A vented cylinder is fed by gravity from a loft tank and is simple, robust and cheap to maintain. An unvented cylinder is sealed and fed from the mains, giving mains pressure at every outlet, and requires safety devices, notification and annual servicing.
The fundamental difference
A vented cylinder has an open vent pipe rising from the top of the cylinder and terminating over the cold water storage cistern in the loft. That pipe is permanently open to atmosphere. If anything overheats, expanding water and steam have an unblockable route out.
An unvented cylinder has no such route. It is a sealed pressure vessel connected directly to the incoming mains. Water expands as it heats and has nowhere to go, so that expansion must be accommodated by an expansion vessel or an internal air gap. If controls fail, pressure and temperature can rise, so a layered set of safety devices exists to prevent it.
Every other difference – the pressure at your taps, the regulations, the servicing, the cost – follows from that single distinction.
The comparison
| Vented | Unvented | |
|---|---|---|
| Fed from | Cold tank in the loft, by gravity | Incoming mains directly |
| Pressure at outlets | Determined by tank height above the outlet – low upstairs | Mains pressure everywhere |
| Loft tank | Required | Not required |
| Notifiable work | No | Yes, under Building Regulations G3 |
| Competence required | General plumbing | Specific G3 competence |
| Safety devices | The open vent | Expansion vessel, PRV, expansion relief, high-limit stat, T and P relief valve |
| Discharge pipework | None required | D1 and D2, with a visible tundish |
| Annual service | Sensible | Required, and a warranty condition |
| Resilience to mains failure | Good – stored water in the tank continues | None – no mains, no water at all |
| Suits marginal mains supply | Yes – tank fills slowly, delivers well | No – limited by what the main supplies |
| Simultaneous outlets | Limited by gravity | Good, if the main supports it |
| Parts cost | Low | Higher |
| Space | Cylinder plus tank | Cylinder only |
Where each genuinely wins
Vented wins
- Marginal incoming mains. A property with poor flow can fill a tank slowly overnight and still deliver good volume in the morning. An unvented cylinder cannot do this – it can only deliver what the main supplies at that moment.
- Resilience. When the water company shuts the main off, you still have the tank and the cylinder. On an unvented system you have nothing at all.
- Simplicity and cost. Fewer components, cheaper parts, a simpler and cheaper annual service, no notification requirement.
- Rural properties on long private supplies, where both of the above apply.
- Where there is genuinely nowhere to route D2 discharge pipework correctly.
Unvented wins
- Pressure at upstairs outlets. The decisive argument for most households. A first-floor shower on a gravity system is fundamentally limited; on an unvented system it is not.
- Simultaneous use. Two showers at once, if the main supports it.
- Space. No loft tank, which frees the loft and removes a freezing risk and an open vessel of stored water.
- Modern fittings. Many contemporary taps and showers specify a minimum pressure a gravity system cannot reach.
- No loft pipework to freeze, which in this county is not a trivial benefit.
The measurement that decides it
Whether an unvented cylinder is viable is determined by the incoming main, and it is measured rather than assumed.
- Static pressure at the stopcock, with nothing running. Tells you the pressure available.
- Dynamic flow rate at the stopcock, measured with a flow cup or a bucket and a stopwatch, with the tap fully open. Tells you the volume available.
- The second number is the important one. An unvented cylinder can deliver mains pressure to every outlet, but only as much water as the main supplies. Good pressure with poor flow gives you a shower that feels forceful and runs out.
- Compare against the cylinder’s requirement. Every unvented cylinder specifies a minimum flow and pressure for satisfactory operation.
- Consider peak demand, not average. The relevant question is what the main delivers at seven in the morning when the street is also using it.
Fitting an unvented cylinder to a supply that cannot feed it produces mains pressure at every outlet and not much of it. The alternatives are upgrading the supply pipe from the boundary – real work, and sometimes the right answer – fitting an accumulator to store mains water under pressure for peak demand, or keeping the vented system and fitting a pump. We would rather tell you that at survey than after installation.
The unvented safety set, briefly
Worth understanding because it explains the annual service.
| Device | Normal job | Acts when |
|---|---|---|
| Cylinder thermostat | Switches the heat source off at temperature | Every cycle |
| Expansion vessel or internal air gap | Absorbs expansion as water heats | Every cycle |
| Pressure reducing valve | Limits incoming mains pressure | Continuously |
| Expansion relief valve | Releases if pressure rises too far | Only if expansion has failed |
| High-limit thermostat | Manually resettable cut-out on the heat source | Only on a control failure |
| Temperature and pressure relief valve | Last line – dumps water | Only if everything above has failed |
Each takes over when the one before it fails. A system where one has silently failed is running with a layer of protection missing, and nobody would know. That is the reason the annual service is not optional – see the unvented cylinder annual service.
Converting
Converting from vented to unvented is a substantial job, not a cylinder swap. It involves removing the loft tank and its pipework, running a new mains cold feed of adequate size to the cylinder position, fitting the full control set, and installing D1 and D2 discharge pipework that complies – which is frequently the constraint on whether it is possible at all.
It is notifiable work requiring the relevant competence, and you should receive a certificate. See converting from vented to unvented and unvented cylinders.
Wondering whether your house suits an unvented cylinder? The flow rate measurement takes ten minutes and it answers the question.
Questions we get asked about this
Which is better, vented or unvented?
Neither universally. Unvented gives mains pressure everywhere and needs a good incoming main. Vented is simpler, cheaper to maintain and keeps working when the mains fails. The incoming flow rate usually decides.
How do I tell which I have?
A tank in the loft plus a cylinder means vented. A cylinder with no loft tank, usually stainless steel with valves and a small vessel beside it, means unvented. A combi has neither.
Is an unvented cylinder dangerous?
Not when correctly installed, serviced and fitted with the full control set. The regulation around it exists because a sealed vessel of very hot water needs those controls – which is also why the annual service matters.
Can I convert from vented to unvented?
If the incoming main supports it and the discharge pipework can be routed correctly, yes. It is notifiable work requiring specific competence, and it is a project rather than a cylinder swap.
Do I lose water in a mains failure with unvented?
Yes, entirely. That is the trade-off for losing the loft tank, and it is worth knowing though it rarely decides the question.
Does a vented system mean poor showers?
It means low pressure at outlets close below the tank, which is most first-floor showers. A correctly specified pump resolves it and is far cheaper than converting.
Need this doing?
Call and describe the problem — you will speak to an engineer, not a call centre. Written quotes, no estimates over the phone.