Types of Boilers: Combi, Heat Only & System Boilers



If you are thinking of installing a new boiler in your home for the upcoming winter season, or just want basic knowledge related to types of boilers, then we have got you covered. Welcome to the ultimate boiler guide, where we will dispel the myths surrounding the three major contenders: Combi, Heat Only, and System boilers. We will also try to include other boiler types for broader understanding and helping you choose the perfect boiler for your home.

As the core of any central heating system, choosing the perfect and proper boiler is very important for your house’s comfort and energy efficiency. In this detailed and comprehensive guide, we will guide you on the working, installation, maintenance, pros and cons of different types of boilers. So, take a cup of tea, relax back, and prepare to be immersed in the fascinating world of boilers!

What is a Boiler?

Before we dive deep into the boiler types, let’s quickly understand what actually is a boiler? A boiler is a vital appliance that is meant to supply heat (and water) to your house. It functions as a beating part of the central heating system. It is the miniature furnace that heats the water that passes past it. This hot water may then be supplied to your central heating system, utilized to provide on-demand hot water, or saved for later use.

A boiler works on a simple principle: it turns energy, such as natural gas, oil, electricity, or even biomass, into heat energy, which is then transmitted to water to increase its temperature. The hot water or steam is subsequently pumped throughout the structure, through radiators or underfloor pipes, providing warmth and comfort.

How are Boilers Categorized?

Now, we know that what is a boiler, let’s see how they are classified into different types. They are categorized on the basis of multiple factors, such as operating pressure, the temperature they operate on, size, design, capacity, fuel type, and the type of heating system they are compatible with.

Types of Boilers

There are many different types of boilers but the most common are:

  • Combi Boilers
  • Heat Only (Conventional) Boilers
  • System Boilers

1. Combi Boilers

Out of all types of boilers, the combi boiler is the most commonly used boiler. Combi boilers, also called combination boilers includes all the components need to both heat and distribute your water, allowing you to have both hot water and heating from the same machine. This makes the installation of combi boilers very easy.

Working of Combi Boilers

Combi boilers heat water directly from the mains supply anytime a hot water tap or central heating system is switched on. A dual-function heat exchanger is used in the process, which swiftly warms the water as it comes through, eliminating the need for a separate hot water cylinder or storage tank.

When you switch on a hot water faucet, the combi boiler rapidly warms the water to the right temperature, providing a constant supply of hot water on demand. Furthermore, when the central heating is turned on, the same heat exchanger distributes hot water via the radiators, offering a rapid and energy-efficient heating option for the house.

Installation and Maintenance of Combi Boilers

Combi boilers are very easy and straightforward to install, but it is essential to let a professional gas-safe engineer do the job for you, so he can follow all safety protocols. The engineer will connect the boiler to the mains water supply and central heating system.

Regular maintenance of combi boilers is very crucial to keep them running smoothly and effectively in top condition. Regular maintenance will include boiler inspection, boiler cleaning, flue inspections, safety checks, etc. If there are any potential issues found, the technician will sort that out for you, giving your boiler a prolonged lifespan and guaranteeing its efficiency, giving you and your loved ones comfort and heat for upcoming winter seasons.

Advantages of Combi Boilers

Some common advantages of combi boilers are:

  • Energy Efficient: Combi Boilers are highly energy efficient, providing up to 90% efficiency.
  • Instant Heating: They instantly heat the water for you to use and take a hot shower.
  • Cost Effective: Combi Boilers are cost-effective providing you with monthly savings on energy bills.
  • Ideal Size: They are available in compact sizes ideal for even smaller homes and properties. They provide a more clean look when installed as everything is packed up in one single unit.
  • Maintenance: As combi boilers are very common, it is very easy to find a gas-safe engineer for the maintenance of combi boilers. The maintenance cost of combi boilers is very cheap.
  • Lower Carbon Footprint: Combi boilers’ energy-efficient operation leads to lower carbon emissions, making them ecologically friendly.

Disadvantages of Combi Boilers

Some common disadvantages of combi boilers are:

  • Reduced Water Flows: When two or more outlets are utilized at the same time, water flow rates are lowered.
  • Water Pressure: Combi boilers rely on mains pressure to operate, thus they won’t perform as effectively if you have low flow rates or incoming water pressure.
  • Solar Incompatibility: Almost all combi boilers are incompatible with solar water heating systems.
  • Initial Cost: The initial cost of acquiring and installing a combi boiler may be more than the cost of standard heat-only boilers.
  • No Backup Water Supply: Combi boilers, unlike system boilers, do not have a reserve hot water supply, leaving consumers without hot water during boiler malfunctions.

2. Heat-Only (Conventional) Boilers

Heat-Only boilers, also referred to as conventional boilers or even regular boilers are one of the types of boilers that has a water tank. They immediately heat your radiators and connect to a separate cylinder or tank to provide hot water. A heat-only boiler has three primary components:

  • Boiler itself
  • Cold water storage tank
  • Hot water storage tank

Working of Heat-Only Boilers

Heat-only or regular boilers have a different way of operating when compared to combi boilers. After being heated by the boiler, the water is either delivered directly to the central heating system or kept in a hot water cylinder to supply hot water as required. Unlike combi and system boilers, cold water is channeled via a cold-water tank, which is often located in the loft or attic, before being delivered directly to the boiler. Even in places with low mains water pressure, this gravity-fed ‘open-vented’ system maintains a reasonable flow rate.

Installation and Maintenance of Heat-Only Boilers

As heat-only boilers are more complex than combi boilers, they require professional and certified gas-safe engineers to do the job and install these boilers in your home. A certified professional will connect the boiler to a separate hot water cylinder and cold water storage tank during installation, ensuring all parts of the system operate properly and effectively.

The maintenance of heat-only boilers is very essential to keep the boiler’s output maximum at all times. Regular inspections are carried out in which potential gas leaks are monitored, all internal parts are thoroughly cleaned, and all safety protocols are checked. By following a regular maintenance plan, homeowners enjoy dependable heating and hot water during the heating season.

Advantages of Heat-Only Boilers

Some common advantages of heat-only or regular boilers are:

  • Usage: Heat-only boilers provide more usage as water can run through multiple taps at once.
  • Greater Flow: Regular boilers provide a greater flow as compared to other boiler types so they are much more reliable for larger properties.
  • Cost Effective: These boilers are solar compatible, hence they will be cost-effective and lower your energy bills.
  • Lower Costs: These boilers are a very cheap option to replace.
  • Water Pressure: Because conventional boilers do not need high main pressure, they are an excellent choice for older heating systems.

Disadvantages of Heat-Only Boilers

  • Compatibility: Heat-only boilers can be incompatible with newer heating systems.
  • Less Efficient: Because heat is lost from the water contained in the cylinder, traditional boilers are less efficient than combi boilers.
  • Water Heating: Hot water does not always come on demand; you must leave time for the cylinder to heat up.
  • Bulk Space: With conventional boilers, you must have separate space for tanks

3. System Boilers

System boilers are also referred to as ‘closed vent’ or ‘sealed system’ boilers. They are an upgraded version of regular boilers. These boilers are able to heat up several rooms and operate effectively providing comfort and heating in multiple bathrooms. This is because these boilers take water directly from the mains.

Working of System Boilers

System boilers are said to be very convenient and efficient. They heat water directly from the mains and store it in a hot water cylinder. A built-in expansion vessel maintains water pressure inside the heating system throughout the procedure. When a hot water tap is turned on, warm water from the cylinder is immediately provided.

The central heating system distributes hot water via radiators or underfloor pipes, delivering constant warmth throughout the building, and is fueled by the same boiler. System boilers do not need a cold water storage tank, making them a space-saving and energy-efficient option for medium to large-sized houses.

Installation and Maintenance of System Boilers

System boiler installation needs professional skills to enable flawless integration with the central heating system. A competent heating expert or a gas-safe engineer will connect the boiler to the mains water supply, the hot water cylinder, and the heating system. In addition to this, the engineer will install the built-in expansion tank to adjust the water pressure.

Routine and scheduled maintenance is vital for the dependable functioning of the boiler. A qualified technician will perform thorough inspections, check for leaks, will clean all internal components, and validate all safety features to ensure the safety of the homeowners. They will also inspect the central heating system, flue, and ventilation. This regular maintenance will expand your boiler’s lifespan and make it more efficient in every aspect making it your best investment in the heating season.

Advantages of System Boilers

Some common advantages of system boilers are:

  • Easy Installation: Because there are fewer components to fit, it may be installed more quickly than a conventional and regular boiler.
  • Efficient Heating: System boilers are more energy efficient than conventional boilers because they provide a steady supply of hot water at mains pressure.
  • Appropriate for Larger Properties: System boilers are appropriate for medium to large-sized houses because they can effectively meet larger hot water needs.
  • Solar Compatibility: These boilers can combine readily with solar water heating systems, supporting environmentally beneficial heating options.

Disadvantages of System Boilers

Some common disadvantages of system boilers are:

  • Initial Pricing: When compared to combi boilers, the initial cost of acquiring and installing a system boiler, as well as the hot water cylinder, might be greater.
  • Pressure Issues: These boilers can cause an issue in operating if provided poor flow rates.
  • Repairing: Repairs or replacements of system boiler or hot water cylinder components might be more complicated and costly.

Different and Reliable Boiler Brands in the UK

There are many reliable and reputable brands in the UK for different boilers. Among the top brands known for their quality and performance are Baxi, Glow Worm, Vaillant, Ideal, and Viessmann. These brands have consistently proven themselves in terms of boilers’ efficiency. All types of boilers from these brands are durable and effective. These top brands also provide quality customer support to their customers in case of any problem.

Other Boiler Types

  • Oil Boilers:

Oil boilers are regarded as one of the most reliable heating solutions for homes without access to mains gas, especially in the UK. Their method of operating is very similar when compared to traditional and regular boilers. These boilers use oil as a fuel source for heating.

Regular maintenance of oil boilers is a bit challenging as compared to other boiler types, as its maintenance requires an Oftec registered engineer instead of a gas-safety engineer. This professional and certified engineer will inspect and fix issues if found in your boiler.

  • Biomass Boilers

Biomass boilers are also called wood boilers as they use wood pellets, chips or logs as fuel to produce heat. The cost of this fuel normally ranges between £400-£600 per year for average medium usage. Pellets are more practical as they can be automatically fed into the system, though you’ll need a supplier for them.

  • Condensing Boilers

Condensing boilers are much more efficient than older non-condensing boilers. They help the fuel you use last longer, resulting in lower carbon emissions and lower heating expenditures.

A condensing boiler utilizes heat captured from these gases to heat water returned from your central heating system. As a result, it uses less heat from the hob and is more efficient.

Regular inspections and service by a skilled expert are required for condensing boiler maintenance to guarantee maximum performance and efficiency. Checking for leaks, cleaning internal components, and testing safety systems are all part of the process.

Which Boiler is Best For You?

Well, the answer is that it depends. It depends on your requirements and what type of heating system will you prefer in your home. Before you start shopping for a new boiler, ask yourself these questions first:

  • How large is your property?
  • What’s your current heating system?
  • What’s the type and condition of your current pipework?
  • What’s the mains water pressure like?
  • What’s your budget?

Only keep in mind, that when you looking to get a new boiler for your home, each house has varied heating needs and each boiler set has unique features. In general, tank-based boilers are perfect for homes in the UK with several bathrooms. We will advise you to get a combi boiler if you own a smaller property with just one bathroom.


Finally, our comprehensive boiler guide has given you useful insight into the many kinds of boilers available in the UK market. Whether you are contemplating a Combi, Heat-Only, or System boiler, each choice has advantages and disadvantages, so it is important to evaluate your unique heating requirements and property size before making a selection.

You can make an educated decision that assures maximum comfort and energy efficiency in your home by studying the operating principles, installation methods, and maintenance needs of different boilers. So, whether you like the ease of a Combi boiler, the flexibility of a Heat-Only boiler, or the efficiency of a System boiler, this guide will help you choose the best boiler for your specific needs and create a warm and cozy atmosphere for the forthcoming winter seasons.

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