Vented vs Unvented Cylinder: How Each One Fails

What is the difference between vented and unvented?

The difference is where the water comes from and what holds it in.

A vented cylinder, also called a gravity-fed or open-vented cylinder, is supplied by a cold water storage tank, usually in the loft. A vent pipe runs from the top of the cylinder back up over the tank. Heated water expands up that pipe, and the system can never build up more than the pressure from the height of water above it. Hot taps are only as strong as the height of that tank allows.

An unvented cylinder is sealed. It is fed directly from the mains, so there is no tank and no vent. Because the water expands with nowhere to go, the cylinder carries its own expansion vessel or internal air gap, an inlet group with a pressure reducing valve, and relief valves with a discharge pipe to a tundish. These are described in are unvented cylinders safe.

The names are about the vent, not the heating. Either type can be heated by a boiler coil, an immersion heater or both. A cylinder with a coil is called indirect, and one with only immersion elements is called direct.

How does each type fail?

Comparing them by brochure gives you pressure and flow. Comparing them by failure gives you a better idea of what ownership looks like.

A vented cylinder fails in simple, visible places. The float valve in the loft tank sticks and the tank overflows. The vent pipe or overflow drips. A copper coil develops a leak and the cylinder starts taking on water from the heating side. An immersion heater element or its thermostat gives out, and the water goes cold while the boiler keeps running. These are mostly mechanical faults with short repair routes, and several are described in the guide on vented cylinder overflow dripping.

An unvented cylinder moves the failure points into the safety chain. The expansion vessel loses its air, the inlet valves scale or drift, the relief valve seat gets scored, or the tundish crusts over. The symptoms are discharge, pressure swings and tripped cut-outs, rather than a leaking loft tank. They are covered in TPRV dripping and unvented cylinder tripped cut-out. Replacement parts are more numerous, and work on the safety devices is for someone with the skills for unvented cylinders, under the Part G guidance.

Both types share the failures that come with age: element and thermostat wear, limescale in hard water, and eventually corrosion of the cylinder shell.

Which gives better pressure and flow?

The unvented cylinder, as long as your mains supply is good. With a vented cylinder the pressure is the head of water between the tank and the tap, which in a typical loft arrangement is small. Many homes fit a pump to compensate, and a pump brings its own noise and wear.

An unvented cylinder delivers hot water at the pressure the cold supply has, reduced to the cylinder setting. Showers and taps run at similar hot and cold pressures, and there is no loft tank to find space for.

The caveat is the supply. The cylinder cannot produce pressure the mains does not have. A weak supply, a narrow or old supply pipe, or a shared supply serving several flats gives a disappointing unvented result. The real figure is the flow at the stopcock, not the pressure alone, and an engineer measures both before recommending a switch. If you already have an unvented cylinder with poor flow, the cause is usually a partly closed valve, a blocked strainer or a pressure reducing valve, as set out in low pressure from taps on an unvented cylinder.

How do I tell which one I have?

Three quick signs settle most cases without touching anything. The separate guide on telling vented from unvented goes further.

• Look in the loft for a cold water storage tank. If a loft tank feeds the cylinder, it is almost certainly vented.
• Look beside the cylinder for a brass valve group, a coloured expansion vessel and a short pipe running to a tundish. That group means unvented.
• Read the label on the cylinder. Unvented cylinders carry a data plate with a pressure rating, and often a warning not to remove the safety devices.

Does a heat pump or a Mixergy cylinder need an unvented one?

Heat pump cylinders are most often supplied as sealed, mains-fed units, partly because the large coil that suits a heat pump is designed around that arrangement. The cylinder manufacturer and the heat pump installer specify the model, so treat any generic statement here as a general pattern, not a rule for your house.

A Mixergy-type stratified cylinder is also a pressurised design in most models, but the brand makes variants, so the data plate and manual are the reference. The Mixergy cylinder guide covers how it heats and fails.

If you are planning a change of heat source, the cylinder question follows from it. The heat pump cylinder guide goes through what changes.

Should I swap from vented to unvented?

Only when the supply and the pipework justify it. The reasons that make sense are better pressure at showers, removing a noisy pump, freeing the loft tank space or avoiding a freeze risk in the loft. The reasons that do not make sense are an assumption that unvented is always better.

Before a conversion, an engineer checks the mains flow and pressure, the condition of old pipework that is about to see higher pressure, and whether the cylinder cupboard can take a discharge pipe route to a safe termination. If any answer is no, a new vented cylinder is a sound choice.

The repair-or-replace question differs too. A vented cylinder with a tired element is a straightforward part swap. An unvented one with a failing vessel and scaled valves is often a bundle of parts, and the decision is set out in repair or replace. An installation of the sealed type is covered at unvented cylinder fitting.

For a fault on either type, the first step is the same: find the fault, then decide.