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 !)

Colin Wilson wrote:
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"

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.

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.

The message .com
from " contains these words:

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.

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.

--
Roger Chapman

"Roger" wrote in message
k...
The message .com
from " contains these words:

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.

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.

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.


--
Bob Mannix
(anti-spam is as easy as 1-2-3 - not)

Colin Wilson wrote:

b) I can`t remember how to work out the volume in a "cylinder"

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.

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

Removes 1 to 1.4 litres of dead-leg volume.

HTH
--
Andy

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?

PeterK wrote:
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?

http://www.fluidmech.net/jscalc/vmd03.htm

Say 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.

"Ian Stirling" wrote in message
...


http://www.fluidmech.net/jscalc/vmd03.htm

Say 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.


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

PeterK wrote:

"Ian Stirling" wrote in message
...


http://www.fluidmech.net/jscalc/vmd03.htm

Say 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.


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?

Generally a good plan.

b) I can`t remember how to work out the volume in a "cylinder"
Removes 1 to 1.4 litres of dead-leg volume.

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