Hot Water Tank Dripping: Is It Failing?

Find exactly where the drip starts

Water runs down pipes and along the outside of the cylinder before it drips, so the wet patch on the floor is rarely where the leak is. Dry everything with kitchen roll, wait ten minutes, then work from the top down with a dry tissue, pressing it against each joint. The first wet tissue is your leak.

Check in this order, because it is the order water travels:

• Pipe joints on top of the cylinder, where the hot draw-off pipe leaves
• The immersion heater boss on the top or side, under the round plastic cap
• Any valves on the pipes: the gate valve on the cold feed, a zone valve, or on an unvented cylinder the brass valve group on the cold inlet
• The coil connections on the side, where the two pipes from the boiler join an indirect cylinder
• The drain cock near the bottom, a small tap with a square head or hose outlet
• The cylinder body, especially the domed top, the seams and the base

If the ceiling below the airing cupboard is sagging or wet near a light fitting, switch that lighting circuit off at the consumer unit, if the unit is dry and safe to reach, and keep people out from underneath it.

What to do right now to stop the water

Switch off the immersion heater at its wall switch and set the boiler hot water to off on the programmer. Then stop the water feeding the cylinder. On a vented cylinder, the one fed from a cold water tank in the loft, close the gate valve on the cold pipe going into the bottom of the cylinder. On an unvented cylinder, which runs straight off the mains, close the isolating valve on the incoming cold supply or the main stopcock.

That stops the tank refilling, so a leak from a joint or the body slows to whatever is already in the pipes. The step-by-step, including what to do if the valve will not turn, is in how to turn off the hot water supply from a cylinder. Put a tray or towels down, and if the leak is from the lower half of the tank you may need to drain it to stop the water completely.

Drips you can usually fix without a new tank

A compression joint that weeps is often just a nut that has worked loose or an olive that has not sealed. A quarter turn can cure it, though overtightening a joint on a thin copper cylinder can crack it, so if a gentle nip does not stop it, leave it for a plumber.

The immersion heater boss is a common source. The element seals against the cylinder with a fibre washer, and years of heating and cooling harden it. The repair is a new washer, and usually a new element while the old one is out, which means draining the tank below the boss.

Gate valves leak from the spindle, where the handle goes in, as the packing dries out. Drain cocks drip from the outlet when grit sits in the seat. Both are cheap parts. On an unvented cylinder, the valve group on the cold inlet, which combines the pressure reducing valve, strainer, check valve and expansion relief valve, can weep from its body or from a failing seal.

Signs the cylinder itself is failing

Copper cylinders corrode from the inside over decades. When they go, they rarely burst. They weep through a pinhole or along a seam, and you see a slow bead of water on the body with every joint above it dry.

The giveaways are green or blue crusty staining on the copper, a damp patch that returns however well you dry it, and water pooling under the base with nothing wet above. A pinhole on a thin old cylinder cannot be soldered reliably, because heat on the surrounding metal tends to open up more holes. At that point the honest answer is a new cylinder, and hot water tank replacement cost sets out the typical ranges.

A weeping cylinder can hold for days, but it will not get better and it can let go more suddenly as the hole widens. Keep the cold feed off between uses if you can, and arrange the replacement rather than living with a bucket.

Water coming out of the tundish on an unvented cylinder

An unvented cylinder has safety valves that release water rather than let pressure or temperature climb too high. Under Building Regulations Part G3 their discharge must pass through a tundish, a short open funnel section, so you can see it happening. Water trickling through the tundish is one of those valves opening.

Cold water trickling usually means the expansion vessel has lost its air charge or the pressure reducing valve is letting full mains pressure through. Hot water or steam discharging means the water has overheated and the temperature and pressure relief valve has opened. That is urgent: switch off the immersion and the boiler straight away, but do not turn off the water supply while it is discharging. The tundish overflowing guide explains each case.

Never cap, block or plug the tundish or the discharge pipe to stop the water. It is the safety outlet for a pressurised tank of hot water. Work on an unvented cylinder’s safety devices is for someone holding an unvented (G3) qualification.

Is it the cold water storage tank in the loft leaking?

If you have a vented cylinder, the leak may be coming from above. The cold water storage tank in the loft feeds the cylinder, and a plastic tank can split along its base, an old galvanised steel tank can rust through, and the float valve can stick open so water pours out of the overflow pipe on the outside wall. A leak from the loft tank shows up as a stain spreading across the ceiling below the tank, not around the cylinder.

An overflow running constantly is a float valve problem, covered in the loft tank ballcock guide. Water on the loft boards under the tank is a leak, and it needs the tank supply turned off at the stopcock until it is fixed.

What the engineer does and what it typically costs

On the visit the engineer traces the leak, isolates the cylinder and tells you whether it is a part or the tank. Remaking a joint or replacing a gate valve or drain cock is usually a short job. An immersion washer and element means a part drain and refill. A failing cylinder is quoted as a replacement.

Typical UK market ranges: a joint, gate valve or drain cock repair sits at the cheap end, an immersion element and washer fitted commonly £130 to £280, and a new vented copper cylinder fitted from roughly £750 upwards, more in London, with unvented replacements considerably more. A cylinder visit has the £60 call-out at any hour, and the repair price is agreed with you on site before any work starts.