Effect of pipe size on water flow rate?
 

I'm told we have a 20mm "poly" water feed to the house, it goes-in to a 15mm
stopcock and then 15mm around the house. The pressure and flow rate at the
upstairs cold tap (mains fed, not gravity!) is very good.
As part of a move to a mains-pressure hot water system it's been suggested
that the stopcock and pipe to the cylinder should be changed to 22mm to
"improve the flow rate" - how can this be?
I would have thought that the limiting factor was the pipe run from the main
to the stopcock and if this has a bore of somewhere near 15mm it doesn't
matter what I do in the house. Am I wrong? Is there any benefit in using
larger pipes downstream of the stopcock?

Dave S

"Dave" wrote in message
...
I'm told we have a 20mm "poly" water feed to the house, it goes-in to a
15mm
stopcock and then 15mm around the house. The pressure and flow rate at the
upstairs cold tap (mains fed, not gravity!) is very good.
As part of a move to a mains-pressure hot water system it's been suggested
that the stopcock and pipe to the cylinder should be changed to 22mm to
"improve the flow rate" - how can this be?
I would have thought that the limiting factor was the pipe run from the
main
to the stopcock and if this has a bore of somewhere near 15mm it doesn't
matter what I do in the house. Am I wrong? Is there any benefit in using
larger pipes downstream of the stopcock?

Dave S

Forgot to ask ...
Is there an easy DIY way to measure the water pressure?

Dave S

Dave wrote:

Forgot to ask ...
Is there an easy DIY way to measure the water pressure?

Yes, with a pressure gauge.
http://www.screwfix.com/app/sfd/cat/pro.jsp?ts=71699&id=82412

--
Grunff

"Grunff" wrote in message
...
Dave wrote:

Forgot to ask ...
Is there an easy DIY way to measure the water pressure?

Yes, with a pressure gauge.
http://www.screwfix.com/app/sfd/cat/pro.jsp?ts=71699&id=82412

--
Grunff

:-) Thanks ;-)
Being resourceful (SWMBO says "tight") I was thinking more of a widget to
adapt a tyre pressure gauge ('seems like it should work) or some other
ingenious way of adapting something I might already have.
Dave S

Dave wrote:

:-) Thanks ;-)
Being resourceful (SWMBO says "tight") I was thinking more of a widget to
adapt a tyre pressure gauge ('seems like it should work) or some other
ingenious way of adapting something I might already have.

Yes, easy enoough to do. But you'll get the inside of your tyre gauge
wet. Depending on the type of gauge, this may not do it much good.

--
Grunff

On Mon, 20 Sep 2004 10:21:12 +0100, "Dave"
wrote:

"Grunff" wrote in message
...
Dave wrote:

Forgot to ask ...
Is there an easy DIY way to measure the water pressure?

Yes, with a pressure gauge.
http://www.screwfix.com/app/sfd/cat/pro.jsp?ts=71699&id=82412

--
Grunff

:-) Thanks ;-)
Being resourceful (SWMBO says "tight") I was thinking more of a widget to
adapt a tyre pressure gauge ('seems like it should work) or some other
ingenious way of adapting something I might already have.
Dave S


Remember that this is really only part of the story. The static
pressure will be constant wherever you measure it (allowing for height
of course). The point in whether or not pipes need to be changed
will be more an issue of the dynaimic pressure and the flow rate.
Flow rate can be measured very cheaply with a stop watch and a bucket.
In practice it is flow rate and flow distribution that is more
important.





..andy

To email, substitute .nospam with .gl

In article , Dave
writes

"Dave" wrote in message
...
I'm told we have a 20mm "poly" water feed to the house, it goes-in to a
15mm
stopcock and then 15mm around the house. The pressure and flow rate at the
upstairs cold tap (mains fed, not gravity!) is very good.
As part of a move to a mains-pressure hot water system it's been suggested
that the stopcock and pipe to the cylinder should be changed to 22mm to
"improve the flow rate" - how can this be?
I would have thought that the limiting factor was the pipe run from the
main
to the stopcock and if this has a bore of somewhere near 15mm it doesn't
matter what I do in the house. Am I wrong? Is there any benefit in using
larger pipes downstream of the stopcock?
It all depends of the relative lengths of the pipes. 20m of 15mm internal
bore will have a greater opposition to flow that 10m of same. If you had
10m of 15mm internal bore outside & 10m of 22mm inside, the opposition
to flow would be less.

If your current internal plumbing is in 15mm then you can get a feel for this
by opening the stopcock fully & doing a timed fill of a 2 gallon bucket from
the closest tap to the stopcock, then repeat for the furthest tap from the
stopcock. If it's far slower then think about changing, otherwise not.

As to the pressure thing, is it really pressure you are interested in or flow,
ie. can I get a healthy shower and can I fill the bath in a reasonable time.
You should be able to get a feel for this by filling the bath with cold only or
rigging up a temp shower head with cold feed only.

Btw, I wouldn't plumb my hot flow on a mains pressure system in 22mm
as it would delay the arrival of hot.

ps: a 20mm poly incomer is tiny . . . .

Dave S

Forgot to ask ...
Is there an easy DIY way to measure the water pressure?

Dave S

--
fred

I would have thought that the limiting factor was the pipe run from the
main
to the stopcock and if this has a bore of somewhere near 15mm it doesn't
matter what I do in the house. Am I wrong?

Only flow rate is really important, provided the pressure is enough for a
good shower. However, there is a reasonable correlation between pressure and
flow rate, as it is the pressure that makes the water move. There are plenty
of exceptions, though.

Provided you are happy with the flow rate provided by the cold mains, and
realise that opening a hot tap might mean that the same flow is shared
between the taps (it might not, of course), then you can use 15mm for the
hot water, too.

There is some benefit to using 22mm within the house even when 15mm is used
outside. You must not think of the system as a "weakest link" effect. Many
people do and find the system odd. It is actually more akin to an electrical
network with resistors. The 22mm has less resistance than 15mm, so having a
system half 22mm and half 15mm is better than a system all 15mm. It doesn't
matter which order the pipework is in.

However, in a hot water system, there is a disadvantage to using 22mm in
that there is more cold water sitting in the pipes to clear, which means a
longer wait for hot water and less efficiency when all the hot water in the
pipe sits around cooling off. This disadvantage doesn't apply to cold water
mains, though.

The ideal is to use 25mm+ MDPE incoming main and then 22mm throughout for
cold, preferably from a manifold system to reduce side effects. The hot is
then run through 15mm, provided flow rate is good, or 22mm if required to
reduce pressure losses leading to slow flow rates (very unlikely to be
required on an instantaneous combi, due to low achievable flow rates).

Christian.

In article , "Grunff"
says...
Dave wrote:

:-) Thanks ;-)
Being resourceful (SWMBO says "tight") I was thinking more of a widget to
adapt a tyre pressure gauge ('seems like it should work) or some other
ingenious way of adapting something I might already have.

Yes, easy enoough to do. But you'll get the inside of your tyre gauge
wet. Depending on the type of gauge, this may not do it much good.

Why would you get water in the gauge? Just get a plastic 2litre Coke
bottle, attach a screw-in Schraeder valve from an old bicyle tyre to a
hole drilled near the top, attach the bottle to the tap with a bit of
rubber hose and a couple of hose clips. Pressurise the air in the
bottle by turning the tap on, check the pressure at the valve.