Unvented vs Vented Cylinder: How They Differ, and Which One You Have

How to tell which one you have, without tools

Look at the cylinder, then look in the loft. Between the two you will know in a minute and you do not need anyone to tell you.

An unvented cylinder is almost always stainless steel under a sprayed or moulded jacket, usually white or grey, with a cluster of brass valves bolted to the cold inlet at the bottom and a sealed vessel about the size of a football sitting near it. The giveaway is a small clear plastic funnel set into a vertical pipe close to the cylinder. That is a tundish, and from it a pipe runs away and terminates outside, normally low on an external wall. There will be no tank in the loft feeding it.

A vented cylinder is copper, either sprayed with yellow foam or wearing a quilted red jacket. A large cold feed enters at the bottom, and a pipe rises from the crown into the loft and bends over to point down into the cold water storage cistern. That is the open vent, and it is why nothing on a vented cylinder is ever under pressure. On older gravity heating there is a second, much smaller tank beside that cistern.

• Tundish, sealed vessel, brass valves at the base, no loft tank: unvented.
• Copper cylinder, vent pipe curling over a loft tank, large cold feed at the bottom: vented.
• A data badge naming a model and a capacity in litres: unvented, almost without exception.

How a vented cylinder works

The loft cistern fills from the mains through a ball valve and feeds the cylinder by gravity. Pressure at the tap is set purely by the vertical drop between the water level in that cistern and the outlet you open. Every metre of height gives roughly a tenth of a bar, so a cistern three metres above a first floor bathroom delivers around 0.3 bar to the shower.

The cylinder is a copper vessel that is never sealed. The vent pipe from the top is open to the atmosphere over the cistern, so when the stored water heats and expands it simply pushes back up that pipe. Nothing can build pressure inside, which is why a vented cylinder needs no safety valve, no expansion vessel and no discharge arrangement. It is an old design and close to unkillable.

Heat arrives one of two ways, usually both: a coil inside carrying hot water from the boiler, and an immersion heater, an electric element screwed into the side or top.

How an unvented cylinder works

An unvented cylinder is a sealed stainless steel vessel connected straight to the incoming cold main. No cistern, no vent pipe. Whatever pressure the street main gives your kitchen tap is broadly what the hot side gives too.

Sealing it creates a problem the vented design never had. Water expands by about four per cent going from cold to storage temperature, and in a sealed vessel that expansion has nowhere to go. So every unvented cylinder has somewhere to put it: an external expansion vessel with a rubber diaphragm and air behind it, or an air bubble trapped inside the top of the cylinder on the designs built that way.

On the inlet sits a control set where each part does one job. A pressure reducing valve brings the main down to the working pressure the cylinder is built for. A check valve stops heated water pushing back into the cold supply. An expansion relief valve opens if pressure climbs past its setting, for instance because the expansion arrangement has lost its air.

Then comes the safety chain, which exists because a sealed vessel of stored hot water holds real stored energy. The cylinder thermostat controls temperature in normal running. Behind it a thermal cut-out kills the heat source if that thermostat fails. Behind that, a temperature and pressure relief valve screwed into the cylinder body dumps water if the contents ever approach boiling. Anything those valves release passes through the tundish, an open air gap you can see a discharge happening in, and out to a visible point outside. Installing or altering one of these is notifiable work for someone qualified for it, and that is as much as needs saying here.

An unvented cylinder installation is notifiable work for that reason, because the safety chain has to be built and commissioned as a whole rather than assembled in parts by whoever is passing.

Servicing and repair both require a G3 qualified plumber, which is a real cost of ownership rather than a formality on a certificate.

Water running through the tundish is the cylinder telling you a safety valve has opened. It is doing its job, but it is not a drip to live with. A tundish that drips has a cause behind it every time, and the cause is what wants finding rather than capping the pipe or catching it in a bucket.

Which gives a better shower, unvented or vented?

The honest headline: an unvented cylinder gives you a better shower and better simultaneous use, right up until your incoming main cannot keep up, at which point it gives you neither.

On a vented system the hot side is gravity fed while the cold is mains fed at the kitchen. That imbalance is why old mixer taps and showers on vented systems wander: the cold pushes harder than the hot. The usual answers are a shower pump, or balanced supplies so both sides come off the cistern.

With an unvented cylinder both sides come off the same main at the same pressure, so mixers behave, thermostatic showers work as designed and a bath fills quickly. Two showers at once stay usable as long as the main can feed them. That clause carries the weight, because no cylinder can create flow the street main does not have. Manufacturers specify these units around a supply of roughly twenty litres a minute at a sensible pressure, and below that an unvented cylinder can feel ordinary through no fault of its own.

How to test your mains flow rate in two minutes

This one you can genuinely do yourself in two minutes, and it settles the question more often than anything a plumber will tell you on the phone. Do it before you pay anyone to quote.

Take a bucket you know the volume of. Go to the kitchen cold tap, which is normally fed directly from the main, make sure nothing else is running, open it fully and time how long it takes to fill, then convert to litres a minute.

Twenty litres a minute or better and an unvented cylinder will perform the way the brochure suggests. Mid teens and it works but feels ordinary with two outlets open. Around ten and you should be asking why the main is poor before spending money on a cylinder. In older London housing the cause is usually a long run of old lead or fifteen millimetre pipe from the stopcock in the street, and improving that supply changes more than any appliance will.

• Run the test at the kitchen cold tap with everything else off.
• Repeat it in the evening if you are in a flat, because pressure varies with what the neighbours are doing.
• A poor result is a supply problem to solve on its own terms, not a reason to write off a cylinder type.

Space: what each type costs you in loft and cupboard

A vented system costs you the loft. The cistern has to be supported, insulated against freezing, lidded against contamination, and reachable at the ball valve. That rules it out for a loft conversion and makes it a nuisance anywhere the loft is the only storage there is. Cisterns in cold lofts freeze, ball valves stick, and an overflow running down an outside wall for a fortnight is a familiar story.

An unvented cylinder costs you nothing upstairs and something downstairs. It needs a cupboard that takes the cylinder, the valve set and the vessel, and a discharge pipe from the tundish running in the right material, at a continuous fall, to a place outside where near boiling water can land without scalding anyone. In a flat or a basement, finding that route is often the hardest part of the job.

That is why the two sort themselves geographically in London. Victorian conversions with a loft above the top flat keep vented systems for decades. Flats above shops and converted attics end up unvented or on a thermal store.

That is also why hot water cylinder sizes are not directly comparable between the two types, since a vented system needs the tank space in the loft as well as the cylinder space downstairs.

What goes wrong with each type of cylinder

A vented cylinder is low maintenance in the literal sense: no annual service, no pressurised parts, nothing that has to be checked to stay safe. What fails is predictable and cheap. An immersion element scales up and burns out, a thermostat drifts, a loft ball valve passes, sludge fouls the coil so reheats take forever, and in the end the copper weeps at a joint.

An unvented cylinder has more parts and they are safety parts, which is why manufacturers specify a yearly check. It is a short visit and what happens during an annual service is worth knowing before you book one, because the valves are only proven by being operated. The expansion arrangement is the one that quietly stops working: lose the air charge and every heat up pushes water out through the relief valve and down the tundish. The relief valves themselves can be held open by grit or by scale on the seat, giving a constant trickle rather than a dramatic discharge.

Scale is the London tax on both. Hard water bakes onto the hottest surfaces first, meaning the immersion element and the boiler coil, and every millimetre means longer reheats and a higher bill. On unvented cylinders it settles on valve seats too, which is why a relief valve quiet for six years suddenly starts passing. What hard water does to a cylinder over its life is worth understanding whichever type you end up with, because it shortens both. Running storage temperature sensibly rather than as hot as the stat allows slows it more than anything else you can do for nothing.

There is a practical consequence to that difference. An unvented cylinder repair has to be carried out by somebody holding the unvented qualification, because the fault is usually in the safety chain itself, whereas most vented faults are ordinary plumbing that any competent plumber can put right.

• Vented: immersion element, immersion thermostat, loft ball valve, fouled coil, eventual copper failure.
• Unvented: expansion arrangement, expansion relief valve, temperature and pressure relief valve, control set, immersion element.
• Both: scale in hard water, and a reheat time that lengthens every year as it builds.

Which is better for your home, vented or unvented?

One bathroom, a working loft cistern, a system that has never bothered you: keep it and repair what breaks. Replacing a working vented system to chase shower pressure is a large job to solve what a decent pump often solves.

Two or more bathrooms, or a household that wants to shower while the bath fills, and a main that passed the bucket test: unvented is the right answer and the reason the design exists. Once that is settled, the next question is capacity, and working out what size of cylinder the household needs matters more to how it performs than the type on the label does.

A weak incoming main: improve the main first. Fitting an unvented cylinder to a supply that cannot feed it buys an expensive cylinder and the same disappointing shower.

Most of how to choose a hot water cylinder comes down to the mains flow rate you measured earlier rather than to a preference between the two designs.

If you are converting rather than repairing, the job is a hot water cylinder replacement plus the pipework and the discharge route, not a straight swap of one vessel for another.

The third option: a thermal store

A thermal store inverts the logic. The vessel holds heated water open to the atmosphere, exactly like a vented cylinder, but your drinking water never mixes with it. The cold main passes through a heat exchanger inside the store and comes out hot at mains pressure on demand.

So you get mains pressure hot water without a sealed vessel of stored hot water and without the same safety arrangements. Units like this are common in London flats fed from a communal heating circuit, where the block supplies the heat. The Pulsacoil is the version most people meet, and how a Pulsacoil works is worth reading before anyone diagnoses one as an ordinary cylinder, because it is neither vented nor unvented and the controls behave nothing like either. They fail in their own way, mostly on controls, pumps and scaled heat exchangers.