Cold Water Tank problems

14 Nov 2017
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United Kingdom
I'd appreciate advice about a problem with the CW tank in our attic. Getting all the information out in one go is a challenge, but here goes:
Our CW tank is in the attic of our listed timber-framed house. It supplies a standard HW cylinder (22mm copper pipe) and an electric pumped shower (15mm). It's plastic and is in fact 2x25 gal. tanks joined together. We don't want to remove the tank by changing the system to a combi-boiler because we get a fair number of power cuts and having a supply of HW is valuable to us.
The attic is becoming increasingly infested with cluster flies. If you don't know what they are, they're like houseflies, black, but clean! They don't look much but are not regarded as a health hazard. They get into the CW tank, under the lids or up the overflow; once they invade an attic they tend to come back year after year. Bits of fly anatomy come through the hot taps and clog the shower mentioned above (we do have other showers). We cannot keep them out of the attic as there are always routes in with the 500-year old roof we have. And we have to be careful because we also have bats!
The problem is how to keep the flies out of the CW tank. I've done most of my own plumbing for years and my plumber is on board with this problem, but we don't have any experience of dealing with this. I'm installing an industrial type electronic fly-killer in the attic and hope to drastically reduce their number this way. We also plan to drain the CW tank, vacuum out the fly residues, and cover the tank with a porous woven membrane to keep them out. And I'm wondering whether to try to put some sort of dry filter in the overflow outlet to stop them getting in.
Does anyone have any experience or advice? Thanks
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Replace the tanks with a variety that have a close fitting lid?

Also, just a point for caution - there has been at least one hotel fire for which the cause was recorded as an electric flyer killer in the roof space. Apparently the thing had been ignored for years & a pile of fly bodies had built up beneath the exterminator. One summers day a falling fly body which had ignited during the 'zapping' fell onto the pile & set it alight. Not wishing to cause alarm, but forewarned is forearmed - it one is aware that this can happen, one can take steps to mitigate it.

edit: found a link to the article....
An appropriate Byelaw 30 kit for the Cisterns should solve the issue. Comes with an air tight lid, filter for the Overflow and a breather vent for the lid, along with an insulating jacket. If you dont know the manufacturer then you may have to look at replacing both cisterns and purchasing Bylaw 30 kits to suit.
Thanks to both Adrian and Hugh for helpful comments. I'm trying to find out just how tightly the tank lid has to fit under Byelaw 30 requirements. Cluster flies need very little space to crawl through into shelter! The tank lids will have to fit tightly all round and the breather pipe from the HW tank will have to be sealed into one of them.
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Would have thought muslin over the top of the tanks (stretched tight and taped round the sides) would do the bulk of the job.
The overflow should be running out of the loft & into the gutter or onto a roof or similar)

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