In article ,
Bill Haines writes:
I have this tank in the roof space. Not sure if it's plastic or
fibre-glass, because it's texture in parts is very uneven, not what I
imagine a moulded PVC tank would usually be like.
There's an obsolete 15mm outlet that used to supply a shower unit. For
some reason, maybe to maximise the pressure, the hole for this outlet
is very close to the bottom edge of the tank, leaving little room for

It's close to the bottom, so if the tank runs empty, the hot
water cuts off first so you don't get burned in the shower.
Doesn't make any difference to the pressure.

any flanged fittings that might be used to seal the hole. The current
union with the tank is a male threaded fitting that's just screwed
into the wall of the tank after being wrapped in PTFE tape. I could
try renewing this with fresh tape but I'd prefer to make a more solid
repair. Can anyone suggest an effective method?

Difficult. If there's really no room for a flange, then you
are restricted to sealing it on the cut edge of the hole.
I wonder if you can get tubeless tyre repairs that big (the
conical rubber things they pull though punctures to reseal
tyres)? There's a French method of fixing suspended ceiling
lights which works by having a short length of very thick-
walled rubber tube which you push into a close fitting hole
in the ceiling, and then compress lengthwise with a screw
and washers on each side so it swells and grips the hole.
Something along these lines might work, with some sealant to
seal around the screwholes in the washers. Make sure any
sealant you use is suitable for water tanks (i.e. not a
silicone sealant with fungicide).

--
Andrew Gabriel