Hot water cylinder leaking, overheating or making noise

Where is the water actually coming from?

A cylinder is a smooth vertical shell wrapped in sprayed foam lagging. Water that escapes anywhere near the top runs down the outside of that foam, or soaks into it and reappears lower down, and it ends up in a puddle at the base. That is why so many cylinders get condemned as leaking from the bottom when the fault is a fitting three feet higher up. Gravity and foam are doing the misleading, not the cylinder.

The way to break that is boring and it works. Dry every surface completely: the top plate, around the immersion cover, the valve bodies on the inlet group, every visible joint, the floor, and the inside of the tundish. Push kitchen paper into the gaps where you cannot get a cloth. Then leave it, use no hot water for an hour or two, and come back. Whatever is wet first is where the water is coming from. If nothing is wet until the boiler or immersion has run a full heating cycle, the fault is pressure or temperature related rather than a failed joint, and that points straight at the expansion side of the system.

Note the timing as carefully as the location. Water that appears only during or just after heating, water that drips constantly whether the system is hot or cold, and water that appears only when a hot tap is running are three different faults with three different parts behind them. An engineer who arrives to find the cylinder already dried and a note saying when it wets is starting the job twenty minutes ahead.

• Dry the top of the cylinder, the immersion cover, the tundish and the floor, then watch what wets first
• Run no hot water for an hour and see whether the drip continues with the system idle
• Run a full heating cycle and check the tundish again while the cylinder is hot
• Check whether the puddle sits under a pipe joint rather than under the cylinder itself
• Feel whether the water is hot or cold, because cold water at a hot cylinder means the cold feed or a mains fitting
• Photograph the inlet group, the tundish and the cylinder label before you call anyone

Stored water can be hot enough to scald on contact. If water is running near the immersion heater cover, the wiring or a fused spur, switch the immersion off at the spur and leave it off until an engineer has looked at it. Never pull an immersion cover with the circuit live.

Hot water cylinder leaking from the top

The top of a cylinder carries the most penetrations, so it produces the most leaks. On an unvented cylinder you typically have the hot outlet, one or two immersion heater bosses, a dry thermostat pocket, and the temperature and pressure relief valve screwed into the shell. Every one of those is a seal that can fail.

The immersion boss is the usual suspect, particularly if the element has been changed at some point. An immersion seats on a thick fibre or rubber washer that is crushed once and is not really designed to be reused. Refit an old element on an old washer, or overtighten a new one, and it weeps a slow trail that only shows when the cylinder is up to temperature and the metal has expanded. The giveaway is a stain radiating out from under the boss flange with the rest of the top plate dry, and it often dries off between heating cycles so it looks intermittent.

The temperature and pressure relief valve is the other common one. It is a safety valve screwed directly into the cylinder body with a sensing probe in the water, and it lifts if the stored water gets too hot or the pressure climbs too far. Once it has lifted even briefly, a fragment of scale or grit can sit on the seat and hold it slightly open, after which it weeps continuously into the discharge pipe. That water goes to the tundish, so the symptom appears there rather than at the valve. A valve that is passing is replaced, not cleaned, because it is a safety device and its seat has already proved it does not seal. If the drip is clearly coming from the valve body rather than from the shell, the valve is the whole fault and the rest of the cylinder is sound, which is worth establishing before anyone opens the airing cupboard any further and starts pricing a vessel.

The thermostat pocket should always be dry inside. If you pull the cylinder stat or the pocket cap and find water, the pocket has corroded through and the cylinder is open to the outside at that point. That is a cylinder fault, not a fitting fault.

• Weep from under the immersion flange: seal or washer, replaced when the element comes out
• Drip from the relief valve body itself: the valve is passing or its thread seal has failed
• Wet hot outlet joint: a compression or push fit connection that has relaxed, often after the pipework has been disturbed
• Water inside the thermostat pocket: the pocket has failed and the cylinder shell is compromised
• Damp foam with no obvious source: the leak is under the lagging and needs the foam cutting back to find

Leaking from the bottom, or a puddle under the cylinder

Start by assuming the water came from higher up, because most of the time it did. Only once the top and the valve group have been proved dry through a full heating cycle is a base leak worth taking seriously.

The single most common genuine base leak is the drain cock. It is a small brass or plastic valve on the cold feed near the floor, and most of them are fine until the day somebody uses one. The washer inside has been compressed in one position for years, it never fully reseats after being opened, and from then on it weeps. A drain cock that has started dripping after a service or a radiator job is not a coincidence, and it is a cheap part to change.

Next come the tappings: the cold feed in at the bottom, and on an indirect cylinder the primary flow and return connections where the boiler pipework enters and leaves the internal coil. These are threaded or brazed connections into the shell. A weeping thread will usually drip at a steady rate that does not care whether the water is hot, whereas a coil connection often wets only when the boiler is firing and the primary side is up to pressure and temperature.

Two things get mistaken for base leaks often enough to check first. Condensation is one: a cold mains pipe in a warm airing cupboard sweats, and the drips collect under the cylinder exactly like a leak. Wipe the pipe, and if the water is on the underside of cold pipework only and the cylinder body is bone dry, it is condensation. The other is the tundish discharge pipe. That pipe has to run to somewhere safe and visible, and when it has been badly installed or has come apart in a cupboard, the discharge that should be leaving the building appears on the floor instead and looks exactly like a cylinder that has split. Where the water is tracking along a pipe buried in screed rather than coming from the cylinder at all, pipe grouting and no dig repair is the method that seals the run from the inside instead of lifting a floor to reach it.

Hot water tank leaking through the tundish

A tundish is the small open funnel in the pipework near the cylinder, with a visible air gap in the middle. It is there so that any discharge from the safety valves is seen rather than piped quietly away, which is also why it is the first place to look when a hot water tank appears to be leaking. A tundish should be dry. Water passing through it is the system telling you something, and what it tells you depends on when it flows and how hot it is. The distinction that decides the whole job is between a discharge that appears only at the end of a heating cycle and then stops, and one that runs continuously whether the system is hot or cold, because those two patterns have different causes behind them and different parts at the end of them.

A discharge that appears near the end of a heating cycle and then stops is almost always expansion. Because an unvented cylinder is sealed, the water inside has to grow somewhere as it heats, and that space is provided either by an external expansion vessel with a bladder and an air charge behind it, or by a bubble of air deliberately trapped at the top of the cylinder on bubble top designs. Both can be lost over time. When the expansion space has gone, pressure climbs sharply every time the cylinder heats through, the expansion relief valve on the inlet group lifts to protect the system, and a small amount of water leaves through the tundish. The cylinder is not leaking. It is discharging, correctly, because it has nowhere to put the expansion.

A discharge that runs continuously, hot or cold, idle or heating, is a valve passing. Either the expansion relief valve on the inlet group or the temperature and pressure relief valve in the cylinder body is not seating, usually because debris or scale got on the seat the last time it lifted. This wastes water slowly and constantly, and on a metered supply in a flat it shows up on the bill long before anyone notices the drip.

A discharge that is scalding hot, or that comes with steam and banging, is the temperature side operating, and that is different in kind. It means the stored water has been driven past its safe temperature and the last line of protection has opened to dump it. Switch off the immersion at its spur and turn the boiler off, leave the water supply on so the cylinder can refill and cool, and have it looked at before it is used again. Unvented hot water is one of the few domestic systems where a control fault has a physical consequence, which is why the discharge arrangement is regulated and why the safety components are only worked on by engineers holding the unvented qualification.

If the tundish is discharging water that is too hot to touch, or you can hear boiling or knocking inside the cylinder, turn off every heat source but leave the cold water supply on. Cutting off the supply to a cylinder that is overheating removes the only thing keeping it cool.

How to tell when the cylinder itself has failed

Modern cylinders are stainless steel, usually a duplex grade chosen because it copes with mains pressure in a thin shell. They fail in a particular way. Chloride in the water, concentrated by heat and by repeated cycling, attacks the steel at points of stress, typically near a weld, a tapping, or a crimped seam. The result is a pinhole rather than a split. Copper cylinders, which are still out there on vented systems, more often fail at a dezincified brass fitting or at the base where debris has collected.

The pattern that tells you the shell has gone is water emerging from under the foam lagging at a point where there is no fitting at all, getting worse as the cylinder heats and the pressure rises, and easing off when the system is cold. You may see a rust coloured or chalky stain on the foam where the water has been travelling for a while, and if you press the lagging it feels saturated rather than damp. At that point the water is coming out of the vessel, not out of a joint on it.

A perforated cylinder is not repairable. It is a pressure vessel operating at mains pressure with hot water in it, and there is no honest way to patch, braze or seal one in place and hand it back as safe. Sealants poured into the system do not work on a shell and they will foul the relief valves. The practical decision moves to replacement, and the useful question becomes whether the same size and type goes back in or whether the system needs rethinking while the airing cupboard is open. The signs that a cylinder has genuinely reached the end of its life are worth knowing before you accept that verdict from anyone, and what a hot water cylinder replacement involves, from the size that goes back in to the discharge arrangement that has to be brought up to standard with it, is set out separately.

It is worth saying what does not mean the cylinder has failed, because these get used as the reason for a quote. Rust on an external fitting, corrosion on the strap of a cylinder stat, discoloured water for a few seconds after a long period of no use, and a bit of surface staining on the outside of the foam are all normal wear and none of them means the vessel is perforated.

Hot water cylinder overheating

Overheating and leaking are frequently the same fault seen from two angles, because water that is too hot expands more, lifts the relief valves, and arrives at your feet as a puddle. The signs of an overheating cylinder are water at the taps that is hot enough to scald without any cold mixed in, a tundish that discharges very hot water, rumbling or knocking from inside the cylinder as water flashes to steam against a hot surface, and on an unvented system an immersion thermal cut-out that has tripped.

The mechanism is always the same: heat keeps going in after the water is up to temperature. On the electric side that is a thermostat that has failed closed, so the element never switches off. On the boiler side it is usually the controls rather than the cylinder. A motorised valve on the hot water circuit that has stuck open, or whose spindle has seized in the hot position, lets the boiler keep pushing heat through the coil long after the cylinder has called off. A cylinder stat strapped to the outside of the shell that has worked loose is another one, because a stat reading cupboard air instead of cylinder metal never gets satisfied.

A thermal cut-out that has tripped has done its job and has told you something in the process. Resetting it once to get hot water back is reasonable. Resetting it repeatedly is not, because each reset restores the fault that overheated the water in the first place and leaves the mechanical safety chain as the only thing standing between the system and a genuinely unsafe condition. If a cut-out trips twice, stop and have the cause found. There is a fuller page on a hot water cylinder overheating if that is the symptom you arrived with rather than the water on the floor, and it works through the thermostat, the motorised valve and the cut-out in the order an engineer would actually check them.

Water stored above about sixty degrees scalds quickly, and small children and older adults are burned by it in the time it takes to react. If the hot taps are running dangerously hot, use the cold tap to fill baths and basins until the fault is fixed, and check any thermostatic blending valve at the same time, since a failed blender produces scalding water from a perfectly healthy cylinder.

Noise from a hot water cylinder, and which noises matter

Cylinders are not silent, and London water makes them less silent than most. Separating the noises that indicate a fault from the ones that are just a hot vessel in a cupboard saves a lot of worry.

Rumbling or kettling, a sound like a boiling kettle building through the heating cycle, is scale. London sits on chalk, the water is hard, and limescale forms a hard crust on whatever surface is hottest: the immersion element or the outside of the internal coil. Water trapped under that crust flashes to steam, and the bubbles collapsing are what you can hear. It is not immediately dangerous but it is a warning. A scaled element cannot shed its heat into the water, it runs hotter and hotter, and it eventually fails or trips its cut-out.

A hard bang or a series of knocks when a tap or an appliance shuts off is water hammer, not the cylinder. A fast closing valve, typically on a washing machine or a lever tap, stops a moving column of water dead and the shock travels through the pipework. Loose or missing pipe clips turn that shock into a noise you hear through the whole flat. Hammer is worth fixing because it is a mechanical shock repeated hundreds of times a week on the same joints, and joints that get hammered are the joints that eventually weep.

Ticking and creaking during heating and cooling is pipework and the cylinder body expanding against joists, clips and the cupboard structure. It is harmless. Hissing is not a cylinder noise at all in most cases, it is water passing somewhere it should not, so trace it to the relief valves and the tundish. Gurgling on a vented system usually means air, either in the primary circuit or drawn in through a cold feed that is running low.

• Kettling or rumbling: scale on the element or coil, worth acting on before the element fails
• Banging on tap shutoff: water hammer in the pipework, check clips and fast closing valves
• Ticking while heating: thermal expansion, no action needed
• Continuous hissing: something is passing, check the relief valves and the tundish
• Boiling or violent knocking inside the cylinder: treat as overheating, turn the heat sources off

Weak hot water pressure from the cylinder

On an unvented system the hot water arrives at the tap at roughly mains pressure, so weak hot water means something between the main and the tap is restricting it. The first check takes thirty seconds and it is free: run the cold tap at the kitchen sink, which is normally straight off the main. If the cold is weak too, the problem is the incoming supply or the stopcock, and the cylinder is a bystander. If the cold is strong and only the hot is poor, the restriction is on the cylinder side.

The usual culprit is the inlet control group, the assembly of valves on the cold feed into the cylinder. It contains a pressure reducing valve with a small strainer in front of it, and that strainer catches debris from the main. In older parts of London with cast iron or lead lined services, and after any street works, that strainer silts up and the flow into the cylinder drops. Cleaning or replacing it restores the hot flow completely and costs nothing like a cylinder.

Scale is the second cause. A cylinder that has been quietly furring up for years has a narrower outlet, a scaled coil that transfers less heat, and less usable hot water before the temperature falls away. That reads as poor pressure at the shower but it is really poor delivery, and the clue is that the hot runs well for the first minute and then fades.

Losing system pressure in the other sense, water slowly disappearing from a sealed system, is different and it usually points back at the expansion side already described: if the relief valve is lifting on every heating cycle and dumping through the tundish, water is leaving the building in small amounts continuously. On a vented cylinder fed from a loft tank there is no mains pressure at all, so weak hot water there is normally an airlock, a blocked cold feed, or a float valve in the tank that is not letting the tank refill quickly enough.

What you can safely check yourself before calling anyone

Some of this genuinely does not need an engineer, and it is better to say so. If the water turns out to be condensation on a cold pipe, or a drain cock that needs a quarter turn to reseat, or a discharge pipe that has come apart in the cupboard, you have found it in ten minutes and saved a visit.

What you should not do is take an immersion cover off with the power on, open any valve on the inlet group to see what happens, poke a relief valve that is discharging to try to reseat it, or reset a thermal cut-out more than once. The inlet group is set to a specific pressure and the relief valves are safety devices, so adjusting them is how a controlled fault becomes an uncontrolled one.

There is also a point where tracing it yourself stops being the sensible move. If the water is arriving faster than a cloth can keep up with, if it is already showing on a ceiling below, or if the foam has soaked through so completely that there is no dry surface left to watch, more observation buys you nothing. What to do in the first minutes of water escaping in the house is a separate question from what has failed, and the two are answered in that order: stop the water first, find the fault afterwards. Where the water turns out not to be coming from the cylinder at all, leak detection and repair is the work of locating it without lifting every floor in the flat, and a burst pipe repair on the feed running into the cupboard is a different job from anything the cylinder itself needs.

When we attend, the order is the same every time: prove where the water is coming from before touching a part, separate a control fault from a pressure fault, check the inlet group and the expansion arrangement, and only then treat the cylinder itself as a suspect. Our rate is fixed before we attend and the total depends on how long the job takes, so a fault that is found quickly costs less than one that has had three parts fitted at it on the way. We have twenty engineers across London and the ones who work on unvented systems hold the qualification required for the work.

• Dry everything and note what wets first, and whether it wets hot or cold
• Check the drain cock at the base of the cylinder for a weep
• Check whether the water is condensation on cold pipework rather than a leak
• Follow the tundish discharge pipe and make sure it is still connected and running outside
• Switch the immersion off at the fused spur if water is anywhere near the electrics
• Find your internal stopcock now, and check that it turns, before you need it in a hurry

If water is escaping faster than a drip, turn off the internal stopcock, open a cold tap downstairs to drain the pressure out of the pipework, and switch off the immersion and the boiler. A cylinder holds a lot of water and a flat above a shop drains straight through a ceiling.