Are Boiler Hot Water Storage Tanks a Necessity?

Whether you need a hot water storage tank depends largely on the type of heating system in your home and how much hot water you use.

A hot water cylinder stores heated water so it is ready for baths, showers and taps. This can be particularly useful in larger households where several outlets may need hot water around the same time.

A combi boiler works differently. It heats mains water on demand rather than storing a separate supply, so a hot water cylinder isn't normally required.

Understanding the differences can help you decide which arrangement is better suited to your home.

What is a hot water storage tank?

A hot water storage tank, commonly called a hot water cylinder, stores water that has been heated ready for use around your home.

Depending on the system, the water may be heated indirectly using a boiler or heat pump, or directly using an electrical heating element such as an immersion heater.

Unlike a combi boiler, which heats hot water when you open a tap, a cylinder keeps a volume of heated water available until it is required.

Hot water cylinders are available in different sizes and configurations, so the appropriate option depends on factors including your heating system, available space and household hot water demand.

Do you need a hot water cylinder with a boiler?

Not always.

Whether your boiler requires a cylinder depends on the type of boiler and heating system installed.

Combi boilers provide central heating and heat hot water on demand, so they don't normally need a separate hot water cylinder.

System boilers are designed to work with a separate hot water cylinder. Many of the heating components are incorporated within the boiler itself, but hot water is stored separately.

Conventional boilers, sometimes called regular or heat-only boilers, also work with a hot water cylinder. Depending on the system, they may also use a cold water storage cistern.

This distinction is important if you're considering replacing your boiler, as changing boiler type can affect other parts of the heating and hot-water system.

Vented vs unvented hot water cylinders

One of the main distinctions between hot water cylinders is whether they are vented or unvented.

Vented hot water cylinders

A vented cylinder is supplied from a separate cold water storage cistern, typically located in the loft.

These systems use gravity to supply water from the cistern to the cylinder. As a result, the hot-water performance at outlets can be influenced by the height of the cold water cistern and the design of the system.

Vented cylinders remain common in properties with traditional heating and plumbing arrangements.

Unvented hot water cylinders

An unvented cylinder is supplied directly from the mains water supply rather than a separate cold water storage cistern.

This can provide strong hot-water performance at multiple outlets where the incoming mains supply has sufficient pressure and flow.

Because an unvented cylinder stores water under pressure and incorporates important safety devices, installation and certain types of work on the system require appropriate competence.

The best option depends on your existing plumbing, available mains supply, property and hot-water requirements.

Direct vs indirect hot water cylinders

Hot water cylinders can also be described as direct or indirect, depending on how the stored water is heated.

Direct cylinders

A direct cylinder heats its water using an electrical heating element, usually an immersion heater.

Because it doesn't rely on a boiler to heat the stored water through a separate primary circuit, a direct cylinder can be suitable for properties without a conventional boiler connection.

Indirect cylinders

An indirect cylinder receives heat from an external source, such as a boiler, through a coil or heat exchanger inside the cylinder.

The boiler heats water within a separate circuit, which passes through the coil and transfers heat to the stored domestic hot water.

Many indirect cylinders also have an immersion heater that can provide an alternative or backup method of heating the water.

What are the advantages of a hot water cylinder?

One of the main advantages of storing hot water is having a volume of heated water available for use.

This can make a cylinder-based system particularly suitable for properties with several bathrooms or higher simultaneous hot-water demand. The appropriate performance still depends on factors including cylinder capacity, recovery time and the design of the wider system.

A cylinder can also provide flexibility over how water is heated. Depending on the installation, stored water may be heated by a boiler, heat pump or electrical immersion heater.

An immersion heater can also provide a useful alternative method of producing hot water on compatible systems if the primary heat source is unavailable.

What are the disadvantages of a hot water cylinder?

The most obvious consideration is space. A cylinder requires somewhere suitable to be installed, whereas a combi boiler avoids the need for separate hot-water storage.

The amount of immediately available hot water is also limited by the cylinder's usable capacity. If a large amount is used in a short period, you may need to wait while the cylinder reheats.

Stored hot water also loses some heat over time. Modern cylinders are insulated to limit these standing heat losses, but appropriate insulation and sensible heating controls remain important.

Choosing an unnecessarily large cylinder can also mean heating and storing more water than the household typically requires, while an undersized cylinder may struggle to meet demand.

Hot water cylinder vs combi boiler

The main difference is how domestic hot water is provided.

A cylinder-based system heats water in advance and stores it until you need it. This gives the household a reserve of hot water and can be useful where several showers, baths or taps may be used around the same time.

A combi boiler doesn't normally store domestic hot water. Instead, it takes water from the mains and heats it when a hot tap or shower is used.

This removes the need for a separate cylinder and can save space. However, hot-water performance depends on the boiler's output and the incoming mains water supply. A combi may therefore be less suitable for a property with particularly high simultaneous hot-water demand.

Neither arrangement is automatically better. The right choice depends on the property and how its occupants use hot water.

What size hot water cylinder do you need?

The correct cylinder size depends on your household's expected hot-water demand rather than property size alone.

Important factors include:

• Number of occupants.

• Number of bathrooms.

• How frequently baths and showers are used.

• Whether several hot-water outlets are likely to be used simultaneously.

• How quickly the cylinder can reheat.

• The available installation space.

A cylinder that's too small may run out of usable hot water during periods of high demand. An unnecessarily large cylinder can take up more space and result in more stored water than the household needs.

Generic litre recommendations can provide a rough starting point, but they shouldn't replace a proper assessment of the household and heating system.

How do you choose the right hot water cylinder?

Start with your existing heating system.

If you have a system or conventional boiler, you'll generally need a compatible cylinder capable of meeting your household's hot-water requirements. The choice between vented and unvented will also depend on factors such as the existing plumbing and incoming mains water supply.

You should also consider cylinder capacity, recovery performance, insulation and the space available for installation.

If you're considering an unvented cylinder, the incoming mains pressure and flow need to be suitable for the expected demand. Simply choosing an unvented cylinder doesn't guarantee strong performance if the incoming water supply can't support it.

A heating professional can assess the property and recommend an appropriate cylinder type and capacity rather than relying on household size alone.

Are hot water cylinders energy efficient?

A modern, well-insulated hot water cylinder can form part of an efficient heating and hot-water system.

However, storing hot water inevitably involves some heat loss. The quality of the cylinder's insulation, the temperature of the stored water and how the heating is controlled can all influence those losses.

This doesn't mean cylinders are inherently inefficient. For households with significant hot-water demand, storing an appropriate quantity of hot water can be a practical way to supply multiple outlets.

The cylinder should also be correctly sized. Heating substantially more water than the household regularly needs can result in unnecessary energy use.

Do hot water cylinders need an immersion heater?

Not every cylinder relies on an immersion heater as its primary heat source.

A direct cylinder normally uses an electrical immersion heater to heat its water. An indirect cylinder is typically heated by an external heat source, such as a boiler, through a coil inside the cylinder.

Many indirect cylinders also include an immersion heater. This can provide an alternative way to heat the stored water if required.

How and when an immersion heater should be used depends on the cylinder and heating system, so follow the manufacturer's instructions for your installation.

Hot water storage tank FAQs

Does a combi boiler need a hot water tank?

No. A standard combi boiler heats domestic hot water on demand and therefore doesn't normally require a separate hot water cylinder.

This is one reason combi boilers can work well in properties where space for a cylinder is limited.

Does a system boiler need a hot water cylinder?

Yes. A system boiler works with a separate cylinder that stores hot water ready for use.

This arrangement can suit homes with greater hot-water demand because the cylinder provides a stored supply rather than relying solely on instantaneous hot-water production.

What's the difference between a hot water tank and a hot water cylinder?

The terms are often used interchangeably when discussing domestic stored hot water.

In UK heating systems, hot water cylinder is generally the more specific term for the vessel used to store heated domestic water.

Is an unvented cylinder better than a vented cylinder?

Not necessarily.

Unvented cylinders can provide strong hot-water performance without requiring a separate cold water storage cistern, provided the incoming mains supply is suitable.

Vented cylinders can remain appropriate for homes with existing gravity-fed systems.

The better option depends on the property's plumbing, water supply, available space and hot-water requirements.

Can you run out of hot water with a cylinder?

Yes. A cylinder stores a finite amount of usable hot water.

If demand exceeds the available stored supply, you'll need to wait for the cylinder to reheat. Choosing an appropriate cylinder capacity and recovery performance helps reduce the likelihood of this happening regularly.

Are hot water cylinders necessary with modern boilers?

It depends on the type of boiler.

A modern combi boiler doesn't normally need a cylinder, while system and conventional boiler arrangements use stored hot water.

The age of the boiler therefore isn't the deciding factor. The design of the heating and hot-water system is what determines whether a cylinder is required.

Choosing the right hot water system for your home

Hot water storage tanks aren't necessary for every home, but they remain an important part of many heating systems.

A combi boiler can provide heating and hot water without a separate cylinder, making it a practical option where space is limited and hot-water demand is suitable. A cylinder-based system can be better suited to properties that need a stored supply of hot water, particularly where several outlets may be used around the same time.

If you're replacing your boiler or changing the way your home produces hot water, consider the existing heating system, available space, mains water supply and household demand before deciding whether to keep, replace or remove a hot water cylinder.