Vented cylinders

How it works, and why that matters

A cold water storage cistern in the loft feeds the cylinder. The pressure at any tap is created purely by the vertical distance between the water level in that tank and the tap itself – roughly a tenth of a bar for every metre of height. A ground-floor tap with the tank five metres above it gets a reasonable push. A shower head on the first floor, a metre and a half below the tank, gets very little.

The cylinder has an open vent pipe rising over the tank, which is what makes it ‘vented’: if anything overheats, the system has a permanent, unblockable route to atmosphere. That is why a vented system needs none of the relief valves, expansion vessels and discharge pipework that an unvented one does, and why it is not notifiable work.

The genuine advantages

• Simplicity. Fewer components, fewer things to fail, and a much cheaper annual service.
• No mains dependency for storage. If the water company shuts the main off, you still have whatever is in the tank and cylinder. On an unvented system you have nothing.
• Kind to marginal mains supplies. A property with poor incoming flow can still fill a tank slowly overnight and deliver good volume in the morning. An unvented cylinder cannot.
• Cheap parts. Ball valves, immersion heaters and cylinder thermostats are inexpensive and universally available.
• No notification requirement, so straightforward replacements are simpler work.

Those are real advantages and they are worth weighing properly rather than being talked out of. Plenty of Norfolk properties – particularly rural ones on long private supplies – are genuinely better served by a vented system.

The genuine drawbacks

• Poor pressure upstairs. The fundamental limitation. A shower directly below the tank will never be good without a pump.
• The loft tank itself. It freezes, it needs a lid and insulation, the ball valve fails, and it is an open vessel of stored water that needs to stay clean.
• Space. Both the tank and the cylinder take up room that people increasingly want back.
• Slower to modernise. Many modern taps, showers and appliances specify a minimum pressure a gravity system cannot reach.
• Two tanks in many houses – one for the cylinder, a smaller one for the heating feed and expansion.

The pump question

Before converting to unvented, it is worth knowing that a correctly specified shower or whole-house pump solves the pressure problem far more cheaply, and it is often the right answer.

A pump works because a gravity system has plenty of *flow* and not enough *pressure* – which is exactly the situation a pump is designed for. The important details are that it must be fed correctly (a dedicated draw-off from the cylinder, and adequate tank capacity), it must be sited where the noise is acceptable, and it must be the right type – a negative-head pump where the pump sits above the tank water level, a positive-head one where it does not.

What we do with vented systems

• Cylinder replacement like for like, with much better insulation than an older cylinder had.
• Immersion heater replacement – in hard water these scale heavily and fail, and it is a routine job.
• Cold tank replacement, typically swapping an old galvanised or unlidded plastic tank for a modern lidded one with proper insulation.
• Ball valve and overflow problems, which are the single most common vented-system fault.
• Pump installation where the pressure genuinely is the problem.
• Conversion to unvented, where the mains supports it and the household wants it – measured first, as always.
• Thermostat and control upgrades, which frequently deliver more improvement than anything else on an old gravity system.