Types of Gas Boilers Explained – Combi, Heat Only & System Boilers

Choosing the right gas boiler for your home

There are three main types of gas boilers you can install in your home: combi boilers, heat-only (also called regular or conventional boilers) and system boilers. The type of boiler most suitable for your home depends largely on your hot water demands and the pressure of your mains water supply. Below we’ll explain the different types of gas boilers available for your central heating system. Always use a gas safe registered company to replace your gas boiler. Combination boilers are the most popular choice of boiler for most homes.

Types of Boiler

Combination Boilers (AKA combi boilers)

A combination boiler, often referred to as a combi boiler, is a type of gas boilers that serves two main functions: providing hot water for domestic use and heating water for central heating systems. Unlike traditional boilers that require separate hot water cylinders or tanks, a combi boiler heats water directly from the mains supply as and when needed, therefore eliminating the need for storage tanks.

Here’s how a combi boiler works:

1. Central Heating

   : When you turn on your heating system, the combi boiler fires up and heats water from the mains supply. This hot water is then circulated through the radiators or underfloor heating system, providing warmth to your home.

2. Hot Water Supply

   : Similarly, when you turn on a hot water tap, the combi boiler heats water directly from the mains supply and delivers it instantly to the tap. This means you have access to hot water on demand, without the need for a separate hot water storage tank.

Key features and benefits of combination boilers include:

• Space-saving

  : Combi boilers are compact units that do not require additional storage tanks, making them ideal for homes with limited space.

• Efficiency

  : By heating water only when needed, combi boilers are highly efficient, reducing energy consumption and lowering utility bills.

• Instant Hot Water

  : With a combi boiler, hot water is available on demand, eliminating the need to wait for a storage tank to heat up.

• Constant Pressure

  : Combi boilers operate at mains pressure, providing a consistent flow of hot water to taps and showers throughout the home..

Overall, combination boilers offer convenience, efficiency, and space-saving benefits, making them a popular choice for modern homes looking to streamline their heating and hot water systems.

When considering a combi boiler, you also need to think about how many water supplies will be used at the same time, for example would you likely run 2 showers at the same time. When using two or more water outlets at the same time, water flow rates are reduced as a combi boiler can only heat the water which is fed to it from the mains. Low water pressure in your area could also be unsuitable for a combi boiler.

Heat Only Boilers (AKA Regular or Conventional Boilers)

Heat-only boilers, also known as regular boilers or conventional boilers, are a type of gas boilers that works alongside a separate hot water storage cylinder and cold water storage tank. Unlike combination boilers, heat-only boilers are designed solely to provide heating for the home and do not directly heat water for domestic use. Instead, they heat water in the central heating system, which is then distributed to radiators or underfloor heating systems.

Here’s how a heat-only boiler works:

1. Heating Water

   : The heat-only boiler heats water in the central heating system. This heated water is circulated through pipes to radiators or underfloor heating systems, providing warmth to the various rooms in the home.

2. Hot Water Cylinder

   : To supply hot water for domestic use, a separate hot water cylinder is installed alongside the heat-only boiler. This cylinder is typically located in an airing cupboard or loft space and is connected to the boiler via a coil or immersion heater. The hot water cylinder stores heated water for use in taps, showers, and appliances throughout the home.

3. Cold Water Storage Tank

   : A cold water storage tank, usually located in the loft, supplies cold water to the hot water cylinder. This tank ensures a constant supply of cold water for the heating system and domestic use.

Key features and benefits of heat-only boilers include:

• Flexible Installation

  : Heat-only boilers are suitable for homes with existing hot water cylinders and cold water storage tanks, making them a versatile option for replacement installations.

• Hot Water Storage

  : By using a separate hot water cylinder, heat-only boilers can supply hot water to multiple taps and appliances simultaneously, making them suitable for larger households with higher hot water demand.

• Compatible with Renewable Technologies

  : Heat-only boilers can be easily integrated with renewable heating technologies, such as solar thermal panels or heat pumps, to further reduce energy consumption and carbon emissions.

• Reliability

  : Heat-only boilers have a simple design and fewer components than combination boilers, making them reliable and easy to maintain.

While heat-only boilers may require more space for hot water storage cylinders and cold water tanks, they remain a popular choice for traditional heating systems, particularly in properties with multiple bathrooms or high hot water demand.

System Boilers

System boilers are a type of gas boilers that works alongside a hot water storage cylinder, but unlike conventional heat-only boilers, they don’t require a separate cold water storage tank. Instead, they incorporate most of the major components of the heating and hot water system into a single unit, making them more space-efficient than heat-only boilers.

Here’s how a system boiler works:

1. Heating Water

   : Similar to heat-only boilers, system boilers heat water in a central heating system. The boiler heats water and pumps it through the radiators or underfloor heating system to provide warmth to the home.

2. Hot Water Cylinder

   : System boilers are connected to a hot water storage cylinder, typically an unvented hot water cylinder. These cylinders are pressurized and do not require a separate cold water storage tank. They are usually located in an airing cupboard or loft space. The unvented design provides a greater flow of hot water for domestic use, including taps, showers, and appliances.

3. No Cold Water Storage Tank

   : Unlike conventional boilers, system boilers do not require a separate cold water storage tank. Instead, they are connected directly to the mains water supply, providing a constant flow of water to the unvented hot water cylinder. This setup eliminates the need for additional storage space and ensures a reliable and consistent supply of hot water throughout the home.

Key features and benefits of system boilers include:

• Space-saving

  : System boilers are compact units that combine the boiler and hot water storage cylinder into one, saving space compared to traditional heat-only boiler systems.

• Efficiency

  : System boilers are highly efficient, as they heat water on demand and only require minimal heat loss through the hot water cylinder. This can lead to lower energy bills and reduced carbon emissions.

• Fast Hot Water Recovery

  : With a dedicated hot water cylinder, system boilers can provide a constant supply of hot water to multiple taps and appliances simultaneously, making them suitable for larger households with higher hot water demand.

• Compatible with Solar Thermal Panels

  : System boilers can be easily integrated with renewable heating technologies, such as solar thermal panels, to further reduce energy consumption and environmental impact.

• Simple Installation

  : Installation of a system boiler is typically quicker and more straightforward than heat-only boilers, as there is no need for a separate cold water storage tank.

Overall, system boilers offer a convenient and efficient heating and hot water solution for homes with moderate to high hot water demand. Their space-saving design and compatibility with renewable technologies make them a popular choice for modern heating systems.

A system boiler is very similar to a Heat only boiler as the boiler heats the central heating system and a separate water storage tank or cylinder is required, however it differs when it comes to the cold-water supply as this is supplied to the boiler from the mains, therefore eliminating the need for a cold-water storage tank in the attic. This is called an unvented cylinder where all the components such as the pump and expansion vessel are internalised.

Great for space saving as there’s no extra component needed in the attic, but the hot water flow rate depends on the main supply so the water pressure in your area also needs to be considered.

Things to consider when choosing the right type of boiler for your home

1. ALWAYS check the engineer installing your new boiler is registered on the Gas-Safe Register
2. If swapping from a combi boiler to a system boiler, consider where you will place the water cylinder
3. How many water outlets does your household use at any one time? Is there more than one shower running in the morning?
4. do you want a long warranty on your new boiler?

What boiler suits my home?

A practical method for determining the appropriate boiler size is to count the number of radiators throughout your home. The quantity of radiators directly influences the boiler’s power requirement to ensure they all maintain optimal heat. As a rough guide, we would advise to consider the following power outputs for different numbers of radiators or by property type in order to determine what size boiler is recommended for your home:

For Combi Boilers:

• Up to 10 radiators: 25– 30 kW
• 10 to 15 radiators: 30–35 kW
• 15 to 20 radiators: 35–40 kW

OR

• 1 – 2 bedroom flat with 1 bathroom  – 25 kw.
• 2 bedroom apartment with 2 bathrooms – 25 – 35 kw ( 35kw will deliver more Litres per minute of hot water )
• 2 bedroom semi with 1 bathroom – 25 kw
• 3 bedroom semi with 1 bathroom – 30 kw
• 3 bedroom semi with 2 bathrooms – 30 – 35 kw
• 4 bedroom semi with 1 bathroom – 30 – 35 kw

For Heat Only or System Boilers:

• Up to 10 radiators: 12–15 kW
• 10 to 15 radiators: 18–24 kW
• 15 to 20 radiators: 30–40 kW

OR

• 3 bedroom semi with 6 radiators and 1 bathroom – 12 kw
• 3 bedroom semi with 8 radiators and 1 bathroom – 15 kw
• 4 bedroom semi with 10 radiators and 2 bathrooms – 18 kw
• 5 bedroom detached with 15 radiators and 2 bathrooms – 30 kw
• 5 bedroom detached with 20 radiators and 3 bathrooms – 40kw

Combi boilers require higher power outputs because they provide instant hot water, whereas system and heat only boilers heat water more gradually by circulating it around the cylinder. Although combi boilers may seem less efficient, they operate differently. Typically, a combi boiler will only operate at maximum capacity for brief periods when performing tasks such as washing, bathing, showering, or washing dishes. In contrast, system and conventional boilers maintain heat for longer periods as they heat up the entire cylinder.

Team Expert Heat hope this blog has helped with what type of gas boilers is best for you and answered the question, What boiler suits my home.