Hi,

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. Is there any rule of thumb/limit on what length of heater you can
use versus the size/height of the tank?

Andy