How Do Unvented Cylinders Work?

In recent years unvented cylinders have been growing in popularity. Since being made legally available in 1985, they have become a common occurrence in large homes and apartments. They were actually invented in 1861! But, at the time, were deemed too dangerous and expensive for British homes. So, what is an unvented cylinder? And, how do unvented cylinders work?

Unvented cylinders explained

Unvented cylinders hold hot water at very high pressure. Providing hot water to taps, showers or baths whenever it is needed. Due to unvented cylinders being fed directly from the mains, they can provide and maintain a very high flow rate. Provided there is an adequate flow rate coming into the property.

Unvented cylinders don’t need to be vented into a feed and expansion tank, unlike gravity fed hot water cylinders. Hence the name – unvented cylinder. The lack of a cold water tank saves space in properties and makes them easier to maintain.

Typically made from stainless steel, they are more than capable of coping with the high temperatures and extreme pressure. Unvented cylinders last for a very long time and generally come with a 25-year warranty.

How do unvented hot water cylinders work?

There are two different types of unvented cylinders, direct and indirect. This refers to the way they are heated.

Direct cylinders – Heated via an element (immersion heater) inside the cylinder. Mostly found in apartments and flats where there isn’t gas central heating.

Indirect cylinders – Heating via a coil inside the cylinder. Hot water from a central heating system or solar panels is passed through the coil, heating the water around it. Generally, indirect cylinders have an immersion heater installed as a backup. This way they can still be heated if the boiler breaks.

Unvented cylinders are under pressure constantly due to the mains water feeding them. When a hot tap is opened, cold water pushes through the inlet pipe at the bottom of the cylinder, pushing the hot water out of the outlet pipe at the top. When the cold pressure is high, this can provide enough hot water for multiple showers at once. Making them perfect for larger homes with multiple bathrooms and kids that tend to use a lot of hot water.

how do unvented cylinders work

Safety features

To cope with hot water expansion, unvented cylinders are fitted with pressure relief valves and expansion vessels. The temperature and pressure relief valve at the top of the cylinder is designed to open if the water ever gets too hot or the pressure gets too high. There is also a pressure relief valve on the inlet control valve. The inlet control valve is designed to reduce incoming water pressure, stop water backflowing from the cylinder and relieve pressure if it ever gets too high.

Both of the pressure relief valves are installed into a tundish, which allows visual checks and lets the water cool if the PRV’s are activated. An expansion vessel installed on the cold inlet gives the water somewhere to go once it has expanded. This is an important feature and helps prevent extremely high pressures. Incorrect installation of the expansion vessel can cause serious problems.

What properties are unvented cylinders suitable for?

Unvented cylinders are suitable for a variety of domestic and commercial properties. As mentioned in the previous section, unvented cylinders are perfect for a larger home. They can provide huge amounts of hot water at 60° all year round, which is something a combi boiler could never do. Most combi boilers only give a 35°-45° temperature rise. So in winter, when the water is colder there can be a noticeable difference. Als, because of the pressure they can provide, hot water can be drawn from multiple outlets at the same time. Without a noticeable drop in flow rate. With extremely larger houses, a hot water circuit can even be installed. This circuit pumps hot water round the house at peak times, meaning there is a much shorter wait time for hot water.

Also, unvented cylinders are a good option in properties where a gas boiler isn’t an option. They can commonly be found in apartments and flats around the country. Especially new ones. In single floor homes, they’re a much better option than gravity fed hot water tanks as the pressure is so much higher. Also, because unvented cylinders are highly insulated they experience much less heat loss. Meaning cheaper energy bills. This can make a considerable difference in properties running solely on electric.

Hotels, leisure centres and salons have also started switching to unvented cylinders in recent years because of the amount of hot water they can provide. In hotels, you can find plant rooms full of large unvented cylinders installed together to provide enough hot water for the entire building.

Solar thermal

A common sight in the UK, solar panels are becoming the go-to way to save money on energy. Most of the population don’t actually realise there are two types of solar panels. Most commonly installed is Solar PV, which harnesses the power of the sun to generate electricity. Solar thermal is a bit different, transferring the heat from the sun to a fluid. It circulates the hot fluid round an additional coil in the unvented cylinder. Heating up the water around it. Resulting in free hot water! This is a great way to save money on energy bills and help the environment.

solar themal panel

Unvented hot water cylinder advantages


High hot water pressure

Unvented cylinders provide very good hot water flow rates, suitable for properties with large hot water demand. Allowing multiple showers and baths to be run at the same time. Also, with the water being stored at 60° they provide hotter water than a combi boiler could. Moreover, because of the additional pressure, unvented cylinders can provide much more powerful showers than their gravity-fed counterparts. This eliminates the need for any shower pumps that may have been installed to increase the pressure. Most shower pumps are extremely noisy, so most people are happy to get rid of them.

Doesn’t require a cold water tank

Because unvented cylinders are mains fed they don’t need a cold water tank to feed them. This eradicates the risk of water becoming contaminated through the cold water tank. Or the pipes feeding the tank freezing in cold winters. Also, because unvented systems doesn’t require a cold water tank they help free up valuable space in the loft. Furthermore, unvented cylinders can be installed anywhere in a home. Whereas with gravity fed cylinders the cylinder has to be installed below the tank.

Energy efficient

Being highly insulated, unvented cylinders keep water hot for a long time. Meaning, the boiler or immersion heating the cylinder has to work a lot less to keep it hot. This helps save money on energy bills. Also, when installed with solar thermal panels, unvented cylinder can provide free hot water all year round. Who doesn’t want free hot water?

Quieter operation

With unvented cylinders being mains fed they are very quiet when being used. Unlike older gravity-fed cylinders which can be very noisy when the tank is being filled. Also, problems can develop with the ballcock feeding the tank resulting in excess noise.

Easier to maintain

Because gravity-fed systems need tanks, maintenance can be difficult. Especially when working on the tank in the loft, they’re never easily accessible or well lit. Also, most of the time the area around it isn’t boarded, making it dangerous for the plumbers working on them. All unvented cylinder parts are located around the cylinder itself making maintenance very easy.

Unvented hot water cylinder disadvantages

Unsuitable for low water pressure

Unvented cylinders are only as good as the water feeding them. Unvented cylinders don’t work well with poor water pressure. Making them unsuitable in properties or areas where there isn’t good enough water pressure. In these cases, gravity-fed systems are a much better option as the pressure is dictated from the height of the cold water tank. Also, gravity-fed cylinders can be installed with shower pumps to increase the pressure, whereas unvented cylinders can’t. It is possible to pump the cold water main feeding the unvented cylinder but this can prove very expensive.

Takes a lot of space

Even though unvented cylinders don’t require water feed tanks, compared to a combi boiler they still take up a lot of space. An unvented cylinder will take up space equivalent to a bedroom wardrobe. In bigger houses, this isn’t an issue. But in smaller homes, it can be difficult finding space to put them. There are multiple different sizes available, but even the small ones are wide and take up a lot of room.

Worcester greenstore unvented cylinder

Discharge pipe required

Because unvented cylinders use pressure relief valves, they require a discharge pipe installing to a drain. When installed next to an outside wall, this isn’t a problem. But, when installed centrally, this can add a lot of additional work. To install the pipe underneath, carpets and floorboards may have to be lifted. This can cause a lot of upheaval.

Problems can have serious implications

When installed correctly, unvented cylinders can be a fantastic addition to any home. But, when installed incorrectly problems can be disastrous. If safety features fail or aren’t fitted correctly the water inside boils. When opening a tap the water would turn into steam instantly, scalding anyone too close. If this continued, the cylinder would eventually explode.

How much does an unvented hot water system cost to install?

Installation costs of unvented systems can vary, depending on the location of the cylinder and the existing system Possibly the biggest factor affecting the cost. Directly replacing an existing system boiler can cost between £800-£1,200 depending on the size required. Replacing a gravity-fed cylinder will cost a bit more due to the additional work needed. But, expect to between £1,000-£1,500.

Upgrading a combi boiler to an unvented hot water system can cost significantly more. To upgrade the system, a system boiler will have to be installed to run the cylinder, which can add a lot to the price of the job. Expect to pay at least £3,000 for everything, sometimes it can be closer to £4,000 if a premium boiler is installed.

Who can install unvented cylinders?

To work on unvented cylinders, a plumber or heating engineer would have to be G3 Qualified. G3 Qualified engineers have undertaken special training to teach them how to install unvented cylinders correctly. An incorrectly installed unvented cylinder can be very dangerous, so always make sure your engineer has the correct qualifications.

Do unvented cylinders need servicing?

Servicing unvented cylinders every year is essential.  Although unvented cylinders are very well made and reliable. Safety devices can fail over time, so it is essential that they are checked annually by a qualified engineer. It’s usually easier if you get your cylinder checked at the same time as your boiler, that way you don’t forget.

Summary: How Do Unvented Cylinders Work?

We hope this guide has helped you understand everything there is to know about unvented cylinders. Unvented hot water systems are brilliant innovations. And, when installed correctly can provide fantastic hot water to homes and commercial properties. If you think we have missed anything or have any questions about unvented cylinders don’t hesitate to get in touch –

If you’re considering getting an unvented cylinder don’t forget to make sure the installer has the correct qualifications.

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