Helpful video reference. We use James Lawrence's tutorial from plumberparts, "HOW TO CHANGE IMMERSION HEATER STEP BY STEP" as the video reference here. James is a UK-based plumber based in Cambridge who runs plumberparts.co.uk, a long-established UK trade resource. The video shows the drain-down, element removal, thread preparation and refill sequence clearly and is firmly based on UK cylinder and fittings standards.
1. Confirm the element has failed
Before you drain anything, test the element with a multimeter set to resistance. A working element shows a resistance between 8 and 30 ohms depending on its wattage - a 3 kW element in a 240 V circuit has a resistance around 19 ohms. An open circuit reading (OL or infinity on a digital meter) means the element has burned through and needs replacing.
If the element reads a reasonable resistance but you have no hot water, the thermostat may be the problem. Thermostats are much cheaper and easier to swap and do not require draining the cylinder. See the testing guide first.
2. Buy the right replacement element
Most UK domestic copper cylinders use a 2¼ inch BSP (British Standard Pipe) element with an 89 mm spanner flat. The element length matters: short elements - 7 inch or 11 inch - are used in oval or horizontal cylinders and heat the top section of water. Long elements - 27 inch or longer - reach down into tall vertical cylinders and heat from the bottom, warming the whole tank.
If you can, take the old element with you to a plumbers' merchant or builders' merchant when buying the replacement. Match the thread, flange diameter, element length and wattage. The wattage is usually stamped on the element flange or on the thermostat housing. Common ratings are 2.25 kW, 3 kW and 3 kW dual-entry.
Buy a new 89 mm immersion heater spanner too, unless you already own one. A standard adjustable spanner will not grip the flange reliably and will round off the lugs - this is a tool worth having.
3. Isolate the electrical supply
The immersion heater is fed from a dedicated circuit, usually through a double-pole fused connection unit (FCU) on the wall near the cylinder. Switch off the FCU if it has a switch, then go to the consumer unit and switch off the MCB for the immersion heater circuit.
Open the cover on the immersion heater head and confirm dead at the flex terminals with your voltage tester before touching anything. The two insulated conductors going into the thermostat head should read zero.
Disconnect the brown live and blue neutral from their terminals. Label them if you are at all unsure which is which, even though the terminal positions should be marked inside the cover.
4. Drain the cylinder
The cylinder must be drained before the element can come out. Find the drain cock - a small tap or valve near the base of the cylinder - and connect a garden hose to its outlet. Run the hose to a drain or outside. Open the drain cock using a flat-bladed screwdriver or a small square key, then open a hot tap on an upper floor to let air into the system. The cylinder will empty by gravity.
On a pressurised (unvented) system rather than a traditional gravity-fed cylinder, the procedure is different and involves a pressure relief valve - do not attempt to drain an unvented system without specific guidance on the model, as mistakes can be dangerous. Call a plumber if you are not certain what type of cylinder you have.
A typical cylinder holds 100 to 200 litres. Allow 30 to 45 minutes for a full drain. You can usually speed it up slightly by opening more hot taps around the house.
5. Remove the old element
Once the cylinder is empty, position the 89 mm immersion heater spanner over the element flange. Turn anticlockwise. On a cylinder that has not been touched for years, the element may be very stiff. Apply steady pressure rather than sharp jerks. If it will not move, a little penetrating oil on the thread and another 20 minutes of waiting sometimes helps.
As the element comes loose, have a towel ready - there will be some residual water in the pocket even after draining, and it will run out when the element clears the thread. Pull the element straight out, noting its orientation if the thermostat cap has a directional clip.
Inspect the boss (the threaded socket in the cylinder wall). If the thread is corroded or cracked, stop here and call a plumber - a damaged boss means the cylinder itself may need replacing.
6. Fit the new element
Clean the boss thread with a rag and check for scale or debris inside the pocket. Apply two or three wraps of PTFE tape to the BSP thread of the new element, winding in the direction the element will be tightened so the tape does not unwrap as it goes in.
Insert the element squarely and hand-tighten as far as it will go. Then use the spanner to tighten firmly - a quarter turn past hand-tight is usually enough on a new copper cylinder. Older cylinders with thin walls need less force, not more. Over-tightening the boss on an old cylinder can split it.
Reconnect the thermostat if you removed it separately, clipping it into the thermostat pocket inside the element housing.
7. Refill and check for leaks
Close the drain cock. Restore the cold water supply to the cylinder - turn on the cold-feed stopcock or open the ball valve if it was closed. Open a hot tap upstairs to let air escape as the cylinder fills. You will hear it filling and eventually water will flow at the tap.
While filling, watch the element flange closely. Any weeping at the thread means the seal is not good - drain down again and add more PTFE or check the element is sitting squarely in the boss.
Once full and confirmed not leaking, close the hot tap.
8. Set the thermostat and reconnect the wiring
Before putting power back on, set the thermostat dial. The correct setting is 60°C. This is the temperature at which Legionella bacteria (which can cause Legionnaires' disease) are killed within two hours in stored hot water. Settings below 55°C are not safe for stored hot water; settings above 65°C risk scalding injuries at the tap and waste energy.
Most element thermostats have a dial with a pointer and a scale marked in degrees. Set it to the 60 mark. Some replacement elements come preset to 60°C - confirm this on the packet.
Reconnect the brown and blue flex conductors to the live and neutral terminals inside the thermostat head. Refit the cover. Go to the consumer unit, switch the immersion heater MCB back on, and switch on the FCU if you switched it off earlier.
The element will begin heating immediately. Allow 45 to 60 minutes for a full cylinder to reach temperature, then check the hot tap. Water should run hot and stay hot throughout a drawn-down.
When to call us
If the electrical side of the job turns out to be more than a like-for-like swap - a wiring problem at the FCU, no RCD on the circuit, or a trip that won't stay reset - Richard can carry out the electrical work and certify it. Small jobs in Sandwich are on the £10 per 10-minute rate.
Electrical problem with your hot water cylinder?
If the immersion heater circuit is tripping, the FCU has failed, or there's no RCD on an older installation, Richard can sort the electrical side and issue a Minor Works Certificate.
Contact Richard