Unvented hot water cylinders have become more popular in the last couple of decades because they draw water directly from the mains, meaning there’s no need for a water tank in the loft. But how much do they cost? We explain everything you need to know about having a new unvented hot water cylinder installed.

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

Replacing an unvented hot water cylinder is a pricey job. The cylinder alone can cost between £450 and £2,500 depending on the make, plus the price of labour, which might include your heating engineer and a helper needing a full day to install and fit the cylinder. 

These prices are just estimates and may not include VAT. Various factors can affect the cost, including the hot water cylinder you choose, whether you are replacing like-for-like and your location.

If your heating engineer is replacing a vented cylinder with an unvented one, then this will cost more. They will need to spend extra time making sure your pipework can handle higher water pressure, so expect to spend up to an extra day’s labour.

How does an unvented hot water cylinder work?

An unvented hot water cylinder receives cold water directly from the mains water supply and supplies all of your hot taps and radiators with hot water at mains pressure.

The cold water comes into the unvented hot water cylinder and is then heated by either an immersion heater or a boiler before being sent to the tap or radiator requesting hot water.

Unvented hot water cylinders can be both direct or indirect:

• direct unvented cylinders use electricity to heat water using immersion heaters
• indirect unvented cylinders use gas from a boiler to heat the water 

As an unvented hot water cylinder is fed from your mains water supply, there’s no need to have a water tank in the loft that feeds the cylinder. This also means that you should get better water pressure as the cylinder is relying on your mains water pressure rather than pressure from gravity. 

Why install an unvented cylinder?

There are lots of reasons to install an unvented cylinder, including reasons related to space, water pressure and renewable energy:

• No need for a gravity-fed water tank in the loft, freeing up space in your home
• Removes the risk of pipes or the tank in the loft freezing in the winter
• Available in slimline options taking up even less space - a well-known brand to look out for is Gledhill's Stainless Lite Plus range which can be installed almost anywhere
• Water is supplied from the mains to give a better, more reliable flow rate your the hot water taps
• Supply multiple showers or hot taps at the same time 
• Can be used alongside solar thermal panels so you can heat water using energy from the sun
• Usually quieter than vented cylinders, ideal if your cylinder is stored in a bedroom cupboard 

A popular unvented cylinder option is Megaflo, manufactured by Heatrae Sadia (often misspelt as Megaflow system or Megaflow boiler). It’s a stainless steel unvented cylinder that stores hot water as part of a central heating system.

Reasons that make the Megaflo range popular are:

• water pressure in the system is consistent even if more than one outlet is being used at the same time, so it is suitable for families
• easy to install, and parts are easier to replace and maintain
• compatible with solar PV
• fitted with several different safety features, including a thermostat to shut the boiler down when it has reached the correct temperature. If it becomes too hot, the Temperature/Pressure Relief Valve (TPRV) would dump water out of the cylinder into a drain to reduce pressure

Disadvantages of unvented cylinders

Unvented hot water cylinders offer excellent water pressure and efficiency, but they do come with some drawbacks:

• Unvented systems have complex components like pressure relief valves and expansion vessels, making them more expensive to install than traditional vented cylinders
• If your property has low mains water pressure, an unvented system may not perform well without additional pumps or pressure-boosting solutions
• If not installed or maintained properly, unvented systems can suffer from pressure build-up, requiring safety mechanisms to prevent damage
• Although they don’t need a loft tank, unvented cylinders still require adequate space for the unit and associated pipework

It’s best to speak to a heating engineer who can advise you on whether an unvented cylinder is the best option for your home.

What size unvented cylinder do I need?

Choosing the right size unvented hot water cylinder is crucial to ensuring you have enough hot water without wasting energy or money on an oversized unit. The ideal size depends on household size, water usage habits and the number of bathrooms in your home.

Here’s a rough guide to help determine the appropriate capacity for your needs:

• 1-2 people (1 bathroom): 120-180 litres
• 3-4 people (1–2 bathrooms): 140-220 litres
• 4-5 people (2 bathrooms): 160-230 litres
• 5+ people (2+ bathrooms): 260+ litres

Factors to consider when you’re choosing a hot water cylinder size

• Number of bathrooms: Homes with multiple bathrooms require a larger cylinder to meet demand, especially if showers or taps are used simultaneously
• Shower type: Power showers and rainfall showers consume more hot water than standard showers, so a larger cylinder may be necessary
• Hot water demand: If multiple people shower at the same time, you’ll need a larger cylinder to prevent running out of hot water
• Reheat time: Some modern cylinders have fast reheat times, meaning you may get away with a slightly smaller unit that can heat water quickly between uses

Can I replace my vented cylinder with an unvented cylinder?

You can replace your vented cylinder with an unvented one, but it will cost more and take longer to install. Your heating engineer may need to replace some of your pipework to ensure that it can handle water flowing at a higher pressure. 

A lot of UK homes prefer to upgrade the system to the unvented hot water cylinder to make use of the space that has been housing a water feed tank. Another idea is if you have a combi boiler, you can also add an unvented hot water cylinder to the system. This will help with demand if your home has multiple bathrooms and where the demand for hot water is spread over different hot taps.

How long do unvented hot water cylinders last?

Unvented hot water cylinders typically have a longer lifespan than vented cylinders, and as long as they are maintained, they should last at least 20 years. However, it’s not uncommon for them to last 25-30 years! 

There are a few factors that will affect how long your unvented cylinder lasts: 

• Material quality: Stainless steel cylinders tend to last longer and resist corrosion better than other materials
• Water hardness: Hard water areas cause limescale buildup, reducing efficiency and lifespan. Installing a water softener can help
• Maintenance: Regular servicing, including checking the expansion vessel and pressure relief valves, extends the cylinder’s life
• Manufacturer & warranty: Higher-end cylinders often come with extended warranties (up to 25 years) and better build quality

If you notice damp patches around the cylinder or any signs of leaks, the cylinder may have a small hole or a crack and it may be time to replace the cylinder before damage is caused to your home.

Which cylinder should be installed, vented or unvented?

Both vented and unvented cylinders come with their own set of advantages and disadvantages. Older properties tend to have vented systems, so it’s usually cheaper to have a like-for-like replacement. 

Newer properties are fitted with either combi systems or unvented ‘sealed’ systems. Unvented is a better choice between vented and unvented for space saving as well as for consistent higher pressure.

Eliminating the need for the water tank in the loft also avoids the damage of another potential area for a water leak and pipes freezing in winter. There is also no concern about the threat of contaminated water in case any debris gets into the water tank in the loft.

However, if the mains water pressure coming into your home is low, then a vented cylinder system would be the better choice. 

Where is the best place to put an unvented cylinder?

If you’re having an unvented cylinder for the first time, you’ve got the luxury of being able to choose where you put it. The best place to put an unvented cylinder is somewhere that is easily accessible for servicing, but that will cause you the least disruption to your home life:

• Airing cupboard: The most common location, as it provides easy access for maintenance and helps retain heat within the home
• Utility room or garage: Suitable if space inside the house is limited, but additional insulation may be required to prevent heat loss
• Under the stairs: A compact solution for smaller homes, though access for servicing must be considered.

The best location depends on available space, accessibility and pipework considerations to ensure optimal performance. Your heating engineer will be able to advise you on the best place.

How long does it take to fit a new hot water cylinder?

The time required to install a new hot water cylinder varies based on the complexity of the job. Typical installation times include:

• Like-for-like replacement: Replacing an old unvented cylinder with a new one usually takes 4–6 hours.
• Upgrading from a vented to an unvented system: This involves removing the loft tank, upgrading pipework, and installing safety components, taking 1-2 days.
• New installations in a different location – If additional pipework, structural reinforcements, or electrical work is needed, the process can take 2-3 days.

Professional installers aim to complete the work as efficiently as possible while ensuring compliance with building regulations.

Can I fit my own unvented cylinder?

No, installing an unvented hot water cylinder is not a DIY job. By law, unvented cylinders must be installed by a qualified professional with G3 certification to ensure safety and compliance with UK regulations.

• Legal requirements: Unvented systems operate under high pressure, requiring safety features such as expansion vessels, pressure relief valves and thermostatic controls. Incorrect installation can lead to dangerous pressure build-up.
• Building Regulations – A G3-certified engineer must notify local building control when fitting an unvented cylinder. DIY installation could lead to non-compliance and potential fines.
• Warranty & insurance risks: Many manufacturers void warranties if a cylinder is not professionally installed. Incorrect installation can also affect home insurance coverage.

While you can carry out basic maintenance tasks, such as checking the pressure or draining the system, full installation must be left to a qualified professional.

If you’re ready to get quotes for a new unvented hot water cylinder, post your job on HaMuch now. We’ll put you in touch with qualified Gas Safe-registered heating engineers in your area who will be happy to help.

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