Unvented cylinder repair

What an unvented cylinder is doing when it works properly

An unvented cylinder is a sealed store of hot water held at incoming mains pressure. There is no cold tank in the loft feeding it and no open vent pipe over the top. Cold mains water enters through a control group on the inlet, the water is heated either by a coil fed from the boiler or by one or more immersion heaters, and it leaves the cylinder at roughly the pressure your main gives you. That is why an unvented cylinder can run a strong shower on the top floor of a converted house when a vented one cannot.

Because the store is sealed, the water has nowhere to go when it expands, and it does expand: heat a full cylinder through and the contents grow by a small but unavoidable amount. Every unvented cylinder therefore carries somewhere to put that expansion, either an external expansion vessel with a rubber bladder and an air charge behind it, or on some models a trapped air bubble held in the top of the cylinder itself. Behind that sits a chain of safety devices in case the expansion is not absorbed: an expansion relief valve on the inlet group, a temperature and pressure relief valve screwed into the cylinder body, and a thermal cut-out on each heat source. All of them dump through a tundish, the small open funnel visible in the pipework near the cylinder.

Nearly every fault worth calling anyone about is a failure somewhere in that chain. Either heat is not reaching the water, or the water is expanding and has run out of places to go. Working out which of the two you have is most of the diagnosis, and it is the part that gets skipped when a cylinder is condemned early.

Unvented hot water is covered by Part G of the Building Regulations, and the safety and discharge components must only be worked on by an engineer holding the unvented hot water qualification. Our page on what a G3 qualified plumber is covers what that qualification allows and what to ask to see before anyone opens the cupboard.

Hot water cylinder repair: the faults we actually find

Unvented cylinder repair and hot water cylinder repair are the same job in most homes, and the parts list is shorter than people expect. These are the components we replace, roughly in order of how often they come up:

• Immersion heater element, failed open circuit or so heavily scaled that it can no longer shed heat into the water
• Immersion thermostat, or the manual reset thermal cut-out built into the same head, tripped or dead
• Cylinder thermostat strapped to the outside of the cylinder body, out of calibration or failed closed
• Expansion relief valve or temperature and pressure relief valve passing and dripping continuously into the tundish
• Expansion vessel that has lost its air charge, or an internal air bubble that has been absorbed into the water over time
• Pressure reducing valve in the inlet group, either set wrong or with a strainer choked by debris from the main
• Motorised valve on the hot water circuit, stuck on its spindle or with a microswitch that no longer makes
• Internal coil, lined with scale, blocked, or in rarer cases perforated so that heating water and stored water mix

Why the wrong part gets fitted first

The reason a good cylinder sometimes gets replaced is that the symptom, no hot water, points at every one of those parts equally. Somebody fits a new immersion heater, the water is still cold, the conversation moves to a new cylinder, and nobody has yet checked whether the hot water circuit is calling for heat at all. On boiler fed systems that last point is the second most common fault we see, and it is a two wire problem with nothing wrong inside the cylinder.

The order matters more than the parts do. Controls and power first, because they are free to check. Then the inlet group, because a choked strainer or a failed pressure reducing valve mimics a dying cylinder. Then the heat sources. Only then does the cylinder itself come under suspicion, and by then there is usually physical evidence for it rather than a process of elimination.

It is also worth separating a hot water problem from a mains problem. If the cold tap at the kitchen sink has gone weak at the same time as the hot water, the fault is upstream and the cylinder is a bystander. That is a thirty second check and it changes the whole job.

Immersion heater and thermostat faults, and how to tell them apart

An immersion heater is a resistive element on a threaded boss, with a thermostat pushed down a pocket alongside it and a manual reset thermal cut-out in the same head. The element heats the water directly. The thermostat switches it off at the set temperature. The cut-out is the backstop, and it is mechanical: if it has tripped, something made the water hotter than the thermostat allowed, and that is information rather than an inconvenience.

Symptoms split neatly. Water that is stone cold throughout, with the immersion switched on, is usually a dead element or a tripped cut-out. Water that heats but never gets properly hot is more often a thermostat that has drifted, or an element so furred that it cannot transfer its heat into the water fast enough. If only the top third of the cylinder is hot, that is normal on a twin element cylinder running the top element alone, and it is what the short element exists to do.

On site an element takes a few minutes to prove. Isolate, pull the cover, check continuity across the element and resistance to earth, then check the stat separately by bridging it. A failed element usually reads open circuit; one reading low to earth will be tripping the RCD every time it is switched on. Getting a scaled element out of a London cylinder is the slow part of the job, not the testing, because the boss seal frequently comes away with it. Our immersion heater repair page goes further into elements, thermostats and what a tripped cut out is telling you.

• Element reads open circuit: failed, replace, and inspect the removed element for the scale that killed it
• Element reads low resistance to earth: failed to earth, which is what has been tripping the consumer unit
• Thermostat dead but element healthy: a thermostat and cut-out head is a fraction of the work of a full element change
• Cut-out tripped with everything else healthy: find out what overheated the water before resetting it

A thermal cut-out trips for a reason. If it trips again after a reset, stop resetting it and have the cause found; repeated overheating of a sealed cylinder is exactly the condition the safety chain exists for.

Tundish dripping, and what the discharge is telling us

A tundish should be dry. If you can see water passing through it, something upstream is discharging, and which valve is doing it narrows the fault immediately. If that is the symptom that brought you here, the tundish dripping page and the pressure relief valve leaking page take each cause in turn and in more detail than this section can.

A drip that appears only after a long heating cycle and then stops points at expansion. The vessel behind the cylinder has a rubber bladder with air charged behind it, and over a few years that charge can bleed away or the bladder can perforate, leaving the vessel waterlogged and useless. On cylinders that use an internal air bubble instead of a vessel, the bubble slowly dissolves into the water and the same thing happens. With nowhere to expand, pressure climbs as the water heats until the expansion relief valve lifts and lets a little water go. Recharging a healthy vessel to match the setting recorded on the inlet group fixes it; a perforated one has to be changed.

A continuous dribble that does not care whether the cylinder is heating usually means a valve is passing. Debris from the main, or scale, lodges on the seat and the valve no longer closes fully. That is a straightforward valve change once we have confirmed which of the two valves is doing it, because the expansion relief valve and the temperature and pressure relief valve share the same discharge pipe and both end up at the same tundish.

The case to take seriously is a temperature and pressure relief valve lifting because the water genuinely is too hot. That valve is the last line before a sealed cylinder becomes dangerous, and if it is operating, the real fault is a control that has failed to switch the heat off. Changing the valve there would be the worst possible repair.

Never cap, plug or restrict a discharge pipe, and do not let anyone route it into a hidden void to stop the mess. If your tundish is dripping, put a container under it, turn off the heat source to the cylinder, and get it looked at.

No hot water from the boiler when the heating still works

On a boiler fed cylinder, the boiler heats water that flows through a coil inside the cylinder and gives up its heat to the stored water. If the radiators are hot and the cylinder is cold, the boiler is fine and the hot water circuit is not being asked for, or is being asked and cannot answer. That exact combination has a page of its own, no hot water but the heating works, which runs the same checks in the order an engineer would do them.

The usual suspects are the cylinder thermostat, the programmer, and the two port motorised valve on the hot water leg. The valve is the interesting one: it has a synchronous motor that drives it open and a separate microswitch that tells the boiler to fire once it is open. The motor and the switch fail independently, so a valve can be sitting wide open with the boiler never firing, or the boiler can fire with the valve stuck shut and send all the heat to the radiators. Listening for the motor and checking whether the switch has made takes a couple of minutes and settles it.

The cylinder thermostat is the other half. It sits strapped to the cylinder body under the insulation, reading the temperature through the foam, and it is meant to break the call for heat when the store is up to temperature. A stat that has failed open never calls, and a stat that has drifted calls at the wrong point. Bridging it briefly proves whether the rest of the circuit responds, and if it does, the stat is the fault.

If the controls all check out and the cylinder still recovers far too slowly, attention moves to the coil. A coil lined with scale cannot transfer heat, and the giveaway is at the pipework: the return coming back from the cylinder is nearly as hot as the flow going in, because very little heat is being absorbed on the way through. A coil that can be flushed is worth flushing; one that is blocked solid, or perforated so that the heating side and the stored water are mixing, is the point at which replacement enters the conversation honestly.

Cylinders that leak, and why that one is different

Most water found around a cylinder is not coming from the cylinder. Immersion boss seals weep, compression joints on the inlet group loosen, relief valves pass, and the drain cock left over from the last service drips. All of those are repairs, and all of them produce a puddle in roughly the same place, so the first job is to dry everything, heat the cylinder and watch where the water reappears.

The one that is not a repair is a perforation in the cylinder body itself. Modern unvented cylinders are duplex stainless steel, and they fail either at a weld seam or through stress corrosion cracking, which in practice shows as a pinhole. The water tracks under the foam insulation and emerges somewhere that has nothing to do with the hole, so the point of the leak is rarely the source. The hot water cylinder leaking page covers how we separate a weeping joint from a perforated vessel without taking the system apart first. Once the vessel is perforated it is finished. There is no compound, no patch and no sleeve that is safe on a vessel holding mains pressure, and anyone offering one should be shown the door.

If you are looking at active water now, the sequence is to close the cold feed at the inlet group, switch off every heat source to the cylinder including the immersions and the hot water programme, and open a hot tap upstairs to drop the level and take pressure off. That stops the leak getting worse and makes the job that follows shorter.

A cylinder discharging hot water is a scalding risk as well as a water damage one. Keep people away from the airing cupboard until the heat sources are off and the pressure has been released.

London water, scale, and what it costs a cylinder

Much of London sits on hard water, and calcium carbonate comes out of solution faster the hotter the water gets, which is why cylinders here wear out their heat transfer surfaces long before anything structural goes wrong.

The damage is predictable. An immersion element furs until it cannot shed its heat, runs hotter internally, and fails early, often after a period of crackling or kettling noises as water flashes to steam inside the scale layer. A coil lines with the same deposit and the recovery time creeps up so gradually that nobody notices until the second shower runs cold. The strainer inside the pressure reducing valve silts up with a mixture of scale and debris from the main, throttling the flow into the cylinder, which presents as weak hot water and gets blamed on the cylinder every time.

The defences are unglamorous. Run the store at a sensible temperature rather than as hot as the thermostat allows, have the strainer cleaned and the expansion charge checked annually, and change an element that has begun to make noise rather than waiting for it to fail. A cylinder serviced regularly behaves very differently in this water from one left alone since installation.

Is it worth repairing an unvented cylinder or replacing it?

Most unvented cylinder faults are repairs, and we will say so. Elements, thermostats, both relief valves, expansion vessels, pressure reducing valves and motorised valves are all stock components, all replaceable in situ, and all of them restore a cylinder that has plenty of service left in it. Replacing a whole cylinder because a thermostat failed is a waste of your money and our time.

The cases where replacement is genuinely the right call are narrower than the sales pitch suggests. A perforated vessel is one. A model old enough that the relief valves and the inlet group are no longer available as parts is another, because a safety component that cannot be replaced is not a system anyone should be maintaining. A coil that is blocked solid or leaking internally is a third. And occasionally the cylinder is simply the wrong size for the household, in which case nothing is broken and the answer is a different cylinder rather than a repair. Where that is the conclusion, the hot water cylinder replacement page sets out what the work involves and how long it takes.

The one to be careful about is repeat failure. A cylinder that eats immersion elements, or trips its cut-out every few months, is usually reporting a separate problem: a control fault, an incorrect setting, or water quality bad enough to be worth treating. Fitting another element to that cylinder is not a repair, it is a subscription. We would rather find the cause once.

On cost, the rate is fixed before we attend and the total depends on how long the job takes plus any parts used. Full terms are on the pricing page, and we will tell you on site if what we have found changes the scope.

What happens when an engineer attends

It helps if you can tell us four things when you book: whether the cylinder is heated by the boiler, by immersions, or both, whether the tundish is wet, when the hot water last worked properly, and what is written on the badge plate on the front of the cylinder. That last one is the make, model and year, and it decides what we carry.

On site the sequence is the same every time. Power and controls first, including the programmer, the immersion switches and the fused spur, because a cylinder cannot be blamed for a circuit that is not calling. Then the inlet group: incoming pressure, the strainer, the pressure reducing valve setting, and whether either relief valve is passing. Then the heat sources, electrically for the immersions and by flow and return temperature for the coil. Then the expansion arrangement. The cylinder body is inspected throughout and judged at the end, not assumed at the start.

The discharge arrangement gets checked whatever the fault turns out to be, because it has to be right and very often is not. If the pipe has been capped, reduced, run uphill or terminated somewhere it should not be, we will tell you and put it right rather than leave a sealed pressure vessel with nowhere to vent.

Common parts travel with the van, so a large share of these faults are resolved in the same visit. Where a part has to be ordered, you will be told what it is and what remains to be done, and the cylinder is left in a safe condition in the meantime.

Which question you are actually asking about your cylinder

People arrive at a cylinder page from five or six different places, and the useful answer depends entirely on which one you are standing in. It is worth naming them, because the wrong reading order is how a repairable cylinder ends up condemned and how a dying one ends up patched twice.

The first question is what you have got. A sealed store at mains pressure and a tank fed store behave differently, fail differently and are worked on under different rules, and if you are not sure which is in the cupboard the unvented vs vented cylinder comparison settles it from what you can see without tools. Everything else on this page assumes the sealed type.

The second is what the symptom means. Water passing through the small open funnel in the pipework is the single most informative thing a cylinder does, and a tundish dripping is worth reading before you touch anything, because the pattern of the drip points at a different component than the drip itself does. Cold water where hot should be, and water appearing on the cupboard floor, each have their own route through the diagnosis above.

The third is whether this cylinder is worth spending money on at all. That is an economic question rather than a technical one, and the honest version of it is set out in is it worth repairing an old cylinder, which weighs the age of the vessel and the availability of its safety parts against what the repair in front of you costs in time.

Closely related but not the same: the symptoms that say a vessel is genuinely near the end are specific, and none of them is simply age. The list in signs your cylinder needs replacing is the one we work to, and a cylinder that shows none of them is a cylinder to keep.

The fourth is prevention, which is the cheapest of all of them and the one nobody calls about. An annual unvented cylinder service checks the expansion charge, the strainer, both relief valves and the discharge arrangement, which between them account for most of what we are called out to fix. In hard water it is the difference between changing a part and losing a cylinder.

The fifth is who is allowed to do the work. Anything touching the sealed side, the relief valves or the discharge pipework needs the unvented hot water qualification, and the question of what a G3 qualified plumber actually holds, and what you should ask to see, is worth settling before anyone opens the cupboard rather than after.

What you can check yourself before calling anyone

Some of these calls end with nobody needing to attend, and it is better for everyone if that becomes clear before an engineer is on the road. Run through the following first.

• Check the programmer is set to hot water and that the current time falls inside an on period; after a power cut many programmers lose their schedule and revert to off
• Check the switch for the immersion heater at the wall and the fused spur next to it, and check the fuse in that spur
• Look for a small red or black reset button on the immersion head under the plastic cover and note whether it has tripped, but read the warning below before pressing it
• Look at the tundish and note whether it is dry, dripping slowly, or running; that single observation changes the diagnosis more than anything else you can tell us
• Run the cold tap at the kitchen sink and see whether mains pressure is normal, which separates a cylinder fault from a supply problem
• Check whether the airing cupboard is warm and whether any pipe leaving the cylinder is hot, which tells us whether heat is arriving at all
• If the heating is on the same boiler, check whether the radiators still get hot, because that isolates the fault to the hot water circuit

If you press the reset on an immersion cut-out and it trips again, leave it alone and turn the immersion off at the wall. It is protecting a sealed cylinder from overheating, and repeatedly resetting it removes the one thing standing between a control fault and a real hazard.