Stainless Steel or Copper Cylinder: Which Material Should You Fit

Stainless steel or copper: which is better for a hot water cylinder

Neither is better in the abstract. Stainless steel is stronger, so it holds mains pressure in a thin wall, which keeps the vessel light and the price reasonable. Copper conducts heat far better and is easy to work, which is why it dominated a century of low pressure vented cylinders and is still the sensible material for one.

The useful comparison is narrower. At mains pressure, stainless steel wins on every measure that matters to a householder: weight, wall thickness, vessel cost and spares availability years later. At gravity pressure, copper wins on the cost of the vessel and on scrap value when it comes out.

One technical point gets quoted more than it deserves. Copper conducts heat several times better than stainless steel, but the heat enters a cylinder through the coil, and the coil is copper in both types of vessel. Recovery time is set by coil surface area, boiler output and flow rate, so compare the stated recovery figure for a model rather than the metal of the shell. What the shell material does decide is pressure rating, and that is not negotiable: a vented copper cylinder is built for the head of water standing above it and can never be connected to the cold main.

There is a third material you will meet in London, particularly in purpose built blocks and in heat pump installations: glass lined or enamelled mild steel. It behaves differently from both of these and depends on a sacrificial anode to survive, which is covered further down because it changes the maintenance picture completely.

Why unvented cylinders are made from stainless steel

An unvented cylinder is a pressure vessel. It sits at whatever the incoming main gives it, and it has to hold that continuously, hot, for decades. Duplex stainless steels have roughly twice the yield strength of the austenitic grades most people picture when they hear stainless, and many times the strength of annealed copper at temperature. That strength is the whole point: it lets a maker hold mains pressure with a thin shell rather than a heavy one.

Copper can be made to hold mains pressure, and copper unvented cylinders do exist. The problem is that it has to be thick to do it. Thickness means metal, metal means weight and cost, and neither is attractive when the vessel has to go up a Victorian staircase and sit in a first floor airing cupboard. That is the practical reason the market moved, not a verdict on copper as a metal.

Fabrication matters too. Duplex shells are welded, and the welds are the part that must not fail, so the seams are machine made and tested in the factory. Nothing about an unvented vessel is a site repair in any material: if the shell itself is weeping, the conversation is about a hot water cylinder replacement rather than a fix.

Work on the sealed side of an unvented cylinder is notifiable and must be carried out by an engineer qualified for unvented systems. That applies equally whether the vessel is stainless steel or copper.

Where a copper cylinder is still the right choice

Copper is still correct wherever the cylinder is fed by gravity from a cold water storage tank, which covers an enormous number of London properties: conversions with a tank in the loft, older houses that were never converted, and flats where the cold store sits above the bathroom ceiling. The cylinder sees only the head of water above it, so there is no case for a pressure rated vessel.

Swapping like for like in that situation is also the least disruptive job available. The pipework, the tank, the vent pipe and the controls all stay, the new vessel goes in the same footprint, and nobody is cutting a new discharge route through a wall. Against that, converting to unvented brings mains pressure to every outlet, removes the loft tank, and frees the space, which is usually why people do it. It also brings an annual safety check the old system never needed.

The point worth making to anyone being pushed one way or the other is that a vented copper cylinder is not obsolete and is not unsafe. It is a different design with a different failure mode, and if the incoming main in your street is not strong enough to run an unvented system properly, it is also the better system. Our comparison of an unvented versus a vented cylinder works through that decision in full.

How each metal corrodes in London hard water

The two metals fail in completely different ways, and knowing which is which tells you what to watch for. Copper corrodes generally and slowly across its surface, and in hard water it usually does not get the chance, because the same calcium carbonate that furs up your kettle lays down a thin protective film on the inside of the vessel. That film is the reason copper cylinders in chalk water areas have such a long working record. Where copper does fail it is pinholing: a localised pit driven through the wall by water chemistry or by erosion from turbulence at a badly made connection, showing as a fine spray or a damp patch rather than a burst.

Stainless steel does not corrode generally at all. It carries a passive chromium oxide layer that repairs itself in the presence of oxygen, which is why a stainless vessel still looks new inside after many years. What can breach that layer is chloride. Chloride ions attack the passive film at a single point and drive a narrow pit downwards, and the process accelerates with temperature and where chloride can concentrate in a crevice or under a deposit. It is the one chemical risk the material has, and it is the reason duplex and higher alloy grades are specified for stored hot water rather than plain 304.

London supply water carries chloride at levels ordinary for a treated public supply, and that is the background these vessels are designed against. What raises the risk is what happens in the cupboard rather than at the works: water stored far hotter than it needs to be, or a vessel left standing full and stagnant through a long void period.

• Copper: slow general corrosion, protected in hard water by a carbonate film, fails as a pinhole.
• Stainless steel: no general corrosion, but vulnerable to chloride pitting at a single point.
• Both: heavy limescale on the coil and the immersion element, which is a performance problem rather than a corrosion one.
• Enamelled steel: protected by an anode that is consumed, so it fails if the anode is never replaced.

Which lasts longer, a stainless steel or a copper cylinder

Neither material is usually what ends the life of the cylinder. Most are removed because the immersion element has gone for the third time, because the coil is so furred that reheat times have become unacceptable, or because the safety components are seized and no longer available as spares. The shell is often the healthiest part of the assembly when it goes in the skip, and how long a hot water cylinder lasts is set far more by water treatment, storage temperature and servicing than by the metal.

Where the metal does decide it, the pattern is this. A copper vented cylinder in hard London water that has never been run excessively hot has a long and well documented working life, and when it fails it tends to fail gently, as a weep rather than a rupture. A stainless unvented vessel has a shorter track record simply because the design is newer here, and when it fails from chloride pitting it also fails as a weep, usually appearing at the base of the cupboard or at the ceiling below.

The single change that helps both is storage temperature. Sixty degrees is the figure to hold: hot enough to control legionella in stored water, cool enough to avoid making limescale and chloride pitting faster than they need to be. A cylinder wound to maximum because somebody once wanted a hotter bath is being aged deliberately, and the fix for a bath that is too cool is almost never the thermostat.

Weight, access and the first floor cupboard problem

Empty, a stainless unvented vessel is markedly lighter than a copper one of the same capacity and pressure rating, because the wall does not have to be as thick. That difference is what two engineers feel carrying it up a narrow Victorian staircase, and it is a genuine reason the material won.

Full, the difference almost disappears, because water is the weight. A cylinder holding two hundred litres holds two hundred kilogrammes of water, and no realistic difference in shell thickness competes with that. So the material is a handling question during the installation and a structural question only in the sense that any cylinder in a first floor cupboard needs a floor that was built to carry it. In converted flats where a cupboard has been formed in a room that was never intended to hold one, that is worth looking at properly before anything is ordered.

Access cuts the other way in older buildings. Many London cylinder cupboards were built around the cylinder, so the doorway is smaller than the vessel that has to come out and the old one is cut down in place. That is one of the few moments where copper is genuinely easier, and it is why the scrap value of an old copper cylinder is a real line in the job.

Do stainless steel cylinders need a sacrificial anode

No. A sacrificial anode is a rod of magnesium or aluminium that corrodes in preference to the vessel, and it exists because glass lined mild steel needs protecting wherever the enamel is imperfect. Stainless steel protects itself with its own passive film and needs nothing sacrificed on its behalf. Copper does not need one either.

This matters because the three materials have completely different maintenance stories, and people transfer advice from one to another. If your cylinder is enamelled steel, the anode is consumable and inspecting it is part of looking after the vessel, so a service that never mentions it is not a complete service. If it is stainless or copper, there is no anode to check and anyone offering to sell you one for it is selling something you do not have a use for.

What every unvented cylinder does need, in any material, is the annual check of the safety components: the expansion arrangement, the temperature and pressure relief valve, the pressure reducing valve and the discharge route. Those are the parts London water actually attacks, because they seal by pressing a soft seat against a machined face and limescale ruins that. An unvented cylinder service is the appointment where those get proved.

A dripping tundish is a safety valve telling you it has operated. The metal the cylinder is made from has nothing to do with it and the discharge pipe must never be capped or concealed.

What drives the cost of each over twenty years

The purchase price of the vessel is the smallest part of what a cylinder costs you, and it is the part people compare hardest. What actually moves the twenty year figure is the labour around it: whether the cupboard has to be opened up, whether a discharge route has to be cut to outside, whether the incoming main needs upgrading to feed an unvented system properly, and how many immersion elements the water chews through in the meantime.

On the vessel itself, copper tracks the copper commodity market and has swung a long way over the years, while stainless prices are steadier. A copper cylinder also has scrap value on the way out, which offsets part of the removal. A stainless unvented vessel brings the cost of an annual safety check that a vented system does not have, and the cost of replacing safety components as they age. None of that is hidden, and all of it should appear in a written quotation before you agree to anything. Our guidance on what a plumbing quote should include sets out what to insist on seeing.

How to tell which cylinder you already have

Look at the data plate first. Every cylinder carries one, usually on the front of the jacket near the top, and it states the capacity, the maximum working pressure and the model. A vessel rated for mains pressure is unvented and will be stainless steel in almost every case. A vessel rated for a head of a few metres is a vented cylinder and is almost certainly copper.

If the plate is unreadable, the pipework tells you. A vented copper cylinder has an open vent pipe rising from the top and turning over above the cold tank in the loft, and there is a cold tank to find. An unvented cylinder has no vent pipe, but it does have a small tundish in the pipework near it, an expansion vessel either built in or hung beside it, and a discharge pipe running to outside. If there is a tundish, it is unvented.

• Tundish and a discharge pipe to outside: unvented, and the shell will be stainless steel.
• Open vent pipe and a cold tank in the loft: vented, and the shell will be copper.
• No coil and only an electric element: a direct cylinder, in either material.
• A visible anode boss on the top with a hex plug: an enamelled steel vessel, not stainless.

What we would fit, and when we would choose differently

For a mains pressure installation we fit a duplex stainless vessel from an established maker, because the strength to weight case is settled and because spares for the safety components need to still be obtainable in ten years. We have no manufacturer affiliation and no reason to steer anyone to a particular badge, so the choice on a given job comes down to what fits the space, what the incoming main can feed, and whose parts we know we can still get hold of. The brand on the jacket matters considerably less than the size, the coil rating and the servicing.

For a like for like vented replacement we fit copper, because it is the right material for the duty and because converting a property to unvented should be a decision the customer makes deliberately, not something that arrives as a surprise on a quotation. There are properties where we would talk someone out of converting: a weak incoming main, no viable route for a discharge pipe, or a flat where the cupboard simply cannot take the additional components.

Where a cylinder is already in the second half of its life and the question is whether to keep spending on it, the material rarely decides it. What decides it is the state of the coil, whether the safety components are still available, and how many separate faults have been paid for already. That is the same calculation as whether it is worth repairing an old cylinder, and it is worth working through before anyone orders a vessel in any metal.