How to replace an immersion heater

Inspired by a helpful YouTube guide. We based this walk-through on the Proper DIY video "Replace Your Immersion Heater Without A Plumber". It covers the draining process and the immersion spanner in detail - the two parts that most written guides gloss over.

1. Check the element is actually the problem

Before you order a new element, rule out the simpler causes. A tripped MCB or blown fuse in the immersion circuit is the most common culprit, and so is a failed thermostat - which is a much cheaper and easier fix than the element itself.

If the element has completely failed - no heat at all despite a live supply getting through to the unit - then replacement is the likely answer.

2. Isolate the electricity at the consumer unit

Immersion heaters run on a dedicated 3 kW circuit. Switch off the MCB or remove the fuse for that circuit at the consumer unit. Lock it off or tape over it if others are in the house. Then double-check with a voltage tester at the immersion unit itself before touching anything.

This step is not optional. An immersion heater runs at mains voltage and the element sits in water. The electricity must be confirmed off, not assumed off.

3. Isolate the cold water supply to the cylinder

Turn off the cold feed to the hot water cylinder. On a gravity-fed system, this is usually a gate valve or isolation valve on the cold pipe feeding into the bottom or side of the cylinder. On an unvented system, it is the mains supply - isolate it at the service valve near the cylinder.

Open a hot tap somewhere in the house to break the vacuum and let the cylinder drain more freely.

4. Drain the cylinder down

Connect a hose to the drain-off valve at the base of the cylinder - or below the immersion boss if there is no separate valve. Run it to a drain or outside. Open the valve slowly.

You do not need to drain the whole cylinder, just enough to bring the water level below the immersion heater boss. On most cylinders, that means draining roughly the top third. It takes longer than you expect - allow 20 to 30 minutes for a standard 120 litre cylinder.

5. Disconnect and remove the old element

The wiring terminal on the element is usually under a small plastic cover. Note which wire goes where - or photograph it - before disconnecting. The terminals are usually marked L (live), N (neutral), and earth.

Use an immersion heater spanner - a large, flat, open-ended spanner specifically for the job - to unscrew the element. It will be tight, especially on an older cylinder. Some light penetrating oil around the thread the night before helps a great deal. Turn anticlockwise. Expect some residual water when it comes free.

6. Fit the new element with a fresh washer

Always fit a new fibre washer - never reuse the old one. The replacement element should come with one, but check before you start.

Thread the element in by hand first to avoid cross-threading, then tighten with the immersion spanner. Firm, but not extreme force - you are seating a washer, not welding it in place. Reconnect the wiring to match your photograph.

7. Refill and test before you re-energise

Close the drain-off valve, restore the cold supply, and let the cylinder refill completely with the hot tap open. Check the area around the immersion boss for drips as the cylinder fills. Once water runs freely from the hot tap and you are sure there are no leaks, restore power at the consumer unit and test.

The water should be hot within an hour or so depending on cylinder size and element rating.

When to call a handyman

Call Richard if the job involves an unvented (pressurised) cylinder - those require a qualified person under building regulations. The same applies if the electrics look non-standard, the drain-off valve is seized, or the element boss itself is damaged.