A vented cylinder is an open hot water storage tank fed by gravity from a cold water header tank, with an open vent pipe instead of sealed safety valves. Delivery pressure comes purely from the height of the header tank — about 0.1 bar per metre of head — so flow is gentler than mains pressure, but the system physically cannot over-pressurise.
How does a vented cylinder work?
A vented cylinder stores hot water at atmospheric pressure plus the head of a cold water header tank mounted above it, usually in the loft. Cold water falls by gravity from the header tank into the base of the cylinder; heated water is pushed out to the taps by that same static head. An open vent pipe rises from the top of the cylinder and terminates over the header tank, guaranteeing the system always has a free path to atmosphere.
The pressure arithmetic is simple: every metre of height between the header tank's water level and an outlet contributes about 0.1 bar. A typical two-storey house gives an upstairs shower only 0.2–0.3 bar — which is why gravity systems so often need shower booster pumps — while ground-floor taps may see 0.5 bar or more.
Why doesn't a vented cylinder need safety valves?
Because the system is open, over-pressure is physically impossible in normal operation. When the stored water heats up and expands — roughly 2% of its volume between cold and 65 °C — the extra volume simply rises up the open vent pipe or pushes back into the header tank. If a thermostat fails and the water approaches boiling, the vent path relieves it to atmosphere rather than letting pressure build.
This is the vented cylinder's defining virtue: safety by geometry instead of by devices. There is no expansion vessel to lose its charge, no relief valve to weep, and no tundish to monitor. The trade-off is that everything depends on the vent pipe and header tank staying unobstructed, clean and frost-free.
What are the drawbacks of a vented cylinder?
The main drawback is low delivery pressure — an unvented cylinder supplies outlets at roughly ten times typical gravity head, which transforms shower performance and allows balanced thermostatic mixing. Vented systems also need loft space and structural support for the header tank, the tank must be protected against freezing and contamination, and long gravity pipe runs refill and deliver more slowly.
| Criterion | Vented cylinder | Unvented cylinder |
|---|---|---|
| Feed | Gravity from header tank | Direct from cold mains |
| Typical outlet pressure | 0.1 bar per metre of head | Commonly 3–3.5 bar |
| Safety principle | Open vent to atmosphere | Multi-layer valve safety chain |
| Loft equipment | Header tank + vent required | None |
| Annual servicing | Minimal | Required (valves, expansion) |
| Shower performance | Often needs a pump | Mains-pressure without pumps |
Where is a vented cylinder still a sensible choice?
A vented cylinder still makes sense where the incoming mains supply is too weak for sealed operation, where a like-for-like replacement avoids re-piping a whole house, and where minimal servicing matters more than shower pressure. Its simplicity — no annual safety-chain service, no dependence on mains quality — keeps it common in older housing stock and in regions with unreliable water pressure.
The same open-vented principle also underpins thermosiphon systems in warm climates, where a roof-mounted tank feeds the house by gravity and solar collectors below it drive circulation without any pump. An electric immersion heater is the usual backup in both cases. For a full decision framework, see vented vs unvented hot water cylinder.
Frequently asked questions
What is the difference between a vented and an unvented cylinder?
A vented cylinder is open to atmosphere via a vent pipe and fed by gravity from a header tank, so pressure is low but over-pressurisation is impossible. An unvented cylinder is sealed and fed from the mains, delivering far higher pressure but requiring a serviced safety-valve chain.
Can I improve shower pressure with a vented cylinder?
Yes, within limits. Raising the header tank increases head by about 0.1 bar per metre, and a shower booster pump can lift pressure substantially. For whole-house mains-pressure hot water, converting to an unvented cylinder is the durable fix.
Does a vented cylinder need servicing?
Very little compared with a sealed system. The essentials are keeping the header tank covered, insulated and frost-protected, confirming the vent pipe is unobstructed, and checking the immersion heater and any sacrificial anode per the manufacturer's schedule.
Can a vented cylinder work with solar thermal or a heat pump?
Yes. Twin-coil vented cylinders accept solar plus boiler input, and low-pressure storage suits thermosiphon solar systems especially well. Heat pumps prefer purpose-built cylinders with large coil surfaces, which exist in both vented and unvented formats.