Is It Worth Repairing an Old Hot Water Cylinder?

Should you repair or replace a hot water cylinder?

Repair it if the fault is a component and the vessel is sound. Replace it if the vessel is leaking, if the fault keeps returning because of the state of the inside, or if parts for the model are no longer made. Age alone is not a reason. A sound copper cylinder with a failed immersion element is worth an element. A vessel weeping at a seam is worth nothing, whatever year it was made.

This gets muddled because people ask about the cylinder when they mean the hot water system. A cylinder is a tank with a coil or an element in it, a thermostat on the side, and on an unvented unit a set of safety and control components on the cold feed. Most faults called a cylinder fault sit in that second group, and those are replaceable in place in an hour or two. The tank is the expensive part, and it fails in a small number of specific ways.

So the first question to put to whoever is looking at it is not how old it is, but whether the vessel is sound. If it is, repair is almost always the better use of money, because a replacement also means new connections, new controls and a day of somebody in the house. If it is not, no amount of parts changes anything.

Where the fault sits decides most of the answer

Work out which part of the unit has failed before anything else. Faults in the vessel wall mean replacement. Faults in anything screwed, clamped or soldered onto it mean repair. There is no middle ground, because there is no legitimate way to patch a copper shell or weld a certified unvented vessel in a cupboard.

Component faults are the common ones, and the list is short. An immersion heater burned out or weeping at the fibre gasket under the boss. A cylinder thermostat that has stopped calling for heat. A temperature and pressure relief valve passing water to the tundish, usually because grit off the tank has lodged on the seat. A motorised valve on the heating side that is not driving open, which is not a cylinder fault at all but gets blamed as one every week. Each of those is a part and an hour, not a new tank.

Vessel faults look different. A damp patch that returns within a day of being dried. Green or blue staining low on a copper shell. Rust streaks down the casing of a foam lagged cylinder, meaning water has been tracking inside the jacket for a while. Brown hot water with clear cold water, which puts the discolouration inside the hot side rather than on the main. Those are the endings.

Almost everything in the component list is an unvented cylinder repair in the ordinary sense of the phrase: a part, a gasket, a valve, and an hour or two of somebody who already knows where the isolation points are and what order to close them in.

Almost everything in the vessel list is a hot water cylinder replacement instead, because there is nothing on a failed shell to replace except the shell. Establishing which of the two lists you are on is the whole job, and it is the reason the visit that answers the question properly is worth more than the opinion that skipped it.

If water is coming from the body of an unvented cylinder, shut the cold supply to it and isolate the immersion and the boiler feed before anything else. A failing sealed vessel with a heat source still firing into it is the version of this that stops being merely wet.

Can you still get parts for that cylinder?

Parts availability quietly decides more of these jobs than condition does. A cylinder nobody makes a thermostat, a relief valve or a control pack for has decided for you, however sound the tank is. Find the data plate, usually on the front of the casing or near the immersion boss, and note the make, model and capacity. That is what a merchant needs to tell you whether parts still exist.

Unvented units from the larger manufacturers are generally well supported, and control packs, valves and thermostats are still available for models long out of production. Own brand cylinders fitted by a developer across a whole block are the awkward ones. Sometimes a generic equivalent will do, and sometimes a valve with a different thread and discharge arrangement forces a rework of the pipework, at which point you are doing half a replacement to keep an old tank alive.

Vented copper cylinders are easier, because almost everything on them is generic. Elements come in standard lengths and threads, thermostats are interchangeable, and the fittings are ordinary compression. That is a real argument for repairing an old vented cylinder that looks uneconomic on paper.

• Make, model and capacity from the data plate: the first thing to establish.
• Control pack and relief valve still listed for that model: repair stays realistic.
• Only a generic substitute available, with different threads: expect pipework changes.
• Nothing available at all: the cylinder has decided for you.
• Vented copper with standard fittings: parts are rarely the obstacle.

What London water has already done to the inside

Most of London is on hard water, and that is the biggest single factor pushing an old cylinder towards replacement. Every time water is heated, calcium carbonate comes out of solution onto the hottest surface available: the coil in an indirect cylinder, the immersion element in a direct one. Neither can be cleaned in place once it is furred.

Scale on a coil acts as insulation. The boiler still pushes hot water round it, but less heat crosses into the tank, so reheat times stretch out. If a full reheat took about an hour and now takes an afternoon with a boiler that has been checked, a furred coil is the usual explanation. A coil is not recoverable in place, and a new coil means a new cylinder.

Scale on an element is more visible. It runs hotter because it cannot shed heat into the water, the scale layer cracks and flakes into the bottom of the tank, and eventually the element fails. Fitting a new element in a scaled cylinder is fair once. Doing it every year or two is the tank telling you what its insides look like, and each flake ends up on a valve seat somewhere.

The honest test is what comes out of the drain point when an engineer empties it. Clear water off a cylinder that is twenty years old argues for another repair. Grit, flakes and grey slurry argue for spending the money once.

What a replacement would change that a repair cannot

Sometimes the strongest argument for replacing is not that the cylinder has failed but that it is the wrong cylinder for the household now. A repair keeps exactly what you have. A replacement is the only chance to change it, and doing it later pays for the labour twice.

The usual case is capacity. A cylinder sized correctly when it went in runs cold halfway through the second shower once a bathroom is added or there are more people in the house. Nothing is broken. It is too small. The opposite happens too, with a large cylinder in a one person flat reheating a volume nobody draws.

The other case is type. A vented cylinder fed from a loft tank delivers hot water at whatever pressure the height of that tank gives, which is why the hot side of a mixer feels weak against the cold. An unvented cylinder works at mains pressure and balances hot and cold at the outlets. If the complaint that started all this was pressure rather than quantity, no repair fixes it and neither does a like for like swap. Going unvented needs the incoming mains to have the flow to support it, and the work must be done by an engineer holding the unvented qualification.

If you are not sure which of the two is sitting in your cupboard, the difference between an unvented vs vented cylinder is worth establishing before anybody prices anything, because it changes what can be repaired, who is allowed to touch it and what the annual obligations are afterwards. Anything on the sealed side of an unvented unit has to be handled by a G3 qualified plumber, and that applies to the safety components as much as to the vessel. A vented copper tank in a loft fed system carries no such restriction, which is one more reason that repairing an old vented cylinder stays economic long after the paper case looks weak.

There is also the boring one: insulation. An older cylinder with a quilted jacket strapped round it loses heat standing still in a way a factory foamed unit does not. Put a flat hand on it a couple of hours after it has finished heating, and it should feel barely warm. A properly warm cylinder, or a hot cupboard, is money going into the airing cupboard every day of the year. That alone does not justify scrapping a sound tank, but it tips a close decision.

How disruptive is the swap in your building?

The labour side varies enormously by property, and it belongs in the decision rather than being discovered halfway through. Getting the new cylinder in is usually easy. Getting the old one out, and getting the pipework to line up, is where the day goes.

A Victorian conversion is usually the awkward one. The cupboard was built around the cylinder, the pipework runs into a boxed corner and disappears, and the feed and vent arrangement was altered by whoever last worked on it. Many purpose built blocks have a thermal store such as a Pulsacoil in a cupboard sized to the millimetre, so what looks like a cylinder replacement is a different job. A new build with liquid screed over underfloor heating has the manifold and controls tied into the cylinder.

That argues both ways. Where a repair is honest, a difficult swap is a real reason to keep the cylinder. Where the vessel is finished, the swap is a difficult day whenever it happens, and doing it at a time of your choosing beats doing it on the morning the cupboard floor is wet.

• Victorian conversion: boxed pipework, altered feed and vent, expect adaptation.
• Purpose built block with a thermal store: not a like for like cylinder job.
• New build with underfloor heating: controls and manifold are tied to the unit.
• Loft hatch or tight landing access: it limits what can physically be fitted.

How long are you staying?

The same cylinder gets a different answer depending on what happens next. If you are staying ten years, a repair on a marginal vessel is borrowed time you pay for later with a second lot of labour. If you are selling in six months, a competent repair on a working system is reasonable, and the buyer decides with a survey in front of them.

Landlords sit elsewhere again. A cylinder that fails repeatedly in a let property costs more in call outs, access arrangements and tenant relations than the gap between a repair and a replacement ever shows on paper. The practical rule there is that the second repeat of the same fault is the point to stop repairing.

Leaseholders have a question to settle first: whether the cylinder is yours at all. In some blocks it is demised to the flat, in others the whole arrangement sits with the freeholder. Establish that before commissioning anything, because paying for a replacement the building was going to do is an annoying way to spend money.

Put the answer against the ordinary numbers rather than against a feeling. How long does a hot water cylinder last is a question with a usable range behind it, and knowing where your unit sits in that range tells you whether a repair is buying five more years or five more months. A tank comfortably inside its expected life with one failed component is an easy repair. A tank already past it, on hard water, with a second fault in two years, is being kept alive rather than maintained.

When repairing an old cylinder is clearly the right call

Repair is the right answer more often than the internet suggests, and the cases are easy to recognise. A sound vessel with one failed component and no history of the same fault. A cylinder that drains clear. A vented copper unit where every part is generic. A cylinder correctly sized and delivering the pressure the house needs. Any of those, with a fault outside the vessel wall, is a repair.

It is also the right answer when the fault has been misdiagnosed, which happens often enough to say plainly. No hot water with the heating still working is usually a control or a valve rather than the tank. Water that runs out quickly is capacity, not a hole. A cylinder blamed for a boiler fault is a cylinder replaced for nothing.

What makes the difference is somebody actually looking. Draining it, pulling the element to see inside the boss, checking coil performance against boiler output and testing the controls takes an hour and answers the question. Replacing on suspicion is the expensive version of guessing.

Once you have decided to keep a cylinder, the thing that determines how long that decision holds is whether it then gets looked at annually. What a hot water cylinder service should include is not a long list, but every item on it is a part that fails quietly and takes the tank with it when it does. A repaired cylinder that goes back into a service routine tends to behave. A repaired cylinder that is closed back into the cupboard and forgotten is the one that produces the same fault a year later and turns a sensible repair into a bad one in hindsight.

We do not quote a cylinder job from a description on the phone, because the answer depends on what the vessel turns out to be and what has to be altered around it. The rate is agreed before we attend and does not move once the work has started.

The decision, in the order to take it

Run the factors in sequence and most cylinders sort themselves out. Start with the vessel, because a leaking tank ends the conversation. Then parts, because a cylinder with none ends it too. Then the history of the fault, because a component failing repeatedly is the tank speaking through it. Only then the softer factors: size, type, insulation, disruption and how long you are staying. Those rarely decide a case alone, but two or three together will tip a marginal cylinder into replacement.

Before calling anybody, gather what an engineer will ask for anyway. Note the make, model, capacity and year off the data plate. Find out where the water actually comes from by drying everything and laying kitchen roll at each fitting, at the immersion boss, at the tundish and against the base. Know how long a full reheat takes now against how long it used to. Write down what has already been replaced and when. That is most of a diagnosis handed over in one go.

The uncomfortable bit, plainly: plenty of cylinders booked in as replacements need an element, a gasket or a valve, and some that people keep repairing should have been changed two faults ago. The failure mode of the trade is replacing on age. The failure mode of the householder is repairing on hope. Deciding on the vessel avoids both.

• Is the vessel leaking? Yes means replace, and nothing further matters.
• Are parts still available for the model? No means replace.
• Has the same component failed before? Twice means the tank is causing it.
• Does it drain clear or does it drain grit? Grit argues for spending once.
• Right size and type for the household now? If not, a repair preserves the wrong thing.
• How long are you keeping the property? Short stays justify a sound repair on a marginal tank.