hot water cylinder HLT

[Peter Smith]

it says thermal rating 70C  anyone pushed these up to 78C  perhaps with a few holes in top for pressure release etc....   pid controller....

[Smiffy]

Assuming it is a copper cylinder (not an enamelled steel one for mains pressure), a higher temperature should not be a problem.

However, if this is an old one, it might be full of lime-scale which might have an undesirable effect on your water.

You do need to keep the top of the cylinder vented (the normal outlet connection will be plenty), so if you used that point to fill you will have to maintain a vent as well, and when you are draining water out of it, if you don't have it vented, the copper cylinder will collapse as the water comes out. 

Smiffy

[Crusader-Rob]

Sounds about right SMiffy.

a Local Chch guy uses a small Cylinder as HLT, and it works well, I've seen his go up to as far as 85 degrees.