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 are:
- 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?