Just fitted some insulation
(https://www.screwfix.com/p/hot-water-cylinder-jacket-18-x-80mm-x-1219mm/43483)
over a hot water cylinder which already had about 25mm of foam
insulation. Of course, the many pipe entries meant it was not easy to
line up the edges of the bags, and I had to cut a hole for the cylinder
thermostat, as the instructions say it should not be covered.

Mine is an old bi-metal type with a knob on the front to set the
temperature (in deg F)

--

Graham.
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