On Thu, 26 Jan 2006 18:58:57 +0000, John Stumbles wrote:

Curious one: had a call to an overflow from a cold water storage tank. The
float valve was OK but there was warm water trickling out of the vent
pipe into the tank. Fairy Nuff, obviously expansion of HW back into the
tank coupled with the vent pipe having a low clearance from the high-water
mark, in a tall house with the HW cylinder in the basement: about 10
metres from water level to base of cylinder. I adjusted the float valve to
lower the level in the tank and extended the vent pipe to give about 500mm
clearance over the water level and thought I'd fixed it but apparently
not: just had customer call back to say the still overflow runs
intermittently: for a few minutes at a time then stops again.

"Hot and Cold water supply" gives the height of the open safety vent as
40mm per metre of system height plus 150mm (for luck?) so my 500mm is a
bit short for 10m. However that should at worst result in some water
running out of the vent pipe when the system is at maximum temperature
difference between the cold feed and vent pipes. For the tank to fill up
(whether from the vent pipe or back up the feed pipe) and overflow the
water must have expanded considerably since the level in the tank is set
to about 100mm below the overflow. The tank is about 600mm x 500mm so
that's a volume of 60 x 50 x 10 = 30 litres. The HW cylinder is a largish
domestic type - say 140 litres - so if the coefficient of expansion of
water is about 0.0005/C then I reckon if its entire contents were raised
by 50C that would only cause an expansion of 0.0005 x 50 x 140 = 3.5
litres. Are my calculations correct? There seems to be an
order-of-magnitude discrepancy between what ought to be happening and what
seems to be happening.

The other possibilities I've considered a

- leakage from the primary to the secondary. However the primary F+E tank
is lower than the CW tank so any leakage would result in the F+E overflowing.

- leakage from mains to the HW through a mixer tap somewhere. I have yet
to check this.

Any thoughts?

Thanks for all so far. Thinking about it, there is a mixer tap in the
basement kitchen but I can't see mains water pushing back the HW against
the head it has down there. I can't remember if the bath on the ground
floor has a mixer but it's certainly not a bath/shower mixer. The shower
itself is pumped off the storage tank.

The flow from the vent pipe is a continuous slow run - not a trickle but
not a gush, certainly not full-bore.

I'm wondering whether I've got my calculations right: anyone like to look
them over?