Change an immersion heater: how-to guide

If you want to know how to change an immersion heater, this easy-to-follow article will guide you through the process. Changing an immersion heater is a straightforward task, although removing the old immersion heater from the hot water cylinder can be really tough.

How to change an immersion heater

Immersion heaters run on mains voltage power. Mistakes with mains voltage power can cause serious injury or death. If you can’t safely work with electrical fittings, please get a qualified tradesman to do it instead. Don’t take any risks.

Many electrical central heating and plumbing fittings carry live current even when they are switched off at their control panel. It is therefore vital that you isolate them at the consumer unit. Make sure that other people in the household know what you have done so that they don’t restore the power supply while are you working. Before working on any electrical fittings, use a pair of electrical testers to verify that there is no current flowing through them.

Change the immersion heater – which size?

While immersion heaters use the same 2 and a quarter BSP size thread, they’re available in several different lengths. If you’re not sure which length you need, there’s a good chance that it’s written on the cap of the heater you’re replacing.

Once you’ve got your replacement lined up and your tools ready, isolate the power supply to the immersion heater and switch off your boiler.

Shut off the water supply

After isolating the power, you’ll need to shut off the water supply to the cylinder. On the pipe which supplies the cylinder with water from the cold water tank in the loft, there should be a gate valve. This valve is usually in the airing cupboard, although it could be in the loft near the tank.

If there is no gate valve, you’ll need to isolate the cold water cistern. You can do this via its isolation valve, if there is one, or by turning off the mains stopcock. You can also tie up the ball valve with a length of wood and a piece of string.

Connect a hose to the draincock

Connect a length of garden hose to the cylinder’s draincock, and bring the other end outside or to a sink downstairs. The draincock will be on an offshoot of pipe from the cylinder, or on the elbow of the supply pipe from the cold tank.

Water often weeps through the spindles of drain cocks when they are open, so put an old towel underneath it, but don’t open it yet.

Open the kitchen hot tap

Open the kitchen hot top and keep it open until it stops dripping. This will drain all the water above the cylinder out of the system, such as the water in the expansion pipe and the water in the storage cistern, if there wasn’t a gate valve on the supply pipe.

Disconnect and loosen the old heater

Remove the metal or plastic cap from the old immersion heater and disconnect all of the wires.

You’ll notice that we haven’t drained the cylinder yet at all, and that’s because it’s good practice to keep the cylinder full while you loosen the old heater. Loosening it is like trying to open the mother of all pickle jars. Over time, the constant heating and cooling of the cylinder results in a fitting that often takes a huge amount of force to remove. With the cylinder full, the mass of water prevents it from warping or crumpling under this force.

Using an immersion heater spanner, loosen the old heater. You will probably have to use all of your strength in order to do this. If that isn’t enough, tapping the handle of the spanner with a hammer may do the trick.

Change an immersion heater – drain the cylinder

Once you’ve loosened the heater, now it’s time to open the draincock. How much water you need to drain depends on which immersion heater you’re replacing. If it’s a top entry one, you’ll only need to drain off a few litres, but obviously, if it’s a bottom entry one, you’ll have to drain the entire tank. There’s a silver lining here though, as draining it completely is a great opportunity to scoop out all of the limescale at the bottom.

Remove the old immersion heater element

Once the water has stopped flowing out of the hose on the draincock, you can fully unscrew the heater and take it out of the cylinder.

It may be a good idea to file away any burrs of metal on the flange and remove any debris stuck to it. This will help to ensure a nice watertight seal.

Install the new heater

Prepare the new immersion heater by applying jointing compound and PTFE tape to it, and then insert it into the hot water cylinder.

Carefully screw it into place by hand, taking care not to cross the thread. Once it’s finger tight, you can tighten it up with the special spanner. You do not need to tighten it as tightly as the old one was.

Check for leaks

Before connecting the immersion heater up, the next thing to do is to check for leaks. Close the hot tap you opened to drain the pipework, close the draincock, and reestablish the supply to the cylinder by opening the gate valve to it or the isolation valve on the cold water tank.

While some leaks may be obvious, it can take a fair amount of time for smaller, weeping leaks to appear. It’s a good idea to leave the cylinder for a good 15 to 30 minutes to see if this happens. You may also want to cut away a small amount of any insulation foam around the heater, as this can easily hide small leaks.

Connecting the immersion heater and setting the thermostat

Once you’re certain there are no leaks, connect the heater to its electrical supply in accordance with the manufacturer’s instructions, and set the thermostat to 55 or 60 degrees. Anything less may encourage the growth of Legionnaire’s disease bacteria in the tank; anything higher isn’t really energy-efficient and increases the risk of scalds.


Once that’s all done, restore the power supply. Before turning the boiler back on, you can use your hot water ‘boost’ function to test that the new heater is working. If it is, turn it off and then turn the boiler back on.

If you have an Economy 7 cylinder and it was the top immersion heater you replaced, you should hear it get to work immediately. However, if it was the bottom one, you’ll have to wait until the next morning to see if it works, as the timer won’t activate it until the cheaper nighttime electricity tariff kicks in.