On Fri, 07 Nov 2008 11:08:15 +0000, Andy Kirkland
wrote:


I've got a hot water tank (approx 4' high) and it's currently got a tiny
immersion heater element (11" from what I can guess off the label and
also the pitiful amount of water that a single heating produces).

I would like to replace it to enable it to heat the majority/entire
tank.

Our (non-std size) direct cylinder was heated for years with an 11" 3kW heater
but it was fitted at the bottom of the tank so could heat the whole contents.
When I fitted a new indirect cylinder last year (standard 900 x 450mm B&Q type)
I also fitted a 27in immersion heater at the top.
I have posted a graph showing the temperatue at the top of the tank and at about
1/3 way from bottom. The lower trace shows a declining temperature (as the
central heating was turned off for the test) while the upper temperature was
cycling.
http://www.diy.110mb.com/imm27.jpg

If your cylinder is also indirectly heated from a CH boiler then the coil /may/
obstruct a longer immersion heater.

Geo