Hot water cylinder replacement

When replacement beats repair, and when it does not

The single question that decides it is whether the fault is in the vessel or on the vessel. A hot water cylinder is a pressure vessel with a set of serviceable parts hung off it. If the vessel is sound, almost every fault you can have is a part swap that takes an hour or two and leaves the cylinder in place for another decade.

Replace the cylinder when water is coming from the body of it rather than from a joint or a fitting. A pinhole in the copper, a split weld seam at the dome, or a corroded patch at the foot where it sits on the base plate cannot be repaired in any way that is worth paying for. Solder and epoxy on a vessel that is carrying mains pressure is a temporary measure at best and we will not sell it as a fix.

Replace it when the indirect coil has failed. The coil is the loop of pipe inside the cylinder that carries hot water from the boiler and heats the stored water without mixing with it. When the coil splits, heating water and stored water mix. You see it as heating pressure that drops every time the hot water runs, a boiler that keeps losing pressure for no visible reason, or discoloured water at the taps. The coil is inside the vessel and is not replaceable, so the cylinder goes.

Replace it when the cylinder is grossly undersized for how the household now lives. A 90 litre cylinder fitted for a couple does not become adequate for a family of five with two bathrooms, and no amount of turning the thermostat up fixes that. Running the stored water hotter to stretch the volume increases the scale rate and the scald risk without adding much usable hot water.

Repair it, not replace it, when the symptoms point at a component. No hot water at all with a working boiler is usually a motorised valve, a cylinder thermostat or a wiring centre fault. Hot water that runs lukewarm and then cold is often a stratification problem or a failed lower immersion element. Water dripping at the tundish is the pressure relief side of an unvented system and points at the expansion vessel or a valve seat, not at the tank. A leak at the immersion boss is a gasket. None of those are reasons to buy a new cylinder, and being sold one on those symptoms alone is the most common way people overspend on hot water.

Two of those repairs cover most of the calls we get on cylinders that are still perfectly sound. A dead element is an immersion heater repair and often a same day one, and the controls and safety parts on a sealed tank are covered under unvented cylinder repair. Both leave the vessel where it is.

• Weeping from the body of the vessel, not from a fitting: replace
• Heating pressure dropping whenever hot water is drawn: failed coil, replace
• Rust coloured or cloudy hot water with clear cold water: replace
• Cylinder badly undersized for the household: replace
• No hot water but heating works: valve, thermostat or wiring, repair
• Dripping tundish or discharge pipe: expansion vessel or valve, repair
• Immersion heater dead: element swap, repair

If you can see water running down the outside of the cylinder from the body itself, turn off the cold feed to it and switch off the immersion heater at the isolator before you call anyone. A slow weep becomes a fast one without warning, and a full cylinder holds enough water to come through a ceiling.

Signs your cylinder is coming to the end of its life

Cylinders rarely fail without giving notice. The pattern we see most often is a run of small faults over a year or two, each one fixed individually, before the vessel itself lets go. Recognising that pattern is what lets you replace on your own schedule rather than on a Sunday night with no hot water.

Look at the foot of the cylinder first. Corrosion almost always starts where the base sits on the floor or the platform, because that is where condensation collects and where insulation has usually been cut away. A green or white crust on the copper, a stain on the boards, or a rippled patch of paint on a painted cylinder all say the same thing.

Listen to it as it heats. A cylinder that ticks and creaks as it warms is normal thermal movement. A cylinder that rumbles, hisses or makes a popping noise like a kettle full of gravel has a scale bed on the coil or on the immersion element, and a heavily scaled coil is working much harder than it should to get the same water hot.

Watch the recovery time. Note how long a full reheat takes now compared with a year ago. A cylinder that used to be hot through in forty minutes and now takes an hour and a half, with no change to the boiler, has lost heat transfer through the coil to scale. That is a slow failure in progress, not a one off bad day.

Check the age if you can find it. Most cylinders carry a data plate with a manufacture date and a model number, usually on the front near the top or on the side beside the immersion boss. Read the plate before you decide anything, because it also tells us the current capacity and whether the cylinder is direct or indirect, which shapes what can replace it.

None of those on its own is a verdict. Taken together they are, and the full list of signs your cylinder needs replacing is worth reading through with a torch in your hand rather than from memory.

Vented, unvented or a thermal store: what goes back in

What you can put back depends on your incoming mains and on what the rest of the system expects. There are three realistic outcomes and they behave very differently at the tap.

A vented cylinder is fed by a cold water storage tank, usually in the loft, and the pressure at your hot taps comes from the height of that tank above them. It is simple, it is cheap to replace like for like, and the parts are commodity items. The downsides are the loft tank itself, weak pressure on upper floors, and a mixer shower that needs a pump to be worth standing under. If your existing setup is vented, working and delivering pressure you are happy with, a like for like vented replacement is the least disruptive job we do.

An unvented cylinder is fed directly from the mains, so hot water comes out at close to mains pressure and you lose the loft tank entirely. Showers improve dramatically, two outlets can run at once without one starving the other, and the airing cupboard becomes the only place in the house with hot water kit in it. The cylinder is a sealed pressure vessel, so it carries a set of safety components: an expansion vessel or internal air gap to take up the expansion of heated water, a pressure reducing valve on the inlet, a temperature and pressure relief valve on the vessel, and a discharge pipe running to a visible tundish and then outside. Those parts are what make the unvented system safe and they are also what it needs serviced once a year.

A thermal store works the other way round. The stored water is the heating water and your cold mains passes through a coil inside it to come out hot on demand. It is useful where the mains flow is good but you also want to take heat from something other than a boiler. Thermal stores are less common in London housing and we only recommend one where there is a clear reason.

There is also the electric only case. Where there is no boiler serving the cylinder, a direct cylinder heated by one or two immersion heaters does the whole job on electricity. A replacement here is straightforward but worth sizing generously, because an immersion reheats far more slowly than a boiler coil and you cannot rush it.

If you are weighing the first two against each other rather than reading this with a failed cylinder in the cupboard, the comparison of an unvented against a vented cylinder sets out how each one feels to live with, which is usually what decides it.

An unvented cylinder must be installed by an engineer who holds the qualification for unvented hot water work, and the discharge arrangement is a building control matter. Our engineers are qualified for the work described here.

Moving from a vented cylinder to an unvented one

This is the upgrade most people actually want when they ask about replacing a cylinder, and it is the one that needs checking before anything is ordered. An unvented cylinder can only give you good pressure and flow if the incoming mains can supply it.

The first thing an engineer does is measure the incoming supply. We check static pressure with a gauge on an outside tap or the kitchen cold, then run a timed flow test into a measured container to get litres per minute with the supply wide open. Static pressure tells us whether the vessel will be happy. Flow rate tells us what the shower will feel like. A property can have perfectly good pressure and still have poor flow if the incoming pipe is an old narrow lead or steel run, and in that case an unvented cylinder will not fix the shower, it will just move the bottleneck.

We check the incoming stop tap and the size of the main too. Plenty of older London houses and conversions still run off a narrow supply, which limits what any mains fed system can do. Where flow is marginal we say so and set out the options, which sometimes means keeping a vented system and pumping it instead.

The other change is upstairs. The loft cold water storage tank and the vent pipe are no longer needed, which frees the space and removes a freezing and contamination risk, but the cold taps then come off the mains and any tank fed appliance needs rerouting. We plan that part before we touch anything.

The mains fed job itself, from the inlet control set through to the discharge arrangement, is described step by step under unvented cylinder installation, which is the page to read if the survey comes back saying your supply can carry one.

• Static pressure measured at the incoming supply
• Timed flow rate in litres per minute, taken with the supply wide open
• Size and material of the incoming main and the internal pipework
• Whether the discharge pipe can reach a safe visible termination
• Whether the airing cupboard floor can carry the filled weight
• What happens to the loft tank, the vent pipe and any tank fed cold outlets

Working out what size cylinder you need

Sizing is the decision people get wrong most often, in both directions. Too small and the cylinder runs out halfway through the second shower. Too large and you pay to keep water hot that nobody uses, and the reheat takes longer than it needs to. Sizing is only one of the things that decide it, and how to choose a hot water cylinder works through the rest of them in the order a survey would.

Start with bathrooms rather than bedrooms. A bath takes a substantial share of a small cylinder in one go, and a modern high flow shower can draw more than an old one because it is delivering more litres per minute, not fewer. Count how many hot outlets can realistically run at once in your household in the busy hour of the day, then size for that hour, not for the whole day.

Then look at recovery. A cylinder fed by a boiler with a decent output reheats quickly, so a slightly smaller vessel with fast recovery can outperform a larger one that is slow to come back. A cylinder heated only by immersion is the opposite: recovery is slow, so the volume has to carry the whole peak on its own and you size up. Recovery is also where the vessels differ from one another rather than on the badge, which is the practical reason to look at what separates the best unvented cylinder makes before settling on a model.

If you want to put a number on it rather than a feel, the sizing guide on what size hot water cylinder do I need works through it by bathrooms and by peak hour demand, which is the method we use on a survey.

Then look at where it has to physically go. Height is the constraint that catches people out. An airing cupboard with a fixed shelf, a sloping ceiling or a doorway at the top of a narrow stair limits what can get in and stand up. We measure the cupboard, the route in and the height available under any fixed obstruction before we specify anything, because a cylinder that fits on paper and not through the door is a wasted day for everyone.

A filled cylinder is heavy. Before a larger one goes into a first floor cupboard we check that the floor and the base can carry it, especially in a conversion where the cupboard is built over a stair void.

What the replacement job actually involves

A straight swap is typically a day, sometimes less on a simple vented like for like and sometimes more where pipework has to be rerouted or a vented system is being converted to unvented. Here is what happens in the property so you know what to expect.

We isolate and drain first. The cold feed is closed, the immersion is switched off at its isolator, and the cylinder is drained down through a hose run to a drain or an outside point. A full cylinder takes a while to empty and draining it properly matters, because a partly full cylinder is far heavier and more awkward to move than an empty one.

The old cylinder is then disconnected: flow and return to the coil, cold feed, hot draw off, immersion wiring at the isolator, and the cylinder thermostat and any strapped on controls. Where the pipework is old and the fittings are seized we cut back to sound pipe rather than force a joint that is going to weep later. On an unvented removal the discharge pipework and the tundish come out with it.

The new cylinder goes in, is levelled on its base and is connected up. On an unvented installation the inlet control set, the expansion vessel, the temperature and pressure relief valve and the discharge pipework to the tundish all go in at this stage. The discharge has to run to a safe visible termination, and the tundish has to be where somebody will actually see it dripping, because a dripping tundish is the system telling you something is wrong.

Then the system is refilled, vented and tested. We check for leaks at every joint under pressure, set the cylinder thermostat, check the operation of the motorised valve and the controls, confirm the discharge arrangement is correct and run the hot water up to temperature. On an unvented system the expansion vessel is charged to the right pressure against the cold inlet pressure, because a vessel that is not charged correctly will push water out of the tundish within days.

Before we leave we show you where the isolation points are, where the tundish is and what it means if it drips, how the controls are set and what the annual service involves. That handover takes ten minutes and saves a call out later.

What changes in the airing cupboard

The cupboard usually ends up tidier than it was, but it does change. A modern unvented cylinder is taller and narrower than the old copper one for the same volume, because it is better insulated and shaped for the space. Slatted shelves often have to come out to get the height, and we will tell you that before the day rather than after we have removed them.

There will be more kit visible on an unvented installation than you are used to. The inlet control group, the expansion vessel and the tundish all sit in the cupboard and they need to be reachable, because the annual service involves checking and recharging the expansion vessel and exercising the relief valves. We position them so an engineer can get a spanner on them without dismantling anything.

If the cupboard doubles as your drying space, be aware that a well insulated modern cylinder gives off much less heat than an old one with a jacket. That is the point of the insulation, but it does mean the cupboard will not dry towels the way it used to. If that matters, say so early and we can look at a heated towel rail or a low output radiator in the cupboard instead of quietly leaving you with a cold airing cupboard.

What we do with the old cylinder

We take it away. The old cylinder leaves with us the same day, and that is included in the job rather than treated as an extra.

Copper cylinders go for scrap copper and stainless steel unvented cylinders go the same route. Nothing is left in the front garden or in a communal bin store.

Getting it out is the part worth planning. An old cylinder is bulky, awkward and often has to come down a narrow staircase or through a flat door with a tight turn. We drain it fully first, protect the route out, and where the stair is genuinely too tight we cut the cylinder down rather than damage the wall or the bannister. This is a normal part of the job in older London housing and it is not a surprise to us.

Hard water and what it does to a new cylinder in London

Most of London is a hard water area and the cylinder is where that shows up first. Calcium comes out of the water when it is heated and lands on the hottest surfaces, meaning the immersion element and the outside of the indirect coil.

That matters for two reasons. Scale on a coil is an insulator, so the boiler has to run longer to get the same water hot and the recovery time creeps up year after year. Scale on an immersion element makes the element run hotter than it was designed to and shortens its life, which is why immersion heaters fail more often here than in soft water parts of the country.

The practical lever you control is storage temperature. Running the cylinder hotter than it needs to be accelerates scaling noticeably, so the sensible position is hot enough for safety and for the household to have a proper supply, and no hotter for the sake of it. Your engineer will set the thermostat correctly when the cylinder goes in and can explain where it sits and why.

Scale reducing treatment on the cold feed is worth discussing when you are already replacing the cylinder, because the pipework is open and it is the cheapest time to fit anything. It does not eliminate the issue, but in a hard water area it slows it down, and a slower rate of scaling is the difference between an immersion element lasting a long time and being replaced twice.

Keeping a new cylinder working

An unvented cylinder needs an annual service. That is not an upsell, it is how the safety components on a sealed pressure vessel are kept working. The service checks the expansion vessel charge, exercises the temperature and pressure relief valve and the expansion relief valve, checks the discharge arrangement is clear, and verifies the thermostat and the thermal cut out. An uncharged expansion vessel is the single most common cause of a tundish that drips, and it is caught in minutes at a service and turns into a call out if it is not.

What actually happens at that visit, and how long it takes, is set out on our unvented cylinder service page, so you can see it is a list of checks rather than a box ticked on a certificate.

A vented cylinder needs far less. Keep the immersion isolator accessible, keep the loft tank lid on and the tank insulated, and keep an eye on the base of the cylinder once a year with a torch.

What decides the cost of a replacement

A cylinder replacement is not one job, so a number in an article would be a guess. What we can do is be clear about what moves it.

The cylinder itself is the largest single item and it varies with type and capacity. A like for like vented replacement is the least expensive version of this job. An unvented cylinder costs more because it is a pressure vessel with a full set of safety controls attached.

Then it is time and pipework. A straight swap where the new cylinder lands on the same footprint with the same pipe entries is quick. A conversion from vented to unvented takes longer because the inlet control set, the discharge pipework and the tundish all have to be installed and the loft tank has to be dealt with. Difficult access, seized old fittings and pipework that has to be rerouted all add time.

Our rate is fixed before we attend and the total depends on how long the job takes. An engineer will look at the cupboard, the pipework and the access and tell you what the job actually is before any work starts, and if the honest answer is that a repair will do it, that is what you will be told.