What Is An Unvented Boiler System?

An Unvented Boiler System is a plumbing system which takes it’s cold water supply directly from the mains water pipe as opposed to being fed from a gravity fed or tank connected to a pump.

In theory, the hot water will run at the same pressure as the cold water so, if you’re lucky enough to have high mains water pressure in your area then if you use this system you’ll get water mains hot water pressure throughout your household.

Unvented boiler systems are generally great for large households with more than one bathroom as it provides a consistent, strong water pressure throughout your home even if more than one water outlet is being used at the same time.

I find that they work best in households with two or more bathrooms and especially when they’re being used at the same time. With a combi boiler system, if two people have a shower at the same time (I’m assuming different shower rooms here lol) then the mains water pressure is effectively split into two.

So you get half the pressure in each shower so the shower pressure is lower. In some situations, if the house hasn’t been piped up properly, one shower would lose all of the pressure while the other shower is ok. There isn’t an equal split of pressure. So for a busy household, this can be really frustrating as you have to have a shower one by one.

An Unvented Boiler or Plumbing System solves this issue as you can have three showers running at the same time and have the kitchen tap open and the pressure is not really affected. You may feel a slight drop but it isn’t noticeable.

Worcester Gas System Diagram
Worcester Bosch Gas System Diagram

Unvented Boiler Systems Explained

A regular boiler usually requires a cold-water tank, which would generally be kept in the loft. When the boiler is turned on, the water in the cold-water tank will be transferred into the hot water cylinder and heated up. The hot water will then be ready for use at the taps.

On the other hand, an unvented boiler system does not require a cold-water tank. This is because the system boiler directly heats your central heating system and produces hot water which is stored in the unvented cylinder. The water comes into the cylinder from the household’s mains supply which can be heated with an immersion heater or renewable energy like solar power.

Differences Between Vented & Unvented Cylinders

Vented Hot Water Cylinders

Vented hot water cylinders require a cold-water storage tank, often in the loft, which supplies water to the hot water cylinder by taking advantage of gravity. The cold water is generally heated in the hot water cylinder with an immersion heater. Vented hot water cylinders tend to be cheaper than unvented cylinders however, they can provide a low water pressure.

Vented Hot Water Cylinders

Unvented hot water cylinders don’t require a cold-water storage tank as it gets its water directly from the household’s mains supply. The water can be heated by an immersion heater, through the central heating system or through renewable energy (like solar energy). They generally deliver a good, consistent water pressure.

Is The Unvented Boiler System Right For You?

To help you decide if an unvented boiler system is suitable for you and your home, have a read through the following advantages and disadvantages to help you make the right decision.

Advantages Of An Unvented Boiler System

Water Pressure

One of the main advantages is you will have a consistent, strong water pressure when taps are being used simultaneously.

With a regular boiler system, gravity plays a big role. Gravity draws the water from the cold-water tank stored in the loft to the hot water cylinder. This can cause problems because the higher in the house you are, the weaker your water pressure. `This is not the case with an unvented boiler system because it draws its water directly from the mains supply so it does not rely on gravity like a regular boiler does.

Performance Improvement

The unvented boiler system may be ideal for you if you have a large household as you may have a high hot water demand. This boiler system allows the use of multiple taps simultaneously without a reduction in pressure, this is due to the fact it uses a large tank of stored water. This is great for busy homes with more than one bathroom as it will improve the performance of your showers and baths while maintaining a consistent water pressure.

Saving Space

Unvented system boilers don’t require a cold-water feed tank or a feed and expansion tank as they can be used with pressurised cylinders. This could potentially save you quite a bit of space and it is ideal for households with limited space or no loft as there is no cold-water tank. Moreover, since the system does not need to be connected to a cold-water tank, you will be less restricted in terms of where to place it in your home.

Your Household Uses Renewable Energy

If your household uses renewable energy and your system boiler breaks down you will still have access to hot water. This is because the water stored in the unvented cylinder can be heated using an immersion heater or by the central heating system. This means it is compatible with any renewable energy sources you may use.

No Frozen Loft Pipes!

Your unvented system boiler won’t need a cold-water tank and this means you won’t have to store one in your loft! This results in there being no risk of frozen loft pipes and therefore there will be no risk of the subsequent flooding when the frozen pipes burst.

No Water Contamination

Unlike a regular boiler, there isn’t any water sitting around in a tank for days or weeks on end. An unvented cylinder system is completely sealed

You Don’t Need Gas

Even if your household doesn’t have a gas supply you will still be able to heat the water with an electric immersion heater.

Worcester Greenstore Product Pack Shot
Worcester Bosch Greenstore Unvented Cylinder Range

Disadvantages Of An Unvented Boiler System

They Take Up Space

Unvented system boilers will still take up some space in your home. Although they don’t need a cold-water tank, they still require the unvented cylinder. Even though a system boiler takes up less space than a regular boiler they still require a larger amount of space than a combi boiler.

The Tank Needs To Be Well Insulated

Any unused hot water wastes energy. If you don’t use up all the hot water in the cylinder, it will eventually go cold, meaning you’ve wasted money and energy heating it. To reduce the amount of heat lost and energy wastage, the unvented cylinder will need to be well insulated. Depending on where your unvented cylinder is installed, you may potentially need to invest in extra insulation for your unvented cylinder such as foamed polyurethane. You could end up spending more money than you initially planned to.

You Don’t Have Unlimited Hot Water On Demand

The unvented cylinder can only hold so much hot water, this means it can potentially run out if lots of hot water is used. The system boiler will then have to go through the process of heating more water and filling the tank again before your home has hot water again. The amount of hot water you can use will be dictated by the size of your unvented cylinder.

The Dependence On The Mains

If your mains system stops working, there is no backup store of water. The pressure of the hot water system will be limited to the pressure of the mains system so if your mains water supply has a weak pressure then the pressure of the hot water system will be equally as weak.

Incompatibility With Solid Fuel Burners

Unvented cylinders aren’t compatible with solid fuel burners or any other boiler without a thermostat unless extra safety measures are installed to combat the risk of the water overheating.

Safety Precautions Of An Unvented Hot Water System

You have to be aware that any system has the potential to become faulty Fortunately, unvented hot water systems are fitted with a series of safety measures to reduce the risk of danger.

These should include:

Temperature Relief

At the top of the tank there is a temperature/pressure relief valve so that water can be vented if it becomes too hot.

Pressure Relief

If the pressure of the system is too high or the expansion of the system is faulty, there are expansion relief valves to ensure the pressure never becomes dangerously high.

Temperature Regulators

The ideal temperature of the water in the unvented cylinder is 60-65 degrees Celsius:

Unvented boiler systems have thermostatic controls, to ensure the temperature of the water in the unvented cylinder does not exceed 100 degrees Celsius. If the water temperature reaches 85 degrees Celsius the energy supply to the boiler is cut off, to prevent a further rise in temperature. However, in the case that the temperature reaches 100 degrees Celsius the temperature/pressure and expansion relief valves are used.

Safety Discharge Pipe

A tundish collects the water from the release valves and feeds it into the safety discharge pipe. This pipe then takes away the hot water and steam and deposit it elsewhere safely outside the building where it cannot be a hazard and burn anyone. The pipe should be made from a high-temperature resistant material like copper. As the hot water flows away, cold water will then proceed into the cylinder. This helps to keep the water temperature down.

How Much Does Installing An Unvented Cylinder Actually Cost?

There are many different unvented boiler systems available so costs can vary. The cost of an unvented cylinder alone can range between £400 and £2000 without installation. It is vital to do research before buying one.

The cost of installation can also vary but, depending on how much work’s involved, be prepared to pay anything between £400 and £1,200 to install just the unvented cylinder from scratch.

If you are just replacing an unvented cylinder you will be looking at spending anything between £275 and £450. The larger your household, the larger your unvented cylinder will have to be, as expected the larger the cylinder the more expensive it will cost.

Remember it is very important for your unvented hot water cylinder to be installed by a specialist engineer with a G3 qualification that they obtained or renewed within the last five years.

Which Unvented Cylinder Should I Buy?

Best Unvented Boiler System Brands

There are many different unvented boiler systems available and research is key when picking the one that’s right for you. Some may be more expensive than others but here are the ones that I recommended.

Which Unvented Cylinder?
Which Unvented Cylinder Should I Buy?

Heatrae Sadia Megaflo

When it comes to electric heating and hot water products, Heatrae Sadia is top of the market. The Megaflo by Heatrae Sadia has been the top of the range Unvented Cylinder on the market for well over a decade and is one of their most popular products. They very simple to install and maintain. What’s unique about them is that they have an internal expansion vessel. This is part of the reason why they’re so tall in comparison with its competitors. However, top of the range has it’s price so this is an expensive option.

Worcester Bosch Greenstore

A very reliable boiler manufacturer and has a 5-star rating on the Trustpilot website. The Worcester Bosch Greenstore Unvented Cylinder is designed to work with its Greenstar boilers and Greenskies solar panels however they can be used with other brands too. They work seamlessly when installed with a Worcester Bosch System Boiler and their range of Digital Boiler Controls which makes them easy to install for the plumber and easy to use for the homeowner.

Telford Tempest

Telford Copper & Stainless Cylinders have been one of the market leaders in copper cylinder manufacturing. Their Tempest Stainless is their flagship stainless steel unvented cylinder. We never have an issue when installing these cylinders. Their aftercare service is also second to none and you pretty much get through straight away every time.

Gledhill Stainlesslite

The Stainlesslite Cylinders are among the lightest on the market. The company have been around since the 1960’s so have firmly established themselves as one of the major players in the cylinder game. They have the Stainlesslite Plus Horizontal Indirect cylinder which is unique as it can be installed horizontally in places where headroom is limited (like lofts).


If you have a large household and a high demand for hot water then an unvented boiler system may be perfect for you, especially if you have more than one bathroom as the performance improvement could be life changing!

However, if you have little competition for hot water in your household then a combi boiler may be a better alternative due as it will supply enough hot water to suit your needs and its more compact design means you will be saving space.

Take the time to consider all the points made above to help you decide on which boiler system is the most suitable for you and the needs of your household.

Whichever boiler system you choose for your home, remember that having a fully qualified, specialist engineer to install your boiler is vital and it is recommended to have your boiler system serviced annually.

Use this information as a guideline only but like I mentioned earlier, always consult an experienced, qualified Gas Safe Registered plumber to help you make the right decision for your home. It is not as simple as it sounds so leave it to us professionals!

If you liked this blog post and found it useful I’d be grateful if you could help by sharing it with your friends and family on Twitter or Facebook.

If you have any questions feel to free to comment down below and I’ll answer them personally.

If you live in South East London or Kent and need some advice in terms of unvented plumbing systems, then feel free to get in touch. We’d love to hear from you.

Richard Costello

Richard Costello, from Dartford in Kent, is the owner of MultiPlumb and has been involved in the plumbing industry installing bathrooms and boiler systems for over 15 years. If you have any questions or suggestions for future blog posts feel to get in touch at richard@multiplumb.co.uk.


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