Are OSO cylinders any good

Yes. OSO builds in stainless steel in Norway and the standard of the fabrication and welding is at the top end of what we see on London jobs. The vessel quality is genuinely a step above much of the market. The trade offs are price and parts availability, since a London merchant is more likely to stock service parts for the common British brands than for an imported cylinder.

Where are OSO cylinders made

OSO Hotwater manufactures in Norway and sells into the UK through merchants and installers. That matters for two practical reasons. The build standard reflects Norwegian manufacturing, which is a good thing, and the cylinders were designed around soft Norwegian water rather than the hard supply that covers almost all of London, so scale protection and element choice deserve extra thought here.

Is an OSO better than a Megaflo

They are both well built stainless cylinders and neither owner is badly served. OSO generally wins on fabrication quality and costs more. Megaflo wins on name recognition and on how easily a London engineer finds parts and someone familiar with the model. Judge them on how each handles expansion and the immersion arrangement on the specific model quoted, since that decides how easy repairs are later.

How long does an OSO cylinder last

A stainless cylinder that is serviced annually, fed through correctly set inlet controls and not run hotter than the household needs should give many years, and the vessel typically outlives several sets of valves and immersion elements. What shortens that in London is hard water scaling the coil and element, excessive storage temperature, and skipped services that let a worn safety valve pass unnoticed for years.

Can an OSO cylinder be repaired or does it need replacing

Most faults are repairs. Relief valves, pressure reducing valves, immersion elements, thermal cut outs and expansion vessel diaphragms are all wearing parts that get changed without touching the vessel. The one genuine replacement is a weeping stainless vessel, which cannot be patched on any brand. Get the fault diagnosed before accepting a replacement quote, because replacing a repairable cylinder is the most expensive mistake here.

Do OSO cylinders need an annual service

Yes, and manufacturer warranties on unvented cylinders are routinely conditional on it being done and documented. Beyond the paperwork, the service is what catches a tired safety valve, a failing expansion vessel or a scaled element before any of them becomes an emergency. In water as hard as London's, the annual visit does more for the life of the cylinder than the choice of brand does.

What immersion heater should go in an OSO in London

Specify an Incoloy or titanium sheathed element rather than the cheapest copper one, because London water scales elements heavily and a scaled element runs hot beneath its own crust and burns out. Check the model manual for the immersion arrangement before ordering, and confirm the element has physical room to withdraw fully, since a boxed in cupboard turns a simple change into a joinery job.

Can I get OSO parts quickly in London

Universal items like an external expansion vessel are usually same day from any merchant. Model specific components, particularly on pre plumbed cabinet units, may need ordering in, so an OSO repair is occasionally a two visit job where a mainstream brand would be one. Having the model reference and serial number from the data plate ready makes the difference between one visit and two.

Is an OSO suitable for a heat pump

Only if you specify a model with a coil designed for low flow temperatures. A heat pump delivers water far cooler than a gas boiler, so the coil needs much greater surface area to transfer the same energy, and no amount of control tuning fixes an undersized one. That decision has to be made at purchase, because coil surface area cannot be added to an installed cylinder.

Should I choose an OSO for a flat above other properties

It is a reasonable place to spend the extra. Where a future failure means water reaching somebody else's ceiling, and where access for any repeat job is difficult, paying for the better built vessel is straightforward arithmetic. Pair it with a correctly routed discharge, properly set inlet controls and a booked annual service, since installation quality still predicts more than the badge does.

OSO Cylinders Explained: What a London Owner Should Know

An OSO cylinder is a stainless steel hot water cylinder made in Norway by OSO Hotwater, sold in the UK mostly as unvented indirect and direct electric models, and generally positioned above the mainstream British brands on build quality and on price. For a London owner the short version is this: the steelwork and the welding are a step above most of the market, but the things that decide whether you are still happy in year twelve are the same as with any cylinder. Which model you have, whether the immersion can be withdrawn, whether the inlet controls are individual parts, and what our water does to the coil all matter more than the badge. We are an independent plumbing and hot water firm with twenty engineers. We work on OSO cylinders across London, we are not an approved or authorised agent for the manufacturer, and what follows is our own view.

What is an OSO cylinder

OSO cylinders are hot water storage cylinders manufactured in Norway by OSO Hotwater and sold across the UK through merchants and installers. Almost everything in the domestic range is stainless steel, and most of what gets fitted in London homes is an unvented cylinder: a sealed, pressurised store that delivers hot water at mains pressure to every outlet in the property rather than relying on a loft tank and gravity.

Within that there are two broad kinds. Indirect models are heated by a coil connected to a boiler or a heat pump, with an immersion heater as backup or as a second heat source. Direct models have no coil at all and are heated entirely by immersion elements, which suits flats with no gas appliance, properties on an off peak electricity arrangement and some new build blocks. There are also pre plumbed and cupboard style units where the controls and valves arrive already assembled on the cylinder, which shortens the installation and removes a lot of the variation that comes from somebody plumbing an inlet group up on site.

Range names move around over the years, so stop guessing from memory and read the data plate. The model reference and serial number printed there are what a parts supplier and an engineer actually need. Photograph it before you ring anybody.

Indirect unvented: heated by a coil from a boiler or heat pump, immersion as backup.
Direct unvented: no coil, heated by immersion elements alone.
Pre plumbed and cabinet units: controls and valves fitted at the factory.
The data plate on the cylinder carries the model and serial you will be asked for.

Why do OSO cylinders cost more than a mainstream British brand

You are paying for the metalwork and the manufacturing, not for marketing. OSO builds in stainless steel and the standard of the welding, the finish and the general fabrication is visibly better than most of what arrives on a London job. Engineers notice it the moment the packaging comes off. Whether that premium is worth it to you is a different question, and the honest answer is that it depends on how long you intend to own the property and what the alternative on the quote is.

Where we think the money is well spent is on a cylinder going into an awkward position: a first floor cupboard in a Victorian conversion where getting the old one out involved taking a door frame off, a loft space that nobody wants to revisit, a flat where any future failure means a ceiling below somebody else. In those places the cost of ever doing the job again is so much higher than the cost of the cylinder that buying the better built unit is straightforward arithmetic.

Where the premium is harder to justify is a ground floor utility room in a house you are selling in three years, with a competent British stainless cylinder available and easier local parts support. A well specified mainstream cylinder installed by somebody who measured the mains supply first will outperform an expensive one installed by somebody who did not.

Is the Norwegian stainless steel actually better

The material itself is not exotic. Nearly every serious unvented cylinder sold in the UK is now some form of stainless steel, and the good British brands use duplex grades that are entirely fit for the job. What differs is the execution: gauge, weld quality, consistency of finish, and how the vessel is formed. On OSO that execution is at the top end of what we see, and a well made weld seam is exactly the place where a badly made cylinder eventually weeps.

It is worth being clear about what stainless does and does not protect you from. It removes the corrosion problem that made enamel lined steel cylinders dependent on a sacrificial anode that somebody had to inspect and replace on schedule, which in practice almost nobody did. It does not make a cylinder immune to scale, it does not stop a relief valve wearing, and it does not stop an immersion element burning out. Those are the failures we actually attend, and they happen on expensive cylinders and cheap ones alike.

The other thing stainless does not do is repair itself. A vessel that weeps is finished, because you cannot patch it the way you could once braze a copper cylinder. That is true of every stainless cylinder sold here, and it is why the vessel is the one part where better manufacturing has a permanent payoff.

A weeping vessel is a replacement, on any brand. A failed valve, thermostat, immersion element or expansion vessel is not. Get a diagnosis before accepting a replacement quote.

How does an OSO handle expansion, and why does that matter

Water expands when it is heated, and because an unvented cylinder is sealed that expansion has to be accommodated somewhere. Most unvented cylinders on the UK market, OSO included, deal with it through an external expansion vessel on the cold feed: a separate component with its own diaphragm, bolted or plumbed alongside the cylinder rather than built into it.

That arrangement is the boring one and we prefer boring. When the diaphragm eventually fails, which it will, an engineer fits a replacement vessel from stock and leaves. Nothing about that repair depends on the cylinder manufacturer still supporting your model, still existing, or still making a proprietary part. For an appliance that has to survive changes of ownership, changes of engineer and twenty years of London, that replaceability is worth more than a tidier looking installation.

Check the specific model quoted, because manufacturers vary the arrangement across ranges and years, and some pre plumbed units house the vessel inside the cabinet where you would not see it. The question for the person paying is simple: if the expansion vessel fails in eight years, is the fix a commodity part or a special order.

External vessel: replaceable from stock, obvious when it fails, engineer friendly.
Cabinet units: the vessel may be inside the casing, so ask where it lives.
A dripping tundish means a safety valve is passing and needs investigating properly.

Can you get parts for an OSO cylinder in London

Yes, but with more planning than for the most common British brands. OSO has UK distribution and technical support and the wearing parts are obtainable, but a London merchant is far more likely to have a Telford or Gledhill service kit sitting on the shelf than an OSO specific component. In practice that means an OSO repair is sometimes a two visit job where the same fault on a mainstream cylinder would be one.

How much that matters depends on which part has failed. The genuinely universal items, an expansion vessel on the cold feed or a generic inlet component, are usually available the same day from anywhere. Anything model specific, and particularly anything on a pre plumbed cabinet unit where the layout is the manufacturer's own, needs the model reference and the serial number and may need ordering in. This is the practical trade off with any imported cylinder and it is a fair thing to weigh up, not a defect.

Before you call anyone about an OSO, photograph the data plate. The model reference and serial number decide whether the right part arrives on the first visit.

What does London hard water do to an OSO cylinder

Hard water attacks the heat transfer surfaces, not the stainless vessel, so an OSO suffers the same scaling as anything else. Almost the whole London supply area is hard, and calcium deposits build on the indirect coil and on the immersion element every time the water is heated. The scale insulates the surface, the household notices slower reheating, and the element eventually runs hot beneath its own crust and fails.

This is the point where a cylinder designed and proven in Norway meets a condition it was not designed around, because Norwegian water is soft. That is not a design fault and it is not a criticism of the product. It is a reason to specify sensibly for where you actually live. We would always fit an Incoloy or titanium sheathed immersion element rather than the cheapest copper one, and we would resist storing water hotter than the household genuinely needs, because temperature is the single biggest accelerator of scale formation.

Scale lands on the coil and the immersion element, not on the vessel walls.
Incoloy or titanium sheathed elements tolerate our water far better than copper.
Storing hotter than the household needs speeds up scaling and buys you nothing.
An annual service catches a worn safety valve before it becomes a wet ceiling.

Which OSO model suits which kind of London property

Match the cylinder to the heat source and the pattern of demand, then to the space. An indirect unvented model paired with a system boiler is the normal answer in a house or a converted flat with gas. A direct electric model makes sense where there is no gas appliance at all, which covers a lot of purpose built blocks and newer flats. If a heat pump is in the plan now or in five years, you want a cylinder with a coil designed for low flow temperatures, and that decision has to be made at purchase because you cannot add coil surface area later.

Space decides more than owners expect. Victorian conversions typically put the cylinder in a first floor cupboard with a door narrower than the cylinder and a boxed in stack behind it. Before any model is ordered, confirm the unit will get up the stairs and through the door, that the discharge pipework from the safety valves can be routed to a safe termination, and that the immersion element can withdraw fully without dismantling joinery.

New builds bring a different problem. Liquid screed over underfloor heating means the heating side wants a much lower flow temperature than the hot water side, so the design has to separate them rather than treating the whole system as one circuit. Purpose built blocks running a thermal store are a different animal again. If your flat has no boiler and no gas meter but does have a large insulated unit fed by a communal supply, do not let anybody quote you an unvented cylinder before establishing which appliance you actually have.

Gas system boiler, one bathroom conversion: indirect unvented, mains flow measured first.
Two bathrooms with simultaneous demand: more volume and a higher output coil.
No gas in the flat: a direct electric model, sized around the tariff and the demand.
Heat pump now or later: a low temperature coil specified at purchase, never retrofitted.

What actually goes wrong with an OSO, and is it repairable

The same components fail on an OSO as on any other unvented cylinder, and most of them are repairable. The wearing parts are the pressure reducing valve, the expansion relief valve, the temperature and pressure relief valve, the immersion element and its thermal cut out, the expansion vessel diaphragm, and on indirect systems the motorised valve on the heating side. None of those is the cylinder itself, and none of them justifies replacing a sound vessel.

The symptoms owners bring us are usually water at the tundish, lukewarm water, slow reheating or noise during heating. Water at the tundish means a safety valve is discharging, and the correct response is to find out why rather than to cap it. Lukewarm water, slow recovery and noise in London are very often scale on the coil or a tired element rather than anything structural. In all of those cases a repair is the right outcome.

The exception, as with every stainless cylinder, is the vessel itself. If the steel is weeping, the cylinder is finished, and at that point you are choosing your next twenty year appliance rather than fixing this one. What should never happen is a functioning cylinder being condemned because a small wearing part is awkward to source. That is a supply chain inconvenience, not a failure of the cylinder.

Discharging safety valves are protecting you. Anyone who suggests plugging or capping one instead of finding the cause should not be working on your cylinder.

What to check before you buy an OSO cylinder

Ask the questions that decide the next two decades, not the ones printed on the brochure. The specification conversation that matters covers the heat source, the measured mains supply, the physical access, the immersion arrangement and the parts position. If an installer cannot answer those without checking, that is fine. If they do not think the questions are relevant, that tells you something.

Measuring the incoming mains flow rate and pressure is the one nobody should skip. An unvented cylinder can only deliver what the supply can feed it, and plenty of London conversions and older blocks cannot deliver what the owner assumes. A beautifully made cylinder on an inadequate supply produces a disappointed household and an argument about whose fault it is.

Finally, get the warranty terms in writing and read what they require of you. Manufacturer warranties on unvented cylinders are routinely conditional on an annual service being carried out and documented, and the condition is not unreasonable. We are saying this so you keep your cover intact, not so you can discount it. Book the service, keep the paperwork, and file the manual somewhere you will find it.

What is the measured mains flow rate and static pressure at this property.
Is the immersion a standard screw in element on this model, and can it withdraw fully.
Are the inlet controls individual parts or one assembly, and is a service kit available.
Where does the expansion vessel sit, and is it a commodity replacement.
Where will the discharge pipework from the safety valves terminate safely.
What does the warranty require of the owner, in writing.

Is an OSO cylinder worth it in a London home

If you are staying put, if the cylinder is going somewhere awkward or above somebody else's ceiling, and if the budget is not the deciding factor, an OSO is an easy cylinder to recommend. The vessel is as well made as anything on the market, the ranges cover gas, electric and low temperature heat sources properly, and buying the best built unit for a position you never want to revisit is sound reasoning rather than indulgence.

If the budget is tight, or the property is being sold soon, or same day parts availability matters more than fabrication quality, a well specified British stainless cylinder from a brand your merchant stocks will serve you perfectly well. That is a different set of priorities, not a downgrade.

What should not decide it is the badge alone. Our twenty engineers see OSO, Telford, Gledhill, Megaflo and a long tail of older cylinders across London every week, and the pattern is consistent: the survey, the specification and the standard of the installation predict far more than the name on the front. Measure the mains, size the cylinder around the demand and the heat source, confirm the unit will physically fit and stay serviceable, and specify an element that survives our water. Do those four things and most cylinders on the market will make your household happy. Skip them and the most expensive cylinder in the catalogue will not.

Questions we get asked

Are OSO cylinders any good: Yes. OSO builds in stainless steel in Norway and the standard of the fabrication and welding is at the top end of what we see on London jobs. The vessel quality is genuinely a step above much of the market. The trade offs are price and parts availability, since a London merchant is more likely to stock service parts for the common British brands than for an imported cylinder.
Where are OSO cylinders made: OSO Hotwater manufactures in Norway and sells into the UK through merchants and installers. That matters for two practical reasons. The build standard reflects Norwegian manufacturing, which is a good thing, and the cylinders were designed around soft Norwegian water rather than the hard supply that covers almost all of London, so scale protection and element choice deserve extra thought here.
Is an OSO better than a Megaflo: They are both well built stainless cylinders and neither owner is badly served. OSO generally wins on fabrication quality and costs more. Megaflo wins on name recognition and on how easily a London engineer finds parts and someone familiar with the model. Judge them on how each handles expansion and the immersion arrangement on the specific model quoted, since that decides how easy repairs are later.
How long does an OSO cylinder last: A stainless cylinder that is serviced annually, fed through correctly set inlet controls and not run hotter than the household needs should give many years, and the vessel typically outlives several sets of valves and immersion elements. What shortens that in London is hard water scaling the coil and element, excessive storage temperature, and skipped services that let a worn safety valve pass unnoticed for years.
Can an OSO cylinder be repaired or does it need replacing: Most faults are repairs. Relief valves, pressure reducing valves, immersion elements, thermal cut outs and expansion vessel diaphragms are all wearing parts that get changed without touching the vessel. The one genuine replacement is a weeping stainless vessel, which cannot be patched on any brand. Get the fault diagnosed before accepting a replacement quote, because replacing a repairable cylinder is the most expensive mistake here.
Do OSO cylinders need an annual service: Yes, and manufacturer warranties on unvented cylinders are routinely conditional on it being done and documented. Beyond the paperwork, the service is what catches a tired safety valve, a failing expansion vessel or a scaled element before any of them becomes an emergency. In water as hard as London's, the annual visit does more for the life of the cylinder than the choice of brand does.
What immersion heater should go in an OSO in London: Specify an Incoloy or titanium sheathed element rather than the cheapest copper one, because London water scales elements heavily and a scaled element runs hot beneath its own crust and burns out. Check the model manual for the immersion arrangement before ordering, and confirm the element has physical room to withdraw fully, since a boxed in cupboard turns a simple change into a joinery job.
Can I get OSO parts quickly in London: Universal items like an external expansion vessel are usually same day from any merchant. Model specific components, particularly on pre plumbed cabinet units, may need ordering in, so an OSO repair is occasionally a two visit job where a mainstream brand would be one. Having the model reference and serial number from the data plate ready makes the difference between one visit and two.
Is an OSO suitable for a heat pump: Only if you specify a model with a coil designed for low flow temperatures. A heat pump delivers water far cooler than a gas boiler, so the coil needs much greater surface area to transfer the same energy, and no amount of control tuning fixes an undersized one. That decision has to be made at purchase, because coil surface area cannot be added to an installed cylinder.
Should I choose an OSO for a flat above other properties: It is a reasonable place to spend the extra. Where a future failure means water reaching somebody else's ceiling, and where access for any repeat job is difficult, paying for the better built vessel is straightforward arithmetic. Pair it with a correctly routed discharge, properly set inlet controls and a booked annual service, since installation quality still predicts more than the badge does.