Unvented Hot Water Cylinders

Mains-Pressure Hot Water, Direct or Indirect

An unvented hot water cylinder is a sealed tank fed straight from the cold mains, so it stores hot water at mains pressure: strong showers, fast-filling baths and more than one tap running at once – with no cold water tank in the loft. Choose a direct cylinder, heated by electric immersion, or an indirect cylinder, heated by your boiler.

PlumbXtra stocks unvented cylinders from 90 to 500 litres – direct models from 90 to 300 litres and indirect models from 90 to 500 litres – delivered UK-wide from our Burnley base, with free delivery on orders over £500.

Choose Your Cylinder Type

Direct Unvented CylindersHeated by electric immersion – no boiler needed. For flats and all-electric homes. 90–300 litres. Indirect Unvented CylindersHeated by a system or regular (heat-only) boiler through an internal coil. 90–500 litres.

What Size Unvented Cylinder Do You Need?

1–2 people	Flat or 1-bed home, one bathroom: 90–120 litres
2–3 people	2-bed home, one bathroom: 120–170 litres
3–4 people	3-bed home, one or two bathrooms: 200–250 litres
4–5 people	3–4 bed home, two bathrooms: 250–300 litres
6 or more people	Larger homes with three or more bathrooms: 300–500 litres (indirect)

Based on the rule of thumb of 45–60 litres of stored hot water per person (Baxi sizing guide). Size up for an en-suite, a second bathroom or high-flow showers, and have your installer confirm the size against your mains pressure and flow rate.

Fitting. Building Regulations require an unvented cylinder to be installed by a competent person – in practice an installer with an unvented hot water (G3) qualification. PlumbXtra supplies cylinders for delivery only; we do not install.

Direct vs indirect, installation rules and FAQs

Direct vs Indirect Unvented Cylinders

	Direct unvented	Indirect unvented
Heated by	Electric immersion heaters inside the cylinder	Your boiler, through a coil inside the cylinder
Best for	Homes with no boiler – flats, all-electric homes, no gas supply	Homes with a system or regular (heat-only) boiler
Sizes we stock	90, 120, 150, 170, 200, 250 and 300 litres	90, 120, 150, 170, 200, 250, 300, 400 and 500 litres
Hot water	Mains pressure	Mains pressure
Installation	G3-qualified installer required for both

Installation & Building Regulations

Because they run at mains pressure, unvented cylinders are fitted with safety controls – a temperature and pressure relief valve and a non-self-resetting thermal cut-out – and any discharge must be piped safely to where it can be seen, normally through a tundish. In England and Wales, Building Regulations Part G3 requires an unvented system to be installed by a competent person, which in practice means an installer holding an unvented hot water (G3) qualification. Fitting a cylinder over 15 litres is usually notifiable work: your installer self-certifies it through a competent person scheme, or it is notified to Building Control. Scotland and Northern Ireland have equivalent rules.

Before you order, ask your installer to check your incoming mains pressure and flow rate – an unvented cylinder can only deliver what your mains supply provides.

Unvented Cylinder FAQs

What is an unvented hot water cylinder?

A sealed hot water tank fed straight from the cold mains instead of from a tank in the loft. Because it runs at mains pressure it gives strong showers and fast-filling baths, and you can run more than one tap at a time. It is fitted with safety controls to handle that pressure, including a temperature and pressure relief valve and a thermal cut-out.

What is the difference between a direct and an indirect cylinder?

How the water is heated. A direct cylinder heats water with electric immersion heaters, so it needs no boiler. An indirect cylinder has a coil inside: hot water from your boiler runs through the coil and heats the stored water. Have a system or regular boiler? Choose indirect. No boiler? Choose direct.

What size unvented cylinder do I need?

Allow roughly 45–60 litres per person, then size up for an en-suite, a second bathroom or high-flow showers. As a guide: 1–2 people 90–120 litres, 2–3 people 120–170, 3–4 people 200–250, 4–5 people 250–300, and larger households 300 litres and up. Your installer should confirm the size against your mains flow rate.

Can I fit an unvented cylinder myself?

No. Building Regulations require an unvented cylinder to be installed by a competent person – an installer with an unvented hot water (G3) qualification – and the work is usually notifiable, either self-certified through a competent person scheme or notified to Building Control. We supply cylinders for delivery only.

Can I use an unvented cylinder with a combi boiler?

Generally no. A combi boiler heats hot water on demand, so it is not designed to heat a separate cylinder. An indirect cylinder needs a system or regular (heat-only) boiler. If your home has no boiler at all, a direct cylinder heated by electric immersion is the alternative.

Do unvented cylinders need servicing?

Yes. An unvented cylinder should be checked every year by a qualified engineer, and most manufacturers make an annual service a condition of the guarantee.

Do you deliver cylinders, and can I collect?

We deliver UK-wide from our Burnley base, with free delivery on orders over £500. Collection from Burnley is available by prior arrangement – call 01282 429061.