Unvented hot water cylinders: what they are and why they are not DIY

An unvented cylinder stores hot water at mains pressure. It is common in newer homes and in homes where a loft tank has been taken out. It gives strong showers without a pump, but it is also a sealed, pressurised vessel full of hot water, which is why the rules treat it differently from an ordinary copper cylinder.

This guide explains what an unvented cylinder is and what you can safely notice. It does not give installation, servicing or adjustment steps, because work on an unvented hot water storage system is specialised and not a DIY job.

Vented and unvented: the difference

A vented cylinder is fed from a cold water tank, usually in the loft. An open vent pipe runs from the top of the cylinder back up over that tank. The system is open to the air, so the pressure inside is only what the height of the tank provides. If the water overheats, it has somewhere to go. Our hot water cylinders and immersion heaters guide covers vented cylinders in more detail.

An unvented cylinder is fed directly from the cold mains, with no loft tank and no open vent. The water inside is under pressure. To make that safe, the cylinder comes with a set of controls and safety devices. These typically include something to control the incoming pressure, something to absorb expansion as the water heats, thermostats and cut-outs to stop it overheating, and relief valves that open if pressure or temperature get too high.

When a relief valve opens, the water has to go somewhere safe. The Building Regulations 2010, Schedule 1, Part G, requirement G3, says a hot water system with a storage vessel must incorporate precautions to "prevent the temperature of the water stored in the vessel at any time exceeding 100°C" and to "ensure that any discharge from safety devices is safely conveyed to where it is visible but will not cause a danger to persons in or about the building".

In practice that visible point is usually a tundish: a small open funnel in the discharge pipework, normally near the cylinder, where you can see if water is coming out.

Not sure which kind you have? See combi, system and regular boilers.

Why it is specialised work

Approved Document G covers this. It is the government's guidance on meeting Part G in England, available from GOV.UK. It says the safety and performance of an unvented system depend on choosing the right system and safety devices and on installing it correctly. It says building owners and occupiers "should therefore take care to choose installers who have the necessary skills to carry out this work". Those skills can be shown, for example, by registration with a competent person scheme for this type of work, or by holding a current registered operative skills certification card for unvented hot water systems.

Approved Document G also says that installing an unvented system is notifiable building work. If the installer is registered with a competent person scheme for unvented hot water systems, they self-certify the work and the owner is given a building regulations compliance certificate, usually issued by the scheme operator. Keep that certificate with the cylinder paperwork.

Approved Document G applies in England. Wales, Scotland and Northern Ireland have their own building regulations and guidance, so check the rules where you live.

The upshot for a householder is simple: do not open up, adjust, re-set or alter an unvented cylinder or its safety devices yourself. That includes the valves, the expansion vessel, the pressure controls and the discharge pipework. If the cylinder is heated by a gas boiler, anything on the boiler side is for a Gas Safe registered engineer. If it has an electric immersion heater, the wiring is for a qualified electrician. Our guide to expansion vessels explains what that part does.

What you can reasonably notice

You can keep an eye on an unvented cylinder without touching any of its workings.

Water coming out of the tundish. A tundish should normally be dry. Water dripping or running through it means a safety device is releasing. A small, occasional drip and a steady flow both need looking at, but they are not equally urgent.

A small drip at the tundish is not an emergency in the same way, but it still means something is not right. Note when it happens (for example, only while the water is heating) and book a visit from a competent engineer.

No hot water. Check the simple, outside things first: is the programmer set to give hot water, is the boiler showing a fault, has the immersion switch been turned off? Some cylinders have a manual reset on an overheat cut-out. Do not reset it yourself; that cut-out tripping is a sign something needs checking.

Weak flow at every tap. An unvented cylinder depends on the incoming mains. If the cold is weak too, the cause may be the supply rather than the cylinder. See our low water pressure guide. The part that controls the incoming pressure is explained in our pressure reducing valves guide, but on an unvented cylinder it stays a job for a competent person.

Water pooling around the cylinder or dripping from fittings. Turn off the cold supply to the cylinder only if there is a clearly labelled valve for that purpose and it is easy to reach. Then call the engineer.

Servicing and paperwork

Makers commonly ask for unvented cylinders to be serviced regularly by a competent person, and the guarantee may depend on it. The interval and what is checked are set by the maker, so look at the paperwork that came with the cylinder or the label on it. When you move into a home with an unvented cylinder, ask the seller for the compliance certificate and the service records.

Can I fit a shower pump to an unvented cylinder?

Not a standard gravity shower pump. The cylinder already supplies water at mains pressure, and standard shower pumps are made for systems with a loft tank and a vented cylinder. See shower pump sizing.

Is a dripping tundish dangerous?

Hot water or steam coming out is urgent. Switch off the heat sources at their normal controls and call a competent engineer. A small cold drip is less urgent but still a sign that something needs checking. Do not block or cap the tundish or the discharge pipe. That would defeat a safety device.

Can a plumber who is not registered work on my unvented cylinder?

Approved Document G says to choose installers with the necessary skills, for example registration with a competent person scheme for unvented systems or a current unvented skills card. Ask to see evidence before work starts.