Immersion Heater size

[wrighty208]

Hi,
I have a 5ft Hot Water tank which is currently heated by gas. (it's 5ft tall and very thin to fit in my airing cupboard)

What size Immersion should I get? - I have just bought a 27" one but was thinking of taking it back and getting a 36" one instead? What would be the differences or would the 27" one be ok?

Thanks in advance.

 

[TicklyT]

Assuming the heater fits almost vertically in the top of the cylinder,The longer the element, the more water it heats, but the longer it takes to heat it.
Immersion heaters generally don't heat the water below the lowest part of the immersion element in the cylinder.

A short element at the top of the cylinder will heat enough water for washing up or a quick shower fairly quickly, whilst a longer one will heat enough for a bath, but takes longer to do it.

There is the practical limit of how far down the cylinder the immersion can reach before it fouls the heat exchanger coil for the gas boiler, or the opposite side of the cylinder.