McDonald Unvented Cylinder


An unvented hot water cylinder is a pressurised hot water cylinder system which is fed directly from the cold mains water feed and therefore can deliver hot water at mains pressure, which is great for improving the performance of your shower and bath.

Unvented cylinders don’t require a cold water storage tank and are suitable for use with a variety of heat sources, including electric, gas, solar thermal and heat pumps.

The POWERflow, copper unvented cylinders are built to deliver superior mains pressure hot water simultaneously for showers, sinks and wash hand basins.


It has revolutionised new build and refurbishment schemes, bringing greater flexibility, increased cost effectiveness and much improved performance.
View Product
The ECOflow copper heat pump unvented cylinder is designed to enhance the performance of Heat Pumps which are increasingly popular as people look to reduce their costs and emissions.


Copper hot water cylinders have been widely recognised as the best and most cost effective solution to use in conjunction with heat pump systems regardless of whether it is an air or ground source.
View Product
STEELflow offers a range of high performance unvented cylinders for those on a budget.


Manufactured from premium grade duplex stainless steel, STEELflow unvented cylinders are produced with McDonald Water Storage uncompromising efficiency and quality standards.
View Product

An Unvented Hot Water Cylinder is a type of hot water storage cylinder that is pressurised and sealed. It is fed directly from the cold mains water feed and therefore can deliver hot water at mains pressure, which is great for improving the performance of your homes shower and bath.

Unvented Cylinders do not require a cold water storage tank, which is ideal if you are pushed for space as well as offering more flexibility when siting the cylinder.

The water can either be heated directly via an immersion heater or indirectly via the gas or oil boiler, solar thermal technology or other renewal energy such as heat pumps.

Pressurised Hot Water Cylinder

A pressurised water tank such as an unvented hot water cylinder requires an expansion vessel to be fitted to allow the expansion of water in the system as it heats up. Unvented cylinders also come with other safety mechanisms, such as pressure relief valves and twin thermostats. 

To ensure safety and to comply with building regulations, it is essential that an unvented hot water cylinder is installed by a G3 qualified installer and serviced annually.

McDonald Water Storage provide a wide range of unvented cylinder solutions, with the POWERflow unvented hot water cylinders being one of the most popular copper unvented cylinders on the UK market today due to their unrivalled performance, with the ECOflow unvented range specifically designed for use with heat pumps.

We also offer the standard STEELflow range of stainless steel unvented cylinders where price is key.

Our technical sales team have produced a number of handy tips and guides regarding unvented cylinder solutions in response to regular questions being asked by our clients. Check them out below.

Our focus is very much on the manufacture of copper hot water cylinders and tanks. Copper provides fantastic benefits including;

  • Ability to kill 99% of bacteria including Ecoli and legionella
  • One of the most efficient conductors of heat
  • Significantly more flexible for manufacturing than other materials
  • Long lifespan due to its amazing resistance to corrosion
  • 100% recyclable

For further information see our brochure The Ultimate Supermetal or visit our Copper Facts page.


Unvented cylinders allow you to free up valuable space in your property and in the majority of cases, provide stronger water pressure than an open vented system. This is due to the unvented cylinder being directly supplied by the mains, however this depends on the strength of the incoming mains pressure.

Unvented hot water cylinders are also perfect space-saving options for the modern home as they do not require a cold feed tank in the loft, freeing up valuable space in the property. Unlike vented cylinders, the unvented cylinder doesn’t rely on gravity to function, so it can be installed almost anywhere in your home, providing superior hot water pressure! This makes it easier to store in a place more suitable for you.

An unvented pressurised hot water cylinder saves on electricity, making it a superb green option which is better for the environment.

A pressurised hot water cylinder is a great way to provide hot water to your home. It is an efficient and cost-effective way to store and heat water for domestic use. The main benefit of a pressurised hot water cylinder is that it provides a steady supply of hot water on demand. This is because the water is stored in the cylinder at a constant temperature.

Unvented cylinders not only come in different materials but are also suitable for use with a wide range of heat sources.

The cylinders can be manufactured from either copper or stainless steel, with copper being the most versatile due to the bespoke manufacturing options. There are also mild steel solutions, that in a domestic application, are glass lined. These are predominantly used outside the UK.

Unvented cylinders can be used with traditional heat sources such as electric immersions or gas/oil boilers. Recent years have seen the compatibility being widened to include solar thermal, solar PV and ground source or air source heat pump technology.

While domestic unvented cylinders can be manufactured from either copper or stainless steel, the bespoke design flexibility of copper provides the perfect solution where standard stainless steel products cannot fit. With a wide range of diameters available to choose from, and the ability to adjust the tapping positions to suit the installation, it has become the perfect choice for installers.

The POWERflow and ECOflow unvented ranges offer slimline models with as little as 400mm overall diameter, with the STEELflow starting at 475mm diameter.

The POWERflow and ECOflow unvented ranges can be specified to operate at either 1.5 bar, 2.1 bar or 3 bar. The STEELflow products are designed to operate at up to 3 bar.

There are many variations of heat sources that can be utilised with an unvented cylinder to provide the mains pressure hot water.

The most common models are direct models which utilise electric immersions and indirect models which are heated via gas or oil boilers.

With the increase in renewable technologies, there are model variants that unvented cylinders are specifically designed for use with include solar thermal, solar PV (with relevant controls) and heat pumps (with a specially designed coil to maximise the lower temperature heat input).

The main difference between a vented and unvented water storage cylinder is that an unvented hot water cylinder takes its water supply directly from the cold mains supply, delivering the hot water at mains pressure. It also stores the water within the cylinder at pressures ranging generally from 1.5 bar to 3 bar, with an expansion vessel (or internal air baffle) allowing this pressure to raise when the cylinder is heated.

A traditional vented cylinder is fed from a cold feed tank as opposed to directly from the mains. It is reliant on the height of the cold feed tank above the cylinder to provide the hot water pressure at the outlets.

You should be aware that an unvented cylinder is a pressurised container and will require regular maintenance just like a gas boiler, to ensure the key safety devices fitted to the system operate correctly.

An annual check is vital to ensure your safety and must be carried out by a qualified engineer. It not only validates the manufacturer’s warranty but also guarantees the cylinder operates efficiently in compliance with industry guidance and Building Regulations.

When it comes to installing an unvented cylinder, strict safety regulations have been put in place due to its high pressure performance. This means that installers must have a G3 qualification if they wish to install any unvented hot water system within a domestic property.

Introduced to cover the inspection, installation, commissioning and servicing of an unvented hot water system, the Building Regulations Approved Document G section G3, also known as an Unvented Hot Water qualification, is a legal requirement and must be adhered to ensure the installer has the knowledge, skills and competence required to carry out the installation.

Combi boilers are often seen as a heating and hot water solution, providing instantaneous hot water without the need for a storage and expansion system. Historically this freed up space within the property where a cylinder was previously located.

However, when specifying for housing where multiple showers or outlets are supplied, an unvented hot water cylinder will be a more appropriate choice.

In addition, with high fuel bills, there is a growing requirement to use low cost renewable heat sources, which is only possible with hot water storage units, whether they are vented, unvented or thermal store products.

When It is generally the homeowner or landlord who would identify when the cylinder is due a service, in the same way as a boiler. However it must be a G3 qualified engineer that carries out the necessary service and maintenance checks on the cylinder. Click here to see the full details of unvented cylinder service requirements.

A system boiler directly heats your central heating and produces hot water for an unvented storage cylinder. They work like a regular (conventional) boiler, in that they need to store hot water, but differ in the way they take in their water supply.

All unvented cylinders are provided with a 25 year manufacturers warranty on the cylinder body against material defects and manufacturing faults, with 2 years on all components.

The main insulation method is high density injection foam, that is then cased to not only maintain the temperature of the store for the maximum period, but also provide an aesthetic finish that is easy to clean. We also offer spray foam insulation on our copper POWERflow and ECOflow where requested.

Unvented cylinders provide many advantages to the homeowner, however the main disadvantage is in retrofit situations due to routing the discharge pipework in compliance with G3 Building regulations. This needs to go to either a heat resistant waste pipe or an outside wall and down to the ground, which can be problematic in apartment buildings.

In rental properties, access also needs to be considered to carry out the annual service.

CPD Enquiry

Enter your details below...

Register Your Product

Please enter your details below to register your product.

Product Details
Homeowner Details
Installer Details
For more information on the warranty details, click here

Installation Service Quote

Please enter your details below to request a quotation for our installation service.

Technical Assistance Form

If you require Technical Assistance from our online cylinder site, please fill in the form below.

Product Request Form

If you cannot find a product or part in our store you can request it below and we will try to source it direct for you.

Product Enquiry

Enter your details below...

Join The Club!

Claim 10% off
mcdonald water storage
spare parts

To celebrate the launch of our online store with genuine McDonald Water Storage spares and free next day delivery (orders over £50), we want you to enjoy a 10% discount on your first order.
To claim your offer simply enter your email address below

By Entering your email address and pressing send, you agree to be happy to receive offers and updates from McDonald Water Storage.