Vented or unvented cylinder, and how each one fails

Which one you have, settled in ten seconds

Go into the loft. If there is a plastic or galvanised tank up there with a float valve in it, you have a vented system. The hot cylinder in the airing cupboard is fed by gravity from that tank, and the pressure at your taps comes from how far the tank sits above them — roughly 0.1 bar for every metre of height.

If the loft is empty and the cylinder in the cupboard is a fat white or copper drum with a bundle of valves on the cold feed and a small funnel-shaped fitting piped away from it, you have an unvented system. It is fed straight off the mains, sealed, and holds stored water at mains pressure. Megaflo is the best-known name for this type; it is a Heatrae Sadia product, and the same principles apply to every unvented cylinder whoever made it.

That one structural difference — open to the air at the top, or sealed under pressure — decides everything about how each of them fails.

How a vented system fails

The float valve that never quite shuts

The commonest vented failure is not the cylinder at all. It is the float valve in the loft tank wearing out, so the tank keeps filling past its working level. The tank is designed for exactly this: a warning pipe (the overflow) takes the excess out through the eaves and drips onto your path, which is the whole point of putting it where you can see it.

Two things go wrong with that design. The overflow can be blocked with silt, or it can have been badly run and discharge into the loft insulation instead of outside. And people ignore the drip. A pipe that drips outside for a fortnight is a nuisance; the same valve failing again in January when the overflow is frozen puts the whole flow into the loft. We wrote about the outside drip separately, in is a leaking overflow pipe dangerous.

The tank and the cylinder

Old galvanised tanks corrode at the waterline. Plastic tanks go brittle where the sun hits them through a roof light and split along a fold. A vented copper cylinder usually fails at a soldered boss or a seam, and because there is almost no pressure behind it, it weeps rather than sprays.

This is why vented failures are so late to show. A weep of a few litres a day goes into the loft floor, through the insulation, and spreads sideways across the ceiling plasterboard before it comes through anywhere. By the time you see a brown ring, the joists above it have usually been wet for weeks. The ceiling under a loft tank is the single most common place we are called to on a vented system — the symptom, not the cause, which is what a water stain on a ceiling almost always is.

How an unvented system fails

The tundish is doing its job, loudly

An unvented cylinder cannot vent expansion into a loft tank, because there isn't one. Heated water expands, and that expansion has to go somewhere. It goes into an expansion vessel, or into a deliberate air bubble held at the top of the cylinder. If that space disappears, pressure rises on every heating cycle until a relief valve lifts and dumps water.

The dumped water goes through the tundish — the small open funnel in the pipework, which under the Building Regulations has to sit close to the safety valve and somewhere you can actually see it — then down a discharge pipe and out to a safe place outside. A tundish with water running through it means a valve has lifted. A discharge pipe dripping outside, warm, whenever the hot water heats, means the same thing.

That is not a leak in the usual sense. Nothing has burst. The system is protecting itself, and the fault is almost always a lost expansion vessel charge or a tired relief valve. The same physics behind a heating system's vessel is covered in expansion vessel failure or a leak. This one is a G3-registered cylinder engineer's job, not a leak detection visit, and we will tell you so on the phone rather than attend.

When it is a real unvented leak

When an unvented cylinder or its pipework does fail properly, it fails at mains pressure and it fails fast. A pinhole on a compression joint in the cupboard sprays rather than weeps. Because the stored water is replaced by the mains as fast as it leaves, there is no tank volume to run out — it keeps going until someone closes the stopcock. Unvented cylinders also sit in cupboards on the ground or first floor far more often than in lofts, so the water lands on a floor you walk on and is found in hours, not weeks.

Failure, side by side

System	Failure mode	First symptom	What it damages
Vented	Float valve passing, tank overfilling	Steady drip from a pipe under the eaves	Nothing, if the overflow runs clear. The loft, if it blocks or freezes.
Vented	Tank split or corroded, or cylinder weeping	Brown ring on the ceiling below the loft hatch	Joists, insulation, ceiling plasterboard, and the room below it
Vented	Immersion boss or cylinder seam	Damp airing-cupboard floor, musty smell	Floorboards, the ceiling under the cupboard, stored linen
Unvented	Expansion vessel lost its charge	Water through the tundish after every reheat	Nothing directly. Wastes hot water and eventually the relief valve.
Unvented	Relief valve seat worn or fouled	Constant warm drip from the discharge pipe outside	Nothing inside. The wall and path below the discharge.
Unvented	Joint, coil or cylinder failure at mains pressure	Audible spray, hot water on the cupboard floor within minutes	Floor, ceiling below, anything in the room — quickly
Either	Discharge or overflow pipe blocked, frozen or wrongly run	Often none until water appears indoors	Whatever is under the point the water actually escapes at

The one that does more damage is not the one you would guess

People assume the mains-pressure cylinder is the dangerous one. In damage terms it usually is not. Both systems are built with a designed escape route — a warning pipe on one, a tundish and discharge pipe on the other — and both route water outside where you can see it. The unvented one fails loudly and gets fixed the same day.

The vented one fails quietly, in a loft nobody goes into, with an overflow drip that people learn to ignore. The bills we see are bigger on vented systems, not because more water escapes, but because it escapes for longer before anyone notices. The worst outcomes on either type come from the same thing: the escape route being the part that failed.

What is worth checking, on each

• Vented: look in the loft twice a year. The tank should be dry outside, the water level well below the overflow, the insulation under and around it dry, and the overflow pipe clear when you flex the float down.
• Unvented: look at the tundish. Dry and clear is correct. Any water in it, any lime scaling in it, or a discharge pipe outside that is ever wet, means a valve is lifting and the cylinder needs servicing.
• Both: read the water meter with everything off overnight. A vented weep that has not reached a ceiling yet still moves the meter.

Where we come in

We are called when the water has already appeared and nobody can say where it came from — a stain under a loft, a wet cupboard floor, a ceiling below a cylinder. The cylinder is one suspect; the hot feed, the vent pipe, the secondary return and whatever else runs through that cupboard are the others. We isolate, pressure-test to confirm the loss is real, then trace it with thermal imaging and, on a drained run, tracer gas, before anything is opened.

The visit is charged at our hourly rate, agreed before we attend, with no call-out fee, and if we do not find the leak you do not pay for the time spent looking. You can see the full terms on our pricing page. Detection visits include a written report prepared to support a trace-and-access claim; your insurer decides what the policy covers, but the report is what they need in order to decide. If a floor, ceiling or tiled area has to come up to reach the fault, our own handymen put it back.

What we will not do is replace a cylinder or commission an unvented system. That is notifiable work under Building Regulations G3 and belongs to a registered installer. If the answer to your problem is a new cylinder, we will say so and stop there.