Can anyone advise me please what is the purpose of a right angled dip tube on the overflow in a loft water tank? The overflow runs direct to the outside in a conspicuous position. As the end of the dip tube was lying under the water level, is there a possibility that in very windy weather it could syphon water out? I ask this because I've evidence of discharge from the overflow pipe yet the water level in the tank, when I checked, seemed normal. Or maybe the old brass ball valve stuck open temporarily? I've replaced it with a Fluidmaster just in case. I suspect that a dip tube should be used with some sort of air gap device?
Given your description it sounds as though your tank has been fitted with a Bylaw 30 Kit. These were brought in in the late 70's early 80's as i recall. They found that open tanks in lofts collected all sort of debris in them everything from dust to spiders to dead rats and birds. Some systems allows a feed for the bathroom cold water to be drawn from these tanks as well as a feed for the hot water cylinder. So in effect the bathroom water becomes drinking water. Now how many people would be happy cleaning their teeth using water that at best has 20 odd years of loft dust in or at worst a dead mouse or rat or bird. The bylaw 30 kit effectively sealed the tanks from the open air with a lid that had an air vent to allow air in the overflow had the dip pipe the the one you described this served two purposes. One Peter has told you about and also as a trap to prevent spiders and small insects entering the tank. So if your system in your bathroom is tank fed and a lot of houses still are check your tanks if there open then clean your teeth at your own risk.
Thanks both of you. I googled Bylaw 30 kit and it does make sense. I think I may have only noticed half the kit! I'm going back into the loft at the weekend to double check the connections on my Fluidmaster installation and will see if there was an 'insect trap' hidden in the insulation around the warning pipe somewhere! In every other way it seems to match up to the bylaw specification. Oh, and thankfully none of the possible sources of drinking water are tank fed
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