what size immersion heater should I use 11" or 27"?

Question

Does this depend on the coil in the tank or what other dependancies are there?

Answer 1

if it is a bottom fitting 11inch
top fitting 27 inch
the cylinders are made so that the heater will not hit the coil inside
thecoil is only on a cylinder that suplies the central heating as well as the hot water
if you use a 11 inch on a top entry you only heat the top of the cylinder and would use all the hot water before you could fill your bath or it would go cold halfway throgh a shower

Answer 2

One good answer from the builder and is very correct. If you are replacing a failed heater then measure the old one just to verify and set your mind at rest.
Be sure to insulate the cylinder as the cost of using the immersion will be horrendous. Don't leave it on all day but use for an hour morning and an hour evening. If you can put it on a time switch then you will not forget it.
Electricity cost around 20p per unit. 1 unit is one kilowatt for 1 hour. The immersion is 3 KW. Therefore 1 hour from cold will cost about 60pence.

Answer 3

If you are talking about a top entry heater, the longest possible is best. If there is a coil in the tank and the flange is angled this will limit the length you can fit. Best to remove the plug after draining and look inside, pass a long piece of wood or something down to see how much clearance you have.