?
My second question is more straight forward. Because of the location of one of the toilets, I need to lay a horizontal run of soil & waste pipe which will have 2 toilets on it, what fall should the pipe have?
And lastly on the above soil & waste pipe, I believe its ok to use a strap-on boss to join the basin waste to it as long as its not within 200mm of the wc connection or does this only apply to vertical soil pipes?
Hope someone can shed some light on these questions.
Many thanks.
Jeff
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As a general rule, I've only used 15mm for basins, toilets and appliances. Showers and baths get 22mm. However, you appear to be installing a combi boiler. If the mains pressure really is low, you should check that it is still enough for a mains pressure installation. What sort of flow rate can be supplied?
Christian.
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Thanks for the prompt reply Christian.
The short answer is I don't know, but you have a point. The longer answer is I was asked to go down and give them a hand, I didn't know they wanted me to plan the system, they've already bought the boiler a Valiant eco 82? the outside temporary tap fills a kettle pretty quickly, but I didn't think to check the flow rate...too many other problems to sort out
Cheers,
Jeff

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If I were doing it, I would use a conventional boiler and have a large hot water storage tank - and an even larger cold water storage tank. I would plumb the hot water system in 22mm - except for a 28mm feeder pipe from the header tank to the hot tank. I would use mains cold water - probably plumbed in 15mm - to the kitchen sink, washbasins and toilets - and to fill the storage tank. I would use stored cold water, plumbed in 22mm for the baths and showers - and would use shower pumps to boost the pressure/flow to the showers.
The above is based on gut feel rather than detailed science - but seems reasonable to me.
I will leave others to comment on what you can and can't do with regard to the waste water connections.
Roger
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On Mon, 1 Dec 2003 11:28:14 -0000, Jeff wrote:

"adequate" in that some water will come out of the taps some of the time. I'd plumb all the water in 22mm T'ing to 15mm on visible feeds, basins, loos etc. Especially with a combi in the equation.

1:40 rings a bell. There are rules about the maximum effective length on "horizontal" soil runs as well. Not effective length each bend counts as x feet etc...
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Cheers snipped-for-privacy@howhill.com
Dave. pam is missing e-mail
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