Unvented Cylinder Installation: What to Expect and What to Ask For

What does a new unvented cylinder installation cost?

A new cylinder installation is a fixed price of £2,400. It includes pipework, installation, commissioning and Building Regulations, which are self-certified through a competent person scheme. No VAT is charged, and there is no call-out fee.

The cylinder is not in that figure. It is priced separately, from a price list by type, size and your preference, and we recommend the right one for your home and your mains supply.

We cover London inside the M25, 24/7.

What should you ask any installer before accepting a quote?

Unvented fitting is regulated work, so a good quote answers a handful of questions in writing. Ask who will notify building control or which competent person scheme they work under, and what you will receive at the end. Ask how they will measure the mains before choosing the cylinder, where the discharge pipe will end, and what the quote assumes about the old pipework.

• Which scheme, or which building control route, covers the notification?
• What will you receive on completion: a certificate or notification evidence, and a commissioning record for the cylinder?
• Has the mains been tested for pressure and flow, not pressure alone?
• Where does the discharge pipe run, and can a discharge be seen?
• Are the valve group, expansion vessel and tundish included, or separate items?

Can my house take an unvented cylinder?

That question comes before the choice of cylinder. An unvented unit takes its pressure from the incoming main, so a weak main means a weak shower whatever the cylinder cost. Every manufacturer publishes its own minimum pressure and flow, and the installation manual for the model you are considering is where to find it. A high pressure reading does not guarantee flow, so both should be measured with the cold taps running.

A house on an old narrow supply pipe, or a flat several floors up, can pass the pressure test and still fail on flow. When it does, the honest answer may be a vented cylinder with a pump, or a thermal store. The comparison is in vented vs unvented cylinders.

The floor matters too. A full 210 litre cylinder holds 210 kilograms of water before the metal is counted, which is why a loft or first-floor cupboard should be checked rather than assumed.

What goes in an unvented installation?

Under Part G of the Building Regulations an unvented hot water storage system must carry safety devices beyond the ordinary thermostat. The usual set is a non-self-resetting energy cut-out that disconnects the heat, and a temperature relief valve or combined temperature and pressure relief valve that discharges safely if the water keeps overheating. The relief valve is normally factory fitted on the cylinder and is not relocated.

On the cold side the inlet group usually holds a strainer, a pressure reducing valve, an expansion relief valve and a non-return valve. The expansion vessel is pre-charged to suit the supply pressure, so a vessel charged for one pressure and fitted to a supply set for another will discharge every time the water heats. The settings come from the manufacturer's manual for that model, not from this page.

An indirect cylinder fed by a boiler also needs a motorised two-port valve wired to its overheat thermostat, so that a failed thermostat cuts the primary flow. No thermal cut-out or safety wiring is ever bypassed.

Why is the discharge route planned first?

The tundish and discharge pipe are where a fault becomes visible, and Approved Document G sets requirements for where the tundish sits, how the pipe falls and where it ends. The exact dimensions are in the current approved document and the manufacturer's manual, and an installer should be able to show you which they have worked to.

A cupboard that looked suitable can be refused on that basis. A boiler room with no drain, a ground-floor flat with no accessible gully or a long run with several bends may fail the sizing table. Finding that out after the cylinder is delivered is expensive, so a competent installer plans the route first.

The finished system should let you see a discharge, because a dripping tundish is the first sign that a valve or the vessel needs attention. See tundish dripping: causes and checks.

Regulations, paperwork and who can do the work

Fitting an unvented cylinder is notifiable work under Building Regulations Part G. It should be carried out by a competent installer and reach building control, either directly before work starts or through a competent person scheme that self-certifies it. You should expect to end up with certificate or notification evidence and a commissioning record. If either is missing, ask for it before you pay.

This is why unvented fitting is outside DIY. The rules are set out in plain terms in unvented cylinder regulations under Part G.

Typical costs are a separate question, answered with attributed market figures in the unvented cylinder installation cost guide. The fixed price for a new installation is under the cost heading above and on the pricing page.