Immersion Heater Leaking? Where the Water Is Coming From

Make it safe first

Turn the immersion off at the switch beside the cylinder, and if water is anywhere near the switch, the flex or the round cap on top of the element, turn the circuit off at the consumer unit as well. Do not take the cap off to look inside while the circuit is live. A leak into the terminals is exactly the situation where a live part can end up wet.

If the leak is a slow weep, that is usually all you need to do tonight. Your hot water may still work from the boiler if the cylinder has a coil, so you are not necessarily without hot water. If water is running rather than dripping, turn the cold supply to the cylinder off as well. On a vented cylinder that is the gate valve on the cold feed coming down from the loft tank. On an unvented cylinder it is the isolating valve on the cold inlet, before the pressure reducing valve, and if you cannot find it, the stopcock will do the same job.

Never run an immersion heater if the cylinder might be partly empty. An element that is not fully covered by water burns out in minutes and can damage the boss it is screwed into.

Find out exactly where the water is coming from

The quickest way to trace it costs nothing. Dry the top of the cylinder, the flange of the immersion, the pipe joints above it and the floor underneath. Tuck a piece of kitchen paper around the edge of the immersion flange. Then leave it alone for an hour with the heating and hot water running as normal, and come back.

Whatever is wet first is your leak. Paper wet all round the flange with the pipework above it dry means the immersion joint. Water sitting on top of the cap but the flange dry underneath means something above is dripping onto it. Water inside the cap, or a breaker that has started tripping, means the element itself. If nothing is wet until the cylinder has been fully up to temperature, the joint only opens up when the metal expands, which is typical of an old or reused washer. If the water turns out to be coming from somewhere else on the cylinder altogether, our guide to a hot water cylinder leaking covers the other fittings.

• Wet all round the flange, pipes above dry: the washer or seal on the immersion boss.
• Water pooling on top of the cap, flange dry: a drip from a joint or valve higher up.
• Water inside the cap, or the RCD tripping: the element sheath has corroded through.
• Wet only after a full heat up: a seal that opens with expansion, often a reused washer.
• Rust or green staining on the cylinder around the boss: the cylinder itself, not the immersion.

The washer under the flange

This is the cause in most cases. An immersion heater screws into a threaded boss on the cylinder and seals on a fibre or rubber washer that gets crushed once when it is tightened. It is not designed to be reused. If the element has ever been changed and the old washer went back in, or a new element was overtightened, the joint can weep for years before anyone notices, usually as a brown trail running down the side of the insulation.

The repair is a new washer, and in practice usually a new element at the same time, because once the element is out it is rarely worth refitting an old one that is scaled and part way through its life. The job needs the water level in the cylinder lowered below the boss, which on a top entry immersion is a partial drain and on a side or bottom entry immersion means draining much more of the tank. That is why a leaking immersion is not a quick tighten with a spanner. Tightening a leaking element in place often distorts the boss and makes the leak worse.

The element itself has corroded through

The element is a resistance wire packed in powder inside a thin metal sheath. In London water, which is hard, the sheath builds up scale, runs hotter than it should and eventually corrodes or splits. Once the sheath is breached, water gets into the packing and can find its way up into the terminal head. The usual sign is not a puddle but the electrics: the immersion trips the RCD or the breaker the moment it is switched on.

An element cannot be repaired. If water has got inside it, it is replaced, and the terminal head and flex are checked at the same time because they may have been sitting in damp for a while. If your immersion has started tripping and there is also water around the cylinder, our guide to an immersion heater not working covers the tripping side in more detail.

Water from above, running down onto the immersion

The top of a cylinder is crowded. There is the hot outlet, the vent or the relief valves, compression joints, sometimes a second immersion, and all of them sit directly above the element head. A weeping olive on a joint above can run down the pipe, land on the immersion cap and look exactly like an immersion leak. It is also the most common way water gets inside the cap from outside.

That is worth knowing before anyone drains the cylinder. The fix for a dripping joint above the cylinder is a new olive or a remade joint, and the immersion may be perfectly sound. If water is coming from a valve rather than a pipe joint, particularly on an unvented cylinder, it may be a relief valve passing, which is a different fault with its own causes, set out in our guide to a pressure relief valve leaking.

When the leak is the cylinder, not the immersion

Sometimes the washer is fine and the leak is the boss itself, or the copper around it. Look for green or white crusting around the flange, or a pinhole in the cylinder wall close to the boss. On an older copper cylinder this is often corrosion that has been working away for years, and a boss that has been strained by several element changes can crack.

That is not something a new washer will fix. A copper vented cylinder with a corroded boss is usually a replacement, and the useful question becomes: is it worth repairing an old cylinder at all, or better to plan for a new one? We will tell you honestly which you are looking at, and a slow weep gives you time to decide rather than forcing the decision on the day.

Vented or unvented makes a difference to the repair

On a vented cylinder, fed from a tank in the loft, changing an immersion heater is ordinary plumbing and electrical work: isolate, drain down below the boss, remove the element with a proper immersion spanner, clean the boss face, fit a new element and washer, refill, vent the air out and test before switching back on.

On an unvented cylinder the stored water is at mains pressure, the immersion is usually a make specific part with its own thermal cut-out built in, and draining and recommissioning the cylinder involves the safety valves that the Building Regulations require to be worked on by someone holding the unvented qualification. If your cylinder has no loft tanks and a small plastic funnel called a tundish on the pipework, it is unvented, and the immersion repair belongs with an unvented cylinder engineer rather than a general handyman.

What we do on the visit

We trace the leak before we drain anything, because a drip from above and a leaking boss look similar and need completely different work. If it is the immersion, we isolate the circuit, lower the water level, remove the element, check the boss face and the cylinder around it, and fit a new element and a new washer. We then refill, bleed the air out, test the element for continuity and insulation, and run a full heat up with an engineer still there, so that the joint is proven at full temperature rather than cold.

We have 20 engineers across London and hot water cylinders are our core work, so an immersion leak on an unvented cylinder is not something we need to pass on. Our rate is on the pricing page and agreed before we attend. If you call, it helps to say whether the cylinder has loft tanks, and to send a photo of the label on the cylinder and of the immersion head if you can.