On Tue, 25 Jan 2005 11:16:42 +0000, Colin Docherty
wrote:

"Aidan" writes:

When you open the hot tap, water flows down the cold feed, up through
the hot water storage cylinder and then off through the hot
distribution pipes, passing the open vent pipe connection. The moving
water possesses a kinetic energy and there is a hydraulic resistance to
the flow of water, which absorbs some more energy. All this energy is
taken out of the water pressure, so that when the water is flowing the
pressure at every point along the pipe will drop. The pressure at any
point now depends on the water height as previously, minus the water
velocity pressure and minus pressure due to the upstream hydraulic
resistance. The pressure at the branch where the open vent pipe
connects will drop, so the water level in the open vent pipe would
normally drop when there is a draw-off.

In your system, the flow rate and/or the upstream hydraulic resistance
is so great that the pressure at the open vent branch has dropped below
atmospheric. The water in the open vent pipe drains out and air is
sucked down the open vent pipe into the flowing water. You can confirm
this by putting your hand over the open vent and getting someone to
turn on the affected bath taps. You should feel your hand being sucked
onto the pipe. Air won't get entrained in the water until you remove
your hand. You'll hear the air hissing in if you restrict the opening.
Alternatively, immerse the end of the open vent in a glass of water.
The water should get sucked out of the glass.

Solutions? Don't put a non-return valve in the open vent, as someone
is bound to suggest.

Hi,

As far as I remember, and I'll check tonight, there is NO non-return
valve on the vent pipe, so maybe this is the solution.

NO!!!!!!!!!!!!!

You will make the system very dangerous.

Read what Aidan says again.

The solutions a

1) Increase the size of pipe between tank and cylinder. Will improve
but still may not be enough

2) Move the cylinder down one floor.

3) Change to an alternative system involving mains fed hot water.
Either a heatbank, a pressurised cylinder or even a combi boiler.
Check that the cold water mains is adequate before contemplating this.




So, if I fit the non-return valve, does this mean that when running
the tap hard the vacuume created by the downward water to my tap will
effectively provide a suction force on my hot water cyclinder, thus
providing sufficient flow to the tap.


It might do, but shouldn't be done because you are risking an
explosion and a shower of boiling water coming through your ceiling.



Thanks for the help.

Cheers!
Colin.


--

..andy

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