Dumb plumbing / maths question :-}

[Colin Wilson]

Most of our hot & cold water pipework is in 22mm (downstairs bathroom, boiler is in a bedroom on the 1st floor) - the bloke who put the boiler in suggested dropping the pipework to 15mm to speed up how quickly the hot water gets delivered.

Given that:

a) i`m crap at maths b) I can`t remember how to work out the volume in a "cylinder"

...how much difference would it make on (at a guess) 6-8 metres of pipework, given the normal crappy combi flow rate ?

TIA :-}

(and sorry for posing the pathetically lame question !)

[Ian Stirling]

Proportional to the square of the diameter.

22/15 = ~1.5 1.5^2 = about 2.15. So, ballpark, it'll have about half the volume of stored water in the pipes.

[dom]

If your pressure is low, 22mm pipework makes a HUGE difference to flow rate. So what you save in volume in the dead-leg, you may loose in flow rate.

[Roger]

The message from " [e-mail]" contains these words:

Given that the boiler in question is a combi the limiting factor is likely to be the heat output of the boiler rather than any pipework restriction.

[Bob Mannix]

The problem I think the plumber was trying to fix was the dead time (with cold water in the pipes) before hot water is delivered. This is certainly fixed by dropping the pipe diameter as the volume goes with the square of the diameter/readius and the flow changes with a lower power of the diameter/radius for a given pressure. This means the hot water will arrive quicker with the smaller pipes. Once hot water has arrived 22mm is "better" of course but as Roger says, with a combi, it's not going to make a lot of difference to the flow, so, yes, there would be some advantage in dropping the hot water pipe sizes in the OP's case, if he thinks the bother and disruption is worth it.

[Andy Wade]

Cross sectional area times length, so internal diameter squared times pi times length divided by four. Then divide by a million to convert from cubic millimetres to litres.

15 mm tube has a wall thickness of 0.7 mm, so the CSA is 145 mm^2, giving a water content of 0.145 litre per metre run of pipe. 22 mm tube has wall thickness of 0.9 mm, so the CSA is 320 mm^2, giving a water content of 0.320 litre per metre run of pipe.

Removes 1 to 1.4 litres of dead-leg volume.

HTH

[PeterK]

With a "mains" hot water pressure reduced down to 5bar (9bar static mains pressure) and a new kitchen to install I'd been thinking of reducing the

10m "hot" leg to the kitchen sink to 10mm plastic. Any comments?

[Ian Stirling]

7mm ID. That's say 4 bar across 10m, or 40000Pa/m. I make that m5/s fluid flow.

7mm has about 36ml/m of volume, so that comes out to about 180ml/sec, or 10l/min. I'd say that's quite adequate - it's about hte same as my kitchen tap, run from a hot water cylinder. And the dead leg will only be a bit over a cup.

[PeterK]

Thanks for the link and your opinion. I presume it would be advisable to try a "lash up" first and get acceptance from SWMBO before committing?

Peter K

[Ian Stirling]

Generally a good plan.

[Colin Wilson]

Thanks - it might be worth having a think about replumbing them then !