Unvented hot water systems explained Re April 26, 2021

Unvented hot water systems explained

Vaillant boiler

There are two main types of water heating systems used in homes, vented and unvented. Vented systems are more common, however, unvented are still a popular choice. 

Unvented hot water systems require less space for installation in your home and provide satisfactory flow rates to your taps and appliances.

You may not know what kind of system you have in your home, or what the difference is. This blog aims to give an in-depth explanation of how an unvented hot water system works in comparison to a vented system and its benefits to help you identify what system you have in your home.

What is an unvented hot water system?

Unvented hot water cylinders store water that is supplied directly from the mains water supply. The cold water from the mains is then heated using the heat from the boiler or using other electrical heating elements, which provides high-pressure hot water to the feeding outlets. The increase in your hot water pressure with help to deliver better quality in your showers, without the need for additional pumps. 

Unvented hot water systems are ideal for homes with limited space, as they remove the need for any additional cold water storage tanks, this frees up space in your home and means that a lot less pipework is required.

How does an unvented hot water system work?

Cold water is supplied directly from your mains. When you open your tap, the high pressure from the cold water enters the unvented hot water tank from the bottom. The pressure then forces the hot water out of the top, which then comes out of your taps and showers.

There are two types of unvented hot water systems, direct and indirect. A direct unvented hot water system uses electricity to heat the water using immersion heaters, whilst a direct unvented cylinder utilises gas from your boiler to heat the water. 

Not sure what an immersion heater is or how it works? We have a blog that explains everything you need to know. 

Because water expands when it is hot, there is a need to account for this additional space needed. An expansion vessel will be attached somewhere to the system to accommodate this. It may be attached to the cylinder, attached to the hot water system somewhere else, or even be part of the cylinder itself. For this reason, you will need a qualified G3 specialist to install your cylinder, which could result in the price of the installation being more expensive compared to a vented cylinder. You will also need to maintain your unvented system regularly to ensure it meets safety regulations.

Your unvented cylinder also includes additional safeguards against overpressure situations, such as a pressure-reducing valve to limit incoming pressure and an expansion relief valve to allow overheated water to be discharged. For added protection, a  temperature and pressure relief valve is also fitted to your cylinder.

What is the difference between an unvented and vented system?

Vented cylinders are the more traditional option. Vented hot water systems have been around for decades, originally being manufactured using copper due to its efficient conductivity of heat. However, over the years, they have now introduced a stainless steel option in addition to the copper option. 

A vented system is fed directly from a tank above, usually installed in the attic of your home. Thanks to the natural gravitational force of the water when entering the system, this enables a flow of hot water for your taps and other outlets throughout your home. 

A vented system requires next to no maintenance but will require enough space in the home to be installed alongside the feed, in your attic. Therefore, vented hot water systems are not recommended for homes with limited space.


Which is better?

It depends on the demands of your home and what is better for you. It is important to space in your home, your budget, maintenance costs and warranty when deciding whether or not a vented or unvented system is more suitable for your home. 

If budget is the main concern, then a vented system may be a better option. However, if space is limited in your home, an unvented system would be a better choice, especially when slimline and horizontal cylinder options are available.

The pros and cons of having an unvented system installed


  • Pressure is much higher on all outlets
  • No loft tanks needed
  • Reduced risk of contamination and legionnaires
  • No risk of frozen pipes in the loft 
  • Very high heat recovery rate after use 
  • Warm-up time is only 15-20 minutes
  • Can be installed almost anywhere in your home
  • Provides better flow rates


  • Hot water pressure is dependant on your mains
  • Shower pumps can not be fitted on a mains water supply 
  • When feeding multiple outlets the pressure can drop. This also applies to vented systems. 
  • Requires a G3 specialist to install 
  • More expensive to install

What size cylinder do I need for my system?

When purchasing a new or replacement hot water cylinder, whether that be vented or unvented, it is important to ensure that the size of your cylinder is correct for the demand you require in your home. 

Most families with between 2-4 people, will only require a 150-litre tank. This will provide hot water for homes with a bathroom, a shower, and an en-suite. For larger households with higher demand, you should be looking at a 200+ litre tank. 

Cylinder capacity is usually measured in litres. The simplest way to work out what you need is to base it on the number of bedrooms, bathrooms, and showers.

The capacity you need can differ depending on whether you are directly or indirectly heating the water; this is because cylinders that are heated by boilers (indirectly) tend to reheat quicker.

The below figures are used by industry professionals as a guideline:

BedroomsShowers/BathroomsIndirect (litres)Direct (litres)
Larger Properties 300+300+

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

On average, a cylinder can cost anywhere from £500-£1000. You should expect to add an extra £275-£400 for labour. The final price will depend on the ease of installation for your heating engineer. Remember that you will need a qualified G3 specialist to install an unvented hot water system for you.

How will I know when I need to replace my hot water cylinder?

There is a good chance that you will not need to replace your hot water cylinder unless you are converting your heating system, moving your existing cylinders or your cylinder is leaking. 

It is important to remember that you will need to have your unvented hot water system regularly maintained in order for it to comply with safety regulations.

When purchasing a new or replacement unvented hot water cylinder, make sure that you consider the warranty. Manufacturers such as Worcester Bosch offer a 25-year warranty on their Greenstore cylinders.

What are the best hot water cylinders?

Unvented cylinders need to be able to store water under pressure so need to strong and as resistant to corrosion as possible. They are usually made from copper, stainless steel, or glass-lined mild steel.

The duplex stainless steel is generally considered the best type of cylinder as it is durable, corrosion-resistant, and lightweight. Models such as the Megaflo (often referred to as Megaflow) by Heatrae Sadia and the StainlessLite by Gledhill are great examples of this.

If a cylinder is made from copper and you should make sure it is well insulated to prevent heat loss. Most cylinders include some level of insulation such as foamed polyurethane, but you can further insulate your hot water tank and pipes using a jacket and foam tubing.

It’s also important that the immersion heater is made from a corrosion-resistant material. Titanium is ideal for a direct cylinder or Incoloy for an indirect. You should also consider whether your water is ‘hard’ or ‘soft’ as this will influence your choice.

There are several top quality brands selling hot water cylinders on the market, however, if you would like some guidance, we have listed a few below:

Cylinder ModelSizes Available (volume in litres)ErP Rating Heating MethodWarranty 
Megaflo Eco by Heatrae Sadia70 / 125 / 145 / 170 / 210 / 250 / 300BIndirect or DirectLifetime
Greenstore by Worcester Bosch90 / 120 / 150 / 180 / 210 / 250 / 300BIndirect only25 years
Vitocell 200-V by Viessmann90 / 120 / 150 /180 / 210 / 250 / 300CIndirect but includes a backup immersion heater25 years against corrosion
StainlessLite by Gledhill90 / 120 / 150 180 / 250 / 300 / 400CIndirect or direct25 years
Tempest by Telford

90 / 125 / 150 / 170 / 200 / 250 / 300

CIndirect or direct25 years

Common hot water cylinder problems

Typically, hot water cylinders are very reliable so you shouldn’t be faced with a problem, however, in the case that there is an issue, here are some of the more common hot water problems.

  • No hot water
  • Not enough hot water
  • Water is too hot
  • Discoloured water
  • Foul smell to the water
  • Loud and strange noises
  • No water at all

If you’re experiencing any of the above problems with your vented or unvented hot water cylinder, please contact a heating engineer who should be able to resolve this for you.

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