people use the same as the ones in the water tanks at home, any plumbing shop will have them.
i know some guys use these for heating there water up and this is great, its cheap to install, if you only use it once or twice its cheap to run compared to a lot of other methods.
as there are a lot of people that are reccomending these to people that might not know what they are doing or unsure of it, i thought i would post up the vital part that hasn't been mentioned, the wiring side of it.
So here is the only safe and legal way to wire one up, if these are not wired up correctly using the correct graded wire, circuit and fuses then the wire can over heat and could cause the plug/socket in the house to catch fire or even the wires in the van to catch fire.
Immersion heaters are usually rated at 3KW which, under normal circumstances would qualify them for being wired into a ring main. However, because immersion heaters can be left on, taking 3kw from a ring circuit would reduce the circuits capacity immensely so Immersion heaters are placed on their own radial circuit.
The mains cable for an immersion heater is a two core and earth 2.5mm cable. This cable should run to a 20amp double pole isolator switch (With neon indicator light) from where a heat resistant 16amp cable or flex should run to the heater.
Since April 2004 new and replacement immersion heaters must be protected by an overheating safety cut out.
Please remember to make tight all electrical connections. Loose connections can cause fires.
Plugs running continuously at 13A have a tendency to overheat and this can be seen by scorch marks on both the plug and the socket.
Whether you use a plug and socket, or an FCU, you will still need to ensure that the immersion is on its own circuit. This means that no other appliances or socket outlets are on the same fuse or breaker at the fuse box.
If you are wiring this from scratch, then do what most electricians do - fit a 16A breaker at the fuse box and then fit a 20A double pole switch at the immersion heater end, instead of a 13A FCU or plug and socket.