What is the best unvented cylinder brand in the UK

There is no single winner, but Telford, Gledhill and OSO are the ones we fit most confidently, with Albion close behind. All use duplex stainless steel, an external expansion vessel and standard immersion bosses, which is what makes them easy to service and repair years later. Megaflo Eco is well built but uses an internal expansion arrangement and a branded immersion, both of which tie you more closely to one manufacturer.

Is a Megaflo worth the money

It is a good cylinder with a strong dealer network and a sound duplex stainless vessel, so nobody should feel badly served by one. Our reservations are practical rather than about quality. The internal air pocket used for expansion needs a manufacturer specific procedure carried out at every annual service, and the immersion heater is a branded part rather than a standard screw in element. If yours will be serviced properly every year, those are minor.

How long should an unvented cylinder last

A duplex stainless cylinder that is serviced annually, fed by a correctly set inlet control group and not run hotter than it needs to be should give many years of service, and the vessel usually outlives several sets of valves and immersions. What shortens that life in London is hard water scaling the coil and element, excessive store temperature, and years of skipped services that let a worn safety valve pass unnoticed.

What size unvented cylinder do I need

Size comes from the number of bathrooms, the pattern of demand and the output of the heat source, not from the size of the old cylinder. A single bathroom flat is usually well served in the 150 to 180 litre range, while a two bathroom household with simultaneous showers needs more volume and, just as importantly, a higher output coil so the store recovers quickly between draw offs.

Does an unvented cylinder need an external expansion vessel

Most designs use one, and that is a good thing. The external vessel is a separate, replaceable, commonly stocked component, so when its diaphragm eventually fails an engineer changes it on the same visit. Megaflo instead holds an air pocket inside the vessel, which removes the external component but requires a manufacturer specific step at every annual service to maintain it.

Which immersion heater should I fit in hard water

Specify an Incoloy or titanium sheathed element rather than the cheapest copper one. London water deposits scale heavily on the element surface, the scale insulates it, the element then runs hotter than it should and fails. A better sheath tolerates that far longer. Also check the element has physical room to withdraw from the cylinder, because a boxed in installation turns a simple change into a joinery job.

Can an unvented cylinder be repaired or does it need replacing

It depends which part has failed. A passing expansion relief valve, a failed pressure reducing valve, a dead immersion, a faulty thermostat or a perished expansion vessel diaphragm are all ordinary repairs on a cylinder with years left in it. The exception is the stainless vessel itself, which cannot be patched, so a weeping vessel is a genuine replacement. Get a proper diagnosis before accepting a replacement quote.

Is a stainless steel cylinder better than an enamel lined one

For domestic unvented work, yes. Duplex stainless resists the corrosion that matters and needs no sacrificial anode. An enamel lined steel vessel relies on an anode that somebody has to inspect and replace on schedule, and in practice almost nobody does. If you are offered an enamel lined unvented cylinder for a house or flat, ask why it is being proposed over stainless.

Does a heat pump need a different unvented cylinder

Yes, and this is the specification mistake we see most often. A heat pump delivers water at a much lower flow temperature than a gas boiler, so the coil needs far more surface area to transfer the same energy. A standard boiler specification cylinder connected to a heat pump reheats slowly and disappoints the household. If a heat pump is in the plan now or later, buy a heat pump rated cylinder at the outset.

What should I ask an installer before buying a cylinder

Ask what the vessel material and grade is, whether expansion is handled internally or by an external vessel, whether the immersion is a standard screw in element, whether the inlet controls are individual parts or one assembly, and where the immersion will physically withdraw to. Then ask what the measured mains flow rate and pressure at the property are. If nobody has measured the supply, the specification is a guess.

Best Unvented Cylinder Brands: An Engineer's View

The best unvented cylinder is a duplex stainless steel unit with an external expansion vessel, a standard screw in immersion boss and a valve set built from parts any merchant stocks. On that test the cylinders we fit most happily across London are Telford Tempest, Gledhill Stelflow and OSO, with Albion Ultrasteel close behind. Megaflo Eco is well made and we fit plenty, but it loses marks for how it handles expansion and for an immersion arrangement that ties you to one manufacturer. What matters is not the brochure. It is whether an engineer who has never seen your airing cupboard can service the thing in an hour in ten years time.

Which unvented cylinder is best

Judged on the things that decide whether you are still happy in year twelve, the best unvented cylinders are the plainly engineered ones: a duplex stainless steel vessel, an external expansion vessel on the cold feed, a standard immersion boss, and a valve set you can rebuild from stock parts. Telford, Gledhill and OSO all fit that description. Megaflo Eco takes a different route on two of those four.

We do not rank on litres, insulation thickness or the recovery figure printed on the label, because between the serious manufacturers those numbers are close enough that they rarely decide anything. What decides it is what happens on the day something goes wrong. A cylinder where the immersion unscrews, the vessel unscrews, the thermostat clips off and the valve set comes apart into named components is a cylinder that gets repaired. A cylinder built around a proprietary assembly with a lead time is a cylinder that gets replaced, and replacing a working cylinder because of a small part is the most expensive thing that can happen to you in an airing cupboard.

The rest of the answer is uncomfortable for brand loyalists. The biggest predictor of how long your cylinder lasts in London is not the badge, it is the water and the installation. A good cylinder on an unmeasured supply, with no scale protection, an inlet valve set wrong and an immersion boxed in behind joinery will fail before an average cylinder installed by somebody who was paying attention. We also ignore any warranty length quoted without mention of the annual service, because the condition attached to that warranty is that the cylinder is serviced every year and the paperwork kept.

Duplex stainless steel vessel, not enamel lined steel relying on a sacrificial anode.
External expansion vessel that can be swapped as a separate component.
Standard screw in immersion boss, with real room to withdraw the element.
An inlet control group of individual parts rather than one moulded assembly.
A coil sized for the heat source you actually have, not the one on the data sheet.

Stainless steel grade matters more than the brand name

Nearly every serious unvented cylinder sold here is now duplex stainless steel, and that is the right material. Duplex resists the chloride stress corrosion that kills lesser grades, and unlike an enamel lined steel cylinder it does not depend on a sacrificial anode that somebody has to remember to inspect. If you are offered an enamel lined unvented cylinder for a domestic job, ask why.

The failure you are avoiding is a pinhole in the vessel itself, and it is terminal. A stainless vessel cannot be patched, so the day it weeps the cylinder is finished no matter how healthy every other part is. Running the store far hotter than the household needs, permanently, does not help either. It accelerates scale on the coil and the element and buys you nothing that correct sizing would not have given you.

A weeping stainless vessel is not repairable. If an engineer says the cylinder itself has failed, that is a replacement conversation. If he says a valve, an immersion or a thermostat has failed, it is not.

How the cylinder handles expansion is the biggest design split

Water expands when heated and an unvented cylinder is sealed, so that expansion has to go somewhere. There are two approaches. Most manufacturers fit an external expansion vessel on the cold feed, a separate component with its own diaphragm. Megaflo instead holds an air pocket inside the vessel behind an internal baffle, removing the external component entirely.

The internal approach is elegant on paper. Nothing hangs off the pipework and there is no external diaphragm to perish. The drawback is that the air pocket is gradually absorbed into the water, and restoring it is a manufacturer specific step carried out at the annual service. It is straightforward for anyone who has done it before. It is also, in our experience, exactly the step that gets skipped when a cylinder has been serviced by whoever was cheapest that year.

The external vessel is the less clever answer and the one we prefer. It is a separate, replaceable, commodity component. When the diaphragm eventually fails, an engineer fits another from the van and leaves. Nothing about that fix depends on the cylinder manufacturer still existing or still supporting the model. For something that has to survive changes of ownership, changes of engineer and twenty years of London, that boring replaceability is worth more than a tidier installation.

External vessel: a separate part, replaceable from stock, obvious when it fails.
Internal air pocket: tidier install, dependent on a specific step at every service.
Either way, a dripping tundish means a safety valve is passing and needs investigating.

Can you actually get the immersion heater out

This question separates cylinders more than anything else on the day of a repair. A cylinder with a standard screw in immersion boss takes an element any merchant stocks, and an engineer carries the box spanner for it. A cylinder using a proprietary plate mounted immersion needs that manufacturer's part, which becomes a different conversation once the model is old. Megaflo is the example everybody meets: the immersion is a branded part rather than a plain screw in element. It is not a bad design and it is not hard to change if you have the part, but it is a dependency, and dependencies turn a two hour repair into a three day wait.

The second half of this has nothing to do with the manufacturer. An element needs physical room to withdraw, and its length can be substantial. We regularly find cylinders set so close to a wall, a boxed in stack or a cupboard frame that the element cannot come out without dismantling joinery or tilting the cylinder. That is an installation failure, not a brand failure, and it is one of the first things we look at. In water as hard as ours we would also always specify an Incoloy or titanium sheathed element rather than the cheapest copper one, because a bare element here scales heavily, runs hot under the scale and then burns out.

Before anyone fits a cylinder, ask them to show you where the immersion will withdraw to. If the answer involves moving the cylinder, the position is wrong.

The coil decides your recovery, not the litres on the label

Recovery time is a function of coil surface area and the heat source driving it, not of how many litres the cylinder holds. A large cylinder with a modest single coil and an average system boiler will reheat more slowly than a smaller cylinder with a generous coil on the same boiler, and the household experiences that as running out of hot water even though the store is nominally bigger.

Read the assumption behind any stated recovery figure, because it is calculated against a specific boiler output. If the sheet assumes an output the property does not have, the real recovery will be slower than the number. On gas fired jobs in a busy household we want the high output coil rather than simply more volume.

For a heat pump the rules change completely, and this is where cheap cylinders get exposed. A heat pump delivers a much lower flow temperature, so the coil must be far larger to move the same energy, and no amount of controls tuning fixes an undersized coil. If a heat pump is anywhere in the plan, even as a future possibility, specify a heat pump rated cylinder now. Twin coil versions for solar are worth it only where the second heat source genuinely exists rather than being a vague intention.

Standard single coil: fine for a gas system boiler with sensible output.
High output coil: worth specifying wherever demand is heavy or simultaneous.
Heat pump coil: much greater surface area, essential at low flow temperatures.
Twin coil: only where a real second heat source exists now.

Are the controls and safety valves standard parts

The parts that actually fail on an unvented cylinder are not the vessel. They are the pressure reducing valve, the expansion relief valve, the temperature and pressure relief valve, the cylinder thermostat with its thermal cut out, and the motorised valve on the heating side. Every one is a wearing component, and the good cylinders let you change each individually.

So look at how the inlet group is built. On the better designs it is a set of discrete components on the cold feed, each with its own part number, so a passing relief valve is a valve change. On weaker designs the inlet control is one moulded assembly, and a single failed element inside it means buying the whole group. Ask the question before you buy, because nobody volunteers the answer. A manufacturer that sells a proper boxed service kit for the model is telling you something useful about how it expects the cylinder to be looked after.

The temperature and pressure relief valve deserves its own mention because it is the last line of defence against a cylinder that has lost control of its temperature. It is a safety device, not a maintenance irritation to be capped, and anyone who suggests plugging a discharging valve rather than finding out why it is discharging should be shown the door.

Megaflo, Telford, Gledhill, OSO and the rest

Megaflo Eco is a well built duplex stainless cylinder with a wide dealer network and near universal name recognition, and there is nothing wrong with the steel. Our reservations are the two dependencies already described: the internal expansion arrangement that needs a manufacturer specific step at every service, and the branded immersion. Serviced properly every year by somebody who knows the model, it is a good cylinder and we fit them without complaint.

Telford Tempest is the one we reach for most often. Duplex stainless, external expansion vessel, standard immersion boss, sensible connection positions and a realistic range of shapes including slimline and horizontal versions that solve awkward London installations. Parts and service kits are easy to obtain. Gledhill Stelflow sits in the same category and is equally easy to live with.

OSO is the Norwegian option and it is properly made. The stainless work is a visible step above most of the market and it generally costs more. Where somebody wants the best engineered cylinder and is not counting every pound, we are happy to fit one. Albion Ultrasteel is conventional in all the right ways and belongs in the same conversation as Telford and Gledhill. Joule is worth a look for heat pump work, where the large coil versions are designed for the job rather than adapted to it.

The cylinders we are cautious about are the unbranded and thinly branded ones that turn up on price led quotes. The vessel may be perfectly adequate. The problem is support: parts in five years, a service kit, a technical line that answers, and a model still in production. This is a twenty year purchase living above your ceiling, and buying it from a name that may not be there in ten is a false economy.

What we would fit in a London property

In a two or three bedroom Victorian conversion with one bathroom and a system boiler, a Telford or Gledhill in the 150 to 180 litre range with a standard coil, an external expansion vessel and an Incoloy immersion. Measure the incoming mains flow and pressure before anything is ordered, because an unvented cylinder is only as good as the supply feeding it and plenty of converted flats cannot deliver what the owner assumes.

In a family house with two bathrooms and simultaneous demand, go up in volume and specify the high output coil rather than simply buying more litres. The complaint we are asked to solve in those properties is almost never storage volume, it is that the cylinder cannot reheat fast enough between the evening showers. In a new build with liquid screed over underfloor heating, the hot water and heating sides want different flow temperatures, so the design has to separate them properly rather than treating the whole system as one.

In a purpose built block running a Pulsacoil or similar thermal store, none of this applies. That is not an unvented cylinder, there is no gas appliance in the flat, and it fails and is serviced completely differently. Do not let anyone quote an unvented cylinder replacement without first establishing which animal you actually have. Our twenty engineers see both across London every week and the two get confused constantly.

One bathroom conversion flat: 150 to 180 litres, standard coil, mains flow measured first.
Two bathroom family house: more volume and a high output coil, not just more litres.
Heat pump now or later: heat pump rated coil, specified at purchase.
Block with a thermal store: a different appliance entirely, not comparable to this.

The verdict

Fit a duplex stainless cylinder with an external expansion vessel, a standard immersion boss and a valve set built from replaceable parts, and you have made the decision that matters. Telford Tempest and Gledhill Stelflow are our default recommendations because they do all of that without fuss. OSO is the one to buy if you want the best made cylinder and the budget allows. Megaflo Eco is a good cylinder with two dependencies you should understand before you commit rather than discover at year six.

What we would not do is choose on storage volume alone, on a warranty headline with no service behind it, or on whichever brand a quote happens to mention. And if your existing cylinder is misbehaving, get it diagnosed before accepting a replacement quote. Lukewarm water, slow recovery, a dripping tundish or noise during reheat are ordinary faults on ordinary parts far more often than they are a dead vessel, and replacing a repairable cylinder is the costliest mistake in this whole subject. In most of those cases an unvented cylinder repair puts the system right without anybody touching the vessel, because the failed part is a valve, a thermostat, an immersion element or a waterlogged expansion vessel rather than the steel.

Where the steel has genuinely gone, a hot water cylinder replacement is the honest answer and the brand decision above becomes the one that matters, because you are choosing the appliance you will live with for the next two decades. The order is diagnosis first, brand second. Doing it the other way round is how a repairable cylinder ends up in a skip.

Questions we get asked

What is the best unvented cylinder brand in the UK: There is no single winner, but Telford, Gledhill and OSO are the ones we fit most confidently, with Albion close behind. All use duplex stainless steel, an external expansion vessel and standard immersion bosses, which is what makes them easy to service and repair years later. Megaflo Eco is well built but uses an internal expansion arrangement and a branded immersion, both of which tie you more closely to one manufacturer.
Is a Megaflo worth the money: It is a good cylinder with a strong dealer network and a sound duplex stainless vessel, so nobody should feel badly served by one. Our reservations are practical rather than about quality. The internal air pocket used for expansion needs a manufacturer specific procedure carried out at every annual service, and the immersion heater is a branded part rather than a standard screw in element. If yours will be serviced properly every year, those are minor.
How long should an unvented cylinder last: A duplex stainless cylinder that is serviced annually, fed by a correctly set inlet control group and not run hotter than it needs to be should give many years of service, and the vessel usually outlives several sets of valves and immersions. What shortens that life in London is hard water scaling the coil and element, excessive store temperature, and years of skipped services that let a worn safety valve pass unnoticed.
What size unvented cylinder do I need: Size comes from the number of bathrooms, the pattern of demand and the output of the heat source, not from the size of the old cylinder. A single bathroom flat is usually well served in the 150 to 180 litre range, while a two bathroom household with simultaneous showers needs more volume and, just as importantly, a higher output coil so the store recovers quickly between draw offs.
Does an unvented cylinder need an external expansion vessel: Most designs use one, and that is a good thing. The external vessel is a separate, replaceable, commonly stocked component, so when its diaphragm eventually fails an engineer changes it on the same visit. Megaflo instead holds an air pocket inside the vessel, which removes the external component but requires a manufacturer specific step at every annual service to maintain it.
Which immersion heater should I fit in hard water: Specify an Incoloy or titanium sheathed element rather than the cheapest copper one. London water deposits scale heavily on the element surface, the scale insulates it, the element then runs hotter than it should and fails. A better sheath tolerates that far longer. Also check the element has physical room to withdraw from the cylinder, because a boxed in installation turns a simple change into a joinery job.
Can an unvented cylinder be repaired or does it need replacing: It depends which part has failed. A passing expansion relief valve, a failed pressure reducing valve, a dead immersion, a faulty thermostat or a perished expansion vessel diaphragm are all ordinary repairs on a cylinder with years left in it. The exception is the stainless vessel itself, which cannot be patched, so a weeping vessel is a genuine replacement. Get a proper diagnosis before accepting a replacement quote.
Is a stainless steel cylinder better than an enamel lined one: For domestic unvented work, yes. Duplex stainless resists the corrosion that matters and needs no sacrificial anode. An enamel lined steel vessel relies on an anode that somebody has to inspect and replace on schedule, and in practice almost nobody does. If you are offered an enamel lined unvented cylinder for a house or flat, ask why it is being proposed over stainless.
Does a heat pump need a different unvented cylinder: Yes, and this is the specification mistake we see most often. A heat pump delivers water at a much lower flow temperature than a gas boiler, so the coil needs far more surface area to transfer the same energy. A standard boiler specification cylinder connected to a heat pump reheats slowly and disappoints the household. If a heat pump is in the plan now or later, buy a heat pump rated cylinder at the outset.
What should I ask an installer before buying a cylinder: Ask what the vessel material and grade is, whether expansion is handled internally or by an external vessel, whether the immersion is a standard screw in element, whether the inlet controls are individual parts or one assembly, and where the immersion will physically withdraw to. Then ask what the measured mains flow rate and pressure at the property are. If nobody has measured the supply, the specification is a guess.