An immersion heater, as the name suggests, is an electrical unit that fits in water storage containers and heats the water. It is a fairly simple device, consisting of a metal element submerged in the water and an external variable thermostat enabling you to control the temperature of the water. A wire connects the immersion heater terminal box to the power supply. If you find your water isn't heating, run a couple of tests before getting a replacement; it may be a wiring or fuse problem that you can fix yourself.
Put on a pair of lightweight rubber gloves before attempting to test your immersion heater. Test the wiring using a multimeter, so the electrical power must be turned on.
Turn off the wall switch to isolate power. Remove the cover from the immersion heater. It's usually held in place by one or two screws. Use a screwdriver to remove the screws, then lift off the cover. You now see wires connected to one end of a terminal block. The wires then split out from the other end of the block and connect separately to the thermostat and the immersion heater element.
Set your multimeter to read up to 120 volts. Turn on the wall socket, so electricity flows to the immersion heater. Carefully place the prong on the end of the black wire from the multimeter onto the screw in the terminal block holding the white or grey wire in place. Place the prong on the end of the red wire from the multimeter onto the screw on the terminal block holding the red, black or orange wire.
Read the multimeter. If it reads 110 volts, then electricity is getting to the terminal block. It there's not a reading, you need to replace the fuse, and you've solved the problem. If there is a reading, continue testing the immersion heater.
Follow the wires that go from the terminal block to the thermostat, then find the terminals that go from the thermostat to the element. Ensure that the thermostat is set to the highest setting. The thermostat has two terminals, and red and black wires connect to them.
Place the prongs of the multimeter on the terminals. Place red on red and black on black. Read the meter. If it reads 110 volts, the thermostat is operating and letting electricity flow to the element. If you have a reading, the element has blown as it's the only item left in the chain. In that case, replace the unit. If the meter doesn't have a reading, the thermostat isn't working and needs replacing.
Remove the multimeter wires, and turn off the electricity. Replace the cover on the immersion heater.
Electricity is dangerous. Only complete this task if you are reasonably proficient at do-it-yourself jobs. If you have any doubts, get a qualified electrician to test your immersion heater.