A Guide to Unvented Hot Water Cylinders

What is an unvented hot water cylinder?

An unvented hot water cylinder is a water vessel which holds heated water under extreme pressure, feeding it out when a hot water tap or appliance is opened and providing instant hot water. Because the system is completely pressurised, there is no need to vent it to the atmosphere – hence the name.

Unvented hot water cylinders have been fitted in the UK since 1986 and have seen a massive growth in popularity over the years. Unlike vented systems, unvented hot water cylinders do not require a cold water tank and so save domestic users space within their homes. The sealed cylinder is instead fed directly from the cold mains, and since the mains water enters under high pressure, the flow rates and consistency from an unvented cylinder tend to be more reliable.

Unvented hot water cylinders are typically made from stainless steel, which is a relatively thin and high tensile material capable of expanding with the hot water and withstanding the extreme pressures and high temperatures. Heavy gauge copper is sometimes used as an alternative, but while these might be better for storing hot water, they are also much more expensive.

How do unvented systems work?

In an unvented system the hot water cylinder is fed directly from the incoming mains water supply, which keeps the water under constant pressure and ready for use. There are two different methods for heating the water within an unvented hot water system. In a directly heated model the cold mains water is warmed up by an in-built immersion heater, while the water in an indirectly heated model is heated by an external source such as a gas boiler or solar panels.

When a hot water tap or appliance in the system is opened, the pressure from the incoming cold water displaces the hot water in the cylinder and pushes it along an outlet pipe from the cylinder, getting it to where it needs to be. Because of the high incoming pressure these systems work well in homes with more than one bathroom, where multiple taps and showers might be in use at the same time.

All unvented cylinders are fitted with pressure and temperature relief equipment, designed to vent the cylinder in the event that it overheats or the internal pressure becomes too great. This is a safety feature designed to minimise the chances of the equipment failing, or the risk of an explosion.

When is an unvented system the right choice?

Unvented hot water cylinders are an ideal choice for domestic properties, and the absence of a cold water tank makes them especially suited to those with limited space. If your house has more than one bathroom, the pressure behind an unvented system means you can run multiple taps and showers at the same time without seeing a discernible drop in flow rates, making them a good option for modern, multi-occupant family homes.

Having grown in popularity in recent years, unvented systems are now the first choice when it comes to new-build properties and refurbishments. Especially in homes with luxury bathroom fittings such as multi-jet showers and jetted baths, the high pressure means large quantities of hot water can be pumped around the system at a steady, regulated flow.

There is also growing demand for unvented systems in hotels, offices, schools and leisure centres, where there are often ‘peak periods’ of demand which require consistent hot water from multiple taps, showers or other appliances. In hotels, for instance, hot water can be guaranteed to all rooms during the morning rush for the showers by installing a large unvented cylinder or multiple cylinders in sequence.

Unvented cylinders are also being more widely used as people become aware of greener alternatives for their homes. Unvented cylinders can be heated by solar panels, with the heat transferred to the cylinder through a coil installed at the base of the specially designed vessel.

The advantages of an unvented system

Because the water enters the system under pressure from the mains, an unvented hot water system does not require a cold water tank. This makes them a good choice for homeowners who have limited space, and there’s the added advantage that no cold water tank in the loft means no danger of it freezing and pipes bursting over the cold winter months.

The constant pressure from the mains also means that flow rates are better than in a vented system, meaning a more consistent flow of hot water and better performance from your shower and taps. This is particularly important for those who have high-powered luxury bathroom fittings such as jetted baths, which require large volumes of hot water fed into them at a steady rate.

Another major benefit the unvented hot water cylinder has over a vented system is that it can be sited anywhere in your home. While vented systems rely on gravity to transport hot water around your home, the high pressure behind the water entering your unvented cylinder means you can locate it just about anywhere.

For those looking for a greener alternative when it comes to heating their water, certain specially designed unvented tanks can be connected to solar panels, with the heat they generate being transferred into the system via a fitted coil.

Finally, unvented systems are much quieter. In a vented system which includes a cold water tank, the cistern must be noisily refilled – no cold water tank means no gurgling pipework, and the sealed system means there’s no risk of the cold water becoming contaminated.

The disadvantages of an unvented system

Of course, every system has its downsides. When it’s heated, water expands in volume, and in a system which already operates under extreme pressure, there is always the risk of an explosion if the safety features of your cylinder are not properly looked after.

Unvented hot water cylinders must be allowed to expand with the heated water to ensure they are kept at a safe pressure. There are two ways this can be done. The first is a so-called ‘bubble top unit’, which produces an internal air bubble which becomes trapped at the top of the cylinder when it is installed and allows the vessel to expand safely. The second is an external expansion unit which contains the expanded hot water.

The major disadvantage of an unvented system is that because it is attached to the mains water supply, if the mains is turned off for any reason, your home is left without access to hot water.

These systems also tend to be more expensive to install than traditional vented systems because of the extreme pressure they operate under. As additional safety features, such as relief pipework, need to be installed, they need to be fitted and maintained by specially qualified technicians with G3 qualifications.

Unvented system costs

An unvented system is almost always more expensive to have fitted than a traditional vented system, because of the additional safety features which must be installed. It is important to remember that unvented hot water cylinders do tend to give more reliable flow rates than their vented counterparts, and when you have one fitted, the high cost is going on ensuring you’re kept safe.

Because unvented systems must be installed and maintained by G3 certified engineers, the higher cost reflects the expertise you’re paying for. Having the relief pipework and other safety mechanisms fitted correctly so that your system is safe to use is of paramount importance, as high pressure and hot water can be a dangerous mix. Fitted and maintained correctly, however, an unvented system is a wise investment which will see you through many years in your family home or your business premises.

Installing an unvented system

You should never attempt to install an unvented hot water cylinder yourself, but should always use a trained and qualified professional with plenty of experience. Unvented cylinders operate under extreme pressure and transport large volumes of hot water, meaning the risks posed by incorrect installation are significant.

If you’re having an unvented system installed, you will need to contact the local Building Control Department and notify them of your intentions. Although they are described as ‘unvented’, these systems do need to have some pressure relief pipework and valves installed which will vent out of the building. This is a precautionary measure taken to ensure the water pressure and temperature do not build to unsafe levels and cause an explosion.

Under the government’s G3 Building Regulations, any vessel containing more than 15 litres of hot water under pressure must be installed by a qualified engineer. These cylinders should only be fitted by a technician who has CITB training and G3 certification, and it’s important that you always ask to see proof of competence before you allow them to carry out the work. They must also give you documentation to say the system has been certified once the work is complete.

Ensuring your unvented hot water cylinder is fitted safely by a qualified individual reduces the chances of the equipment failing and causing serious injury.

Servicing you unvented hot water system

It is important that you keep your unvented hot water cylinder serviced and well-maintained, prolonging its working life and ensuring it remains safe to operate. Under the BS2870 health and safety regulations, you need to have your unvented hot water cylinder and system checked over by a qualified technician at least once a year.

Although the vast majority of unvented systems are extremely advanced and reliable, as with any plumbing or heating system, things can go wrong. If elements of your hot water cylinder begin to fail, there could be a build-up in the levels of pressure the water is under, which in the worst case scenario could result in an explosion. While the relief pipework and vents are designed to reduce the risk of this happening, ensuring your system is well maintained is key to keeping you safe.

You should have your system thoroughly checked once a year to catch any potential issues before they become too serious, and too costly.

What to do next

If you’re comparing vented and unvented systems for your hot water supply, it is always worth seeking expert advice to establish which system would be best suited to your home and the practicalities of having it installed. With some of the most experienced and highly qualified engineers in the region, Ambient are ideally placed to offer you guidance on choosing and installing an unvented hot water cylinder in your home.

Once you have decided you want an unvented cylinder installed, you should notify the local Building Control Department of your local council before work begins.

With a specialist team serving Bristol and the surrounding areas, Ambient Heating and Plumbing are able to install unvented hot water cylinders to both domestic and commercial properties. All our plumbers have the necessary BPEC G3 qualifications to correctly fit unvented cylinders, ensuring they are safely installed and ready for use.

We also offer annual service packages to see that your unvented system is kept in good, safe working order and is properly maintained according to the manufacturer’s instructions. This not only gives you the peace of mind in knowing that your system is kept in the very best condition, but also ensures your insurance policy is valid.

For more information on unvented hot water cylinders and for further advice on having one installed in your home or business, get in touch with Ambient today.