An unvented cylinder is a sealed, mains-pressure hot water store. Installing one is notifiable work under Building Regulations Part G3 and must be done by someone holding the relevant competence. The safety devices and the D1/D2 discharge pipework are not optional details – they are the whole reason the system is safe.
What an unvented cylinder actually does
A traditional vented system stores hot water in a cylinder fed by a cold tank in the loft. The pressure at your taps comes from the height of that tank above them, which is why upstairs showers on gravity systems are so often disappointing and why a first-floor bathroom directly below the loft is the worst case of all.
An unvented cylinder does away with the loft tank. The cylinder is connected directly to the incoming mains and is sealed. Hot water leaves it at mains pressure, which means every outlet in the house gets the same pressure as the kitchen cold tap, showers work properly upstairs and down, and you can run two of them at once if the incoming supply can feed it.
The consequence of sealing the vessel is that the water has nowhere to expand into as it heats, and nowhere to vent if a control fails. That is what all the safety equipment is for.
The safety devices, and why there are so many
An unvented cylinder has a layered set of protections, each of which takes over if the one before it fails. Understanding the order makes the annual service make sense.
| Device | What it does | When it acts |
|---|---|---|
| Cylinder thermostat | Normal operating control – switches the heat source off at temperature | Every heating cycle |
| Expansion vessel or internal air gap | Absorbs the expansion of water as it heats, so pressure does not climb | Every heating cycle |
| Pressure reducing valve | Limits incoming mains pressure to what the cylinder is rated for | Continuously |
| Expansion relief valve | Opens if pressure rises beyond the set figure | Only if expansion has failed |
| High-limit thermostat (energy cut-out) | Manually resettable – cuts the heat source if the control thermostat fails | Only on a control failure |
| Temperature and pressure relief valve | The last line. Dumps water if temperature or pressure exceed safe limits | Only if everything above has failed |
Every one of those is checked at an annual service, and a system where one has failed is a system running without a layer of protection – which is precisely why the service is not optional and why manufacturers make the warranty conditional on it.
The discharge pipework
This is the part that gets installed wrongly most often, and it is the part a building control inspector looks at first.
If a relief valve opens, it discharges near-boiling water. That water has to be taken somewhere safe, which means it must not scald anyone, must not damage the building, and must be visible so that somebody notices it is happening. Building Regulations Approved Document G sets this out as two sections: D1, the short length from the valve to a tundish, and D2, the run from the tundish to the point of discharge.
- The tundish is an open air break with a visible gap. It must be fitted vertically, within 500mm of the valve, and be visible – in the same room, where somebody will see water running through it. A tundish hidden inside a sealed boxed cupboard defeats the object.
- D2 must be at least one pipe size larger than D1, and must fall continuously from the tundish.
- There is a maximum permitted length, reduced by every elbow in the run – each bend counts against the allowance. A long run with several bends very often needs upsizing again, and this is the calculation most installations skip.
- It must terminate safely – low level to a gully below a grating, or a visible external position where discharge cannot scald anyone, including people at a window or on a path.
- Metal pipe, because it is carrying water above 100 degrees in the worst case.
Look at the tundish. Water passing through it means a relief valve is discharging, and the single most common reason is an expansion vessel that has lost its air charge. It is a repair, not a disaster, but it should not be left – see expansion vessel failure.
Whether your house suits one
The honest limiting factor is almost never the cylinder. It is the incoming water main.
An unvented cylinder can only deliver what the mains supplies. If the flow rate at your stopcock is marginal, you will get mains pressure but not much of it, and the system will disappoint precisely when two outlets are running. We measure static pressure and dynamic flow rate before specifying anything, because that measurement determines whether the answer is a cylinder, a cylinder plus a supply pipe upgrade, an accumulator, or leaving the vented system alone and fitting a pump.
The other requirements are practical: somewhere for the cylinder that allows the discharge pipework to be routed correctly, an adequate cold feed, and a heat source sized to recover the cylinder in a sensible time. There is more in converting from vented to unvented.
What we do
- Installation of new unvented cylinders, and conversions from vented systems, with the flow rate measured first rather than assumed.
- Annual servicing – all relief valves checked and exercised, expansion vessel charge tested and recharged, thermostats verified, discharge arrangements inspected, and the benchmark record completed.
- Fault diagnosis on systems that are discharging, not heating, or losing pressure.
- Bringing non-compliant installations up to standard, which in practice usually means the discharge pipework.
- Cylinder replacement where a vessel has failed or is undersized for how the household actually uses hot water – see cylinder replacement.
Cylinder discharging, or never been serviced? Both are worth dealing with before they become a replacement.
Questions we get asked about this
Is an unvented cylinder notifiable work?
Yes. Installing one is notifiable under the Building Regulations and must be carried out by a person with the relevant competence, who can self-certify it. You should receive a certificate – keep it, because a solicitor will ask for it when you sell.
How often does it need servicing?
Annually. That is the manufacturer’s requirement and it is what keeps the warranty alive, but the substantive reason is that the expansion vessel loses charge gradually and the relief valves need exercising. An unserviced unvented cylinder is a safety system nobody has checked.
Water is dripping from a pipe outside. Should I worry?
It means a relief valve has opened, and the usual cause is a failed expansion vessel. It is not dangerous in itself but it should not be ignored – a valve that keeps discharging will eventually fail to reseat, and you will have a continuous leak.
Can I have one if my water pressure is low?
It depends on the measured flow rate, not on how the shower feels. We measure at the stopcock before recommending anything. If the mains cannot support it, we will say so rather than fitting one that underperforms – sometimes the right answer is a supply pipe upgrade first.
Do I lose my loft tank?
Yes, and for most people that is a benefit: the space back, one less thing to freeze, and no open tank of stored water. It does mean you have no water at all if the mains fails, which a stored system would ride out – worth knowing, rarely decisive.
What size cylinder do I need?
It depends on how many people, how many bathrooms and how they overlap in the morning, not on the size of the house. Undersizing is the most common mistake and the one nobody notices until February. See cylinder sizing.
Is it noisy?
A correctly installed one is not. Noise on an unvented system is usually water hammer from high mains pressure, an incorrectly set pressure reducing valve, or pipework that is not clipped. All are fixable.
Regulations and standards referenced on this page
- Approved Document G: sanitation, hot water safety and water efficiency — Ministry of Housing, Communities and Local Government
What happens when you call
Call and describe it
You speak to an engineer, not a call centre. Most jobs can be narrowed down on the phone.
We look at it properly
A real diagnosis before a price, and a straight answer if it is not worth doing.
A written quote
Itemised, so you can see what you are paying for.
The job, and the paperwork
Certificates where the work needs them, and what will need attention next.
Where we do this work
Across Norwich and the whole of Norfolk. If you are on the edge of the county, call and ask — we will tell you honestly whether we are the right people.
- Norwich
- Wymondham
- Dereham
- Attleborough
- Aylsham
- North Walsham
- Cromer
- Diss
- Loddon
- Long Stratton
- Watton
- King's Lynn
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.