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Last updated: 13th September, 2023


Boiler Parts and Their Functions

Boiler Parts and Their Functions

Key takeaways

  • Boilers consist of multiple components, each serving a specific function for efficient operation.
  • Different types of boilers, such as electric, gas, and biomass, cater to various heating needs and industries.
  • Proper understanding and maintenance of boilers are essential for optimal performance and safety.

Understanding the different parts of a boiler and their functions is essential for homeowners and heating professionals alike. Our guide provides an overview of the key components of a boiler, including the burner, heat exchanger, pump, and controls, and explains how they work together to heat your home.

Boilers play a vital role in various industries and households, providing hot water and steam needed for a range of purposes. 

These complex systems consist of numerous components, each serving a specific function to ensure efficient operation. 

By understanding the parts of a boiler and their functions, individuals will gain valuable insights into this essential piece of equipment, ultimately enabling improved maintenance, safety, and optimal performance.

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At the core of any boiler is the combustion process, where fuel mixes with air and ignites to produce heat. The diverse array of boiler types includes electric, gas, and biomass options, each with a unique set of components designed to suit varying heat-producing needs. 

With a myriad of designs, materials, and applications in which boilers are utilised, it is crucial to have a comprehensive understanding of these key elements and their roles in the efficient functioning of the boiler system as a whole.

Fundamentals of Boilers

Boilers are essential components in various industries and residential settings, serving as a central heating system. They operate by heating water, turning it into steam, and utilising the steam to transfer heat throughout a building. Boilers can function with several types of fuel sources, including natural gas, oil, and biomass.

In essence, boilers consist of three primary components: the heat-emitting system, the control system, and the distribution system. Each component plays a vital role in ensuring the overall effectiveness and efficiency of the boiler.

One of the most critical boiler components is the heat exchanger. The heat exchanger is responsible for transferring heat from the combustion of the fuel to the water inside the boiler, subsequently heating the water and transforming it into steam. This steam then circulates throughout the distribution system, providing heat to different areas of the building.

Speaking of fuel, combustible fuel is an essential element of a boiler's operation. The common fuels utilised are gas, oil or biomass. Their presence ensures the generation of the necessary heat needed to raise the water temperature. The burner, where fuel and air mix for combustion, is a crucial boiler part controlled by the boiler's control system.

Adding to the list of prime parts, a boiler's control system manages the heating process by turning the burner on and off based on thermostat settings or water temperature. It acts as a safeguard, ensuring the boiler runs optimally and prevents overheating.

Finally, the distribution system plays an integral role in transferring steam produced in the boiler to various parts in a building. Radiators or underfloor heating circuits serve as heat distribution devices, conveying the heat from the steam to individual space in the structure.

All in all, boilers serve as reliable heating systems that operate through a seamless interplay of essential components, including heat exchangers, fuel, and control systems. Understanding their fundamentals allows for better maintenance and optimisation of boiler processes, ultimately leading to more efficient heating solutions.

Different Types of Boilers

The world of boilers comprises various types, each designed to cater to specific needs and preferences in terms of their functionality, efficiency, and suitability for different environments. This section delves into three main types of boilers: combi, heat-only, and system boilers. Additionally, it briefly explores other boiler categories such as water tube, fire tube, steam, electric, and commercial boilers.


Combi, or combination boilers, are highly popular among homeowners due to their compact size and their ability to provide both central heating and hot water on demand. These boilers eliminate the need for a separate hot water tank, making them ideal for properties with limited space. 

Since the hot water supply is instant, there is no risk of running out of hot water with a combi boiler. However, combi boilers may struggle to generate hot water when used simultaneously in multiple taps.


Heat-only boilers, also known as conventional or regular boilers, operate using a traditional heating system. These boilers require a separate hot water storage cylinder and a cold-water storage tank, typically located in the loft, to function effectively. 

Heat-only boilers have proven suitable for larger homes with multiple bathrooms and those with an older heating system. Despite their larger size and the need for additional storage tanks, these boilers provide a reliable source of hot water and can cater to multiple taps at once.


System boilers are designed to cater to properties that previously relied on a conventional boiler but now require an upgrade. Similar to heat-only boilers, system boilers utilise a separate hot water storage cylinder to supply hot water. 

However, unlike heat-only boilers, they do not necessitate a cold-water tank as the major components are built into the boiler, reducing the need for additional storage space. System boilers can efficiently supply hot water to multiple taps simultaneously, making them ideal for larger homes.

While combi, heat-only, and system boilers are the most prevalent options, other boiler types include water tube, fire tube, steam, electric, and commercial boilers. In a water tube boiler, water runs through tubes heated externally by fire, providing high-pressure steam or hot water. 

Conversely, in a fire tube boiler, hot gases produced by combustion pass through tubes surrounded by water, generating steam or hot water. Steam boilers are primarily used for industrial purposes, while electric boilers are perfect for small-scale domestic heating. Commercial boilers, as the name suggests, cater to larger spaces like office buildings and retail centres.

Key Components and Their Functions

The Combustion Chamber

The combustion chamber is where the fuel source undergoes combustion, producing heat. Different fuels, such as kerosene, heating oil, and liquid propane, can be used. This process is essential in heating up the heat exchanger to several hundred degrees.

The Burner

Mixing of air and the fuel source occurs at the burner, initiating the combustion reaction in the system. The thermostat sends an electronic signal to the burner, which plays a crucial role in determining the overall efficiency of the boiler.

The Heat Exchanger

The heart of the boiler, the heat exchanger, is responsible for transferring heat from the combustion chamber to the circulating water. It maintains high temperatures without permitting the fluid to boil, rendering it a safe and efficient energy source.

Safety Valve

The safety valve offers critical protection by releasing excess pressure that may build up within the boiler. If left unchecked, such pressure could lead to disastrous consequences like explosions.

Pressure Gauge

Monitoring the boiler's internal pressure is paramount. The pressure gauge enables the operator to maintain the pressure within safe operating limits and ensures the overall efficiency of the system.

Feed Pump

The feed pump is essential in supplying the boiler with the required water. It pumps water into the boiler at a constant pressure, ensuring proper circulation and heat transfer.


The economiser recovers residual heat from flue gases before they leave the boiler. This heat is reused, increasing the overall efficiency and reducing energy consumption.


Installing a superheater elevates the steam temperature above its saturation point, enhancing the system's overall energy output. The final product, superheated steam, possesses higher thermal energy which can be advantageous in particular applications.

Circulator Pump

A circulator pump moves hot water from the boiler to the radiators, providing even heat distribution throughout the system. This crucial component maintains optimal water flow and temperature.

Expansion Tank

Boilers produce expanding hot water, which the expansion tank accommodates. By allowing space for the heated water to expand and control the pressure within the system, the expansion tank ensures the boiler's safe operation.

Control System

The control system is a centralised hub that manages various aspects of the boiler's operation. It monitors temperature, pressure, and water levels, as well as determining when the burner and pumps should operate to maintain optimal efficiency.

Backflow Valve

A backflow valve prevents the reverse flow of water, ensuring it moves only in one direction. This one-way valve thus protects the boiler, providing an additional layer of safety.


Aquastats are vital for monitoring water temperature in the system. They help control the boiler's operation by activating the burner or circulating pump as required to maintain consistent water temperatures within the system.

Boiler Materials and Design

When it comes to boiler materials and design, several factors play a pivotal role in ensuring optimal efficiency and longevity. To begin with, let's consider the primary materials used in boiler construction: cast iron, steel, and copper.

Cast iron has been a popular choice for boilers due to its excellent heat retention properties and durability. Boilers made of this material can last for decades, making them a dependable option. However, cast iron's heavyweight and the need for larger sections can be a disadvantage, especially in terms of installation and transportation.

On the other hand, steel boilers offer a more modern and lightweight alternative. Steel is known for its high tensile strength and corrosion resistance, making it ideal for high-pressure applications. These boilers are typically constructed with welded steel plates, ensuring a reliable and secure structure. However, steel's heat transfer properties are not as favourable as those of cast iron, which may affect efficiency.

Copper is an intriguing option for boiler construction, primarily due to its exceptional thermal conductivity. Copper boilers tend to be lighter and smaller than their cast iron and steel counterparts, making them easier to install in tight spaces. Nevertheless, copper is more susceptible to corrosion, requiring diligent maintenance and the use of protective measures, such as coatings and inhibitors.

The boiler design process takes into account factors such as heat transfer, pressure, and fluid flow to ensure optimal performance. 

Typically, boilers include drums, tubes, flues, ducts, auxiliary equipment, and their associated supports, all of which must withstand stressors from expansion and contraction during operation. Additionally, they should be designed to endure elevated temperatures in service.

To accomplish this, engineers use methods such as computational fluid dynamics (CFD) simulations and stress analyses to predict thermal and mechanical stresses throughout the boiler. These assessments help in selecting appropriate materials and design features to achieve the desired performance and service life.

In conclusion, boiling materials and design play a crucial role in the overall performance and longevity of a boiler. By carefully selecting appropriate materials and employing well-considered design principles, it is possible to create a reliable and efficient boiler system that meets both current and future needs.

Heating Applications and Industries

A diverse range of industries and applications employ boilers for various heating purposes, making them essential components of central heating systems. From domestic setups to large-scale commercial and industrial facilities, boilers play a vital role in providing consistent and efficient heat.

In residential applications, boilers are often used for central heating and water heating purposes. They can be connected to a network of radiators throughout the living space, providing comfortable and homogenous warmth. Furthermore, they supply hot water to various outlets, such as taps and showers, ensuring an uninterrupted supply for daily household activities.

Commercially, boilers find widespread use in space and water heating applications, including comfort heating, freeze protection, and radiant floor heating. In establishments like hotels, restaurants, and hospitals, consistent water heating is crucial for tasks like dishwashing, laundry, and maintaining hygiene levels. Additionally, industries, such as car washes and laundromats, rely on boilers to provide an ample supply of hot water and steam during their operations.

In industrial settings, the use of boilers expands beyond mere space and water heating applications. The generation of steam for power plants, for instance, is a key aspect of boiler functionality. 

The steam produced in boilers under these circumstances enables the production of electricity as part of a conventional steam power plant. Fossil fuels or waste fuels are often incinerated to facilitate this process, showcasing the versatile nature of boiler systems.

To summarise, boilers are integral components in heating systems for various applications and across multiple industries. 

From residential central heating to commercial hot water needs and industrial power generation, the functions of boilers are expansive and indispensable in our daily lives.

Energy and Fuel Sources

A boiler's operation depends on various energy and fuel sources. For different types of boilers, these sources are essential to power the heating process and generate steam or hot water. 

This section will examine those sources, such as gas, oil, electricity, coal, wood, natural gas, and fuel oil.

Gas boilers are prevalent and rely on natural gas, a clean and affordable source of energy. The central element in a gas boiler is the burner, which is controlled by a heating system. 

The thermostat allows the system to switch the burner on and off, maintaining an optimal temperature for the boiler's operation.

Oil boilers, on the other hand, utilise fuel oil or kerosene as their source of energy. Fuel oil is often used in industrial settings, while kerosene is more common in domestic environments. The combustion chamber, or firebox, of these boilers is usually made of cast iron to withstand the heat and pressure generated within.

Electric boilers utilise electricity from the national grid or an on-site supply to run their heating elements. These boilers don't require fuel because their heating process is entirely electric, making them a greener and more environmentally friendly option for energy generation.

Coal-fired boilers are popular in large industrial settings, requiring an abundant and stable source of energy. The boiler utilises coal as fuel, combusting it in a combustion chamber to produce steam. Despite coal's reduced usage in modern systems, it remains an essential energy source in certain industries.

Wood-fuelled boilers utilise wood chips or pellets as a renewable and sustainable fuel source. These boilers are often designed for smaller scale operations, such as in residential homes or small businesses. 

As an eco-friendly option, wood boilers are becoming more popular due to reduced emissions and lower operational costs.

Furthermore, some boilers use a combination of fuel sources to provide maximum efficiency and adaptability. A dual-fuel boiler allows users to choose between two fuel types, such as natural gas and fuel oil, depending on availability and economic factors. This flexibility enables the boiler to meet various energy needs and maintain consistent operations.

In conclusion, different boiler types rely on a range of energy and fuel sources to function effectively. 

The choice of fuel source often depends on the boiler's intended application, efficiency, and environmental concerns. As the industry continues to evolve, there's a growing emphasis on sustainable and cleaner energy options for boiler systems.

Working Process of Boilers

The working process of a boiler involves several essential elements, starting with the combustion, water heating, heat transfer, steam generation, and ending with exhaust stack. Boilers primarily function to produce either hot water or steam for various applications, such as domestic heating, commercial uses and industrial operations.

In the combustion chamber, fuels like kerosene, heating oil, and liquid propane are burned. This process heats the heat exchanger up to several hundred degrees, which acts as a crucial component in transferring heat from the burning fuel to the water in the boiler. During this combustion process, the heat is effectively transferred to the water, resulting in an increase in temperature.

After the heat transfer, the hot water reaches a high enough temperature to convert it into steam. This steam has multiple uses. For instance, it powers turbines for electricity generation in industrial settings or provides heat distribution in residential and commercial buildings. 

Alternatively, hot water generated by the boiler could be used directly for heating purposes, such as in a central heating system, eliminating the need for a steam conversion process.

The exhaust stack is the final integral element of the working process of a boiler. It releases the by-products of combustion, including gases and smoke, into the atmosphere. 

The exhaust stack ensures the proper operation of the boiler by removing these by-products in a safe manner, minimising the environmental impact and maintaining ideal combustion conditions within the boiler system.

By understanding the essential components in the working process of a boiler, one can recognise the intricacies within this integral appliance for heating and energy generation applications. 

The processes of combustion, water heating, heat transfer, steam generation and exhaust stack, all contribute to the efficient functioning of the boiler system.

Importance of Boiler Maintenance

Boiler maintenance is not only crucial for optimum performance but also for ensuring the safety and longevity of the system. Regular maintenance helps in reducing the risk of the boiler failing or breaking down, extending its life and keeping it running efficiently.


One key aspect of boiler maintenance is checking the safety valve. This essential component serves to release excess pressure from the boiler when it reaches a certain level, preventing potential accidents and damage. Ensuring the safety valve is functioning correctly and not obstructed is imperative for maintaining a safe boiler system.

The pressure gauge, another critical component, must also be monitored regularly. It displays the boiler's internal pressure, allowing operators to detect any pressure-related issues in the system. Verifying the accuracy of the pressure gauge is paramount, as inaccuracies could lead to false readings and the possibility of undetected issues.

In terms of the boiler's control system, it is vital to ensure that all sensors, thermostats, and electronic components are functioning correctly. Regular inspections and cleaning of the control system help prevent issues such as ineffective temperature monitoring or unresponsive thermostats. 

Additionally, conducting routine checks on the expansion tank guarantees that the system can adequately accommodate changes in water volume due to temperature fluctuations.

Another important element to consider during maintenance is the backflow valve. It prevents contaminated water from flowing back into the boiler system, maintaining the cleanliness and quality of the water. Inspecting and cleaning the backflow valve is essential to keep it functional and help extend the life of the entire boiler system.

Implementing a daily maintenance checklist can assist in keeping the boiler in proper working order by addressing issues as they arise. Some daily tasks include opening drainage and blowing out water from pipes when not in use, monitoring fuel consumption, temperature, and pressure, and keeping an eye on any changes in the system's performance.

In conclusion, boiler maintenance is indispensable for the safety, efficiency, and life of the system. By regularly inspecting and servicing critical components like the safety valve, pressure gauge, control system, expansion tank, and backflow valve, the boiler's optimal and safe operation can be ensured.

Notable Boiler Manufacturers


Viessmann is a highly regarded boiler brand, boasting impressive performance and rigorous German efficiency. Being Europe's 2nd largest boiler business, with a turnover of £2.4 billion, the company holds a significant presence in the market.

Viessmann offers a wide range of gas combi boilers which are designed to provide users with reliability and high-quality performance. A key feature is the user-friendly LCD display, making it simple to understand and operate these boilers. Moreover, Viessmann provides generous 10-year warranties and excellent aftercare services, ensuring customer satisfaction and peace of mind.

One of Viessmann's standout products is its Vitodens range, lauded for its innovative design and updated features. This range is known for its high efficiency and durability, making it an attractive choice for many homeowners seeking a stable and trustworthy boiler system.

As a brand, Viessmann stands alongside other globally renowned boilers manufacturers like Worcester Bosch, Alpha, Ideal, Vaillant, and Baxi. Each brand brings something different to the table, but Viessmann excels in performance, efficiency, and customer satisfaction.

In summary, Viessmann merits a strong reputation as a reliable boiler brand, providing cutting-edge technologies and exceptional customer service. Choosing a Viessmann boiler is an excellent investment for those seeking a trustworthy and efficient heating system.


Alpha has earned its place among the top boiler brands in the UK. Known for their high-quality craftsmanship, Alpha boilers are designed to provide a reliable solution for your heating needs. As a trusted manufacturer, Alpha has a wide range of boiler options to suit various preferences and requirements.

The unique selling point of Alpha boilers lies in their energy efficiency. These boilers are designed to reduce energy consumption, ultimately lowering the costs of heating your home. Advanced technology and engineering techniques contribute to the efficiency of Alpha boilers, ensuring they are a smart choice for environmentally-conscious homeowners.

Customer satisfaction is at the heart of Alpha's brand, with many customers praising the brand for its superb after-sales support and customer service. Such a level of care sets Alpha apart from other boiler manufacturers, as purchasers have peace of mind knowing that assistance is merely a phone call away.

Additionally, Alpha boilers tend to have a more competitive price point compared to other top brands, making them a popular choice for budget-conscious homeowners. Despite lower prices, Alpha does not compromise on reliability or performance, delivering excellent value for money.

In summary, Alpha offers energy-efficient and cost-effective boiler solutions without sacrificing quality or reliability. With a strong focus on customer satisfaction, Alpha boilers are a wise choice for homeowners looking to invest in a long-lasting, efficient heating system.


Ideal boilers is a brand that has earned a strong reputation for reliability and innovation in the UK market. Manufacturing a range of gas and oil boilers, Ideal designs products that cater to various households and requirements. Let's delve into some aspects that make Ideal a reliable boiler brand.

Firstly, Ideal emphasises energy efficiency and cost-effectiveness in their products. With the inclusion of advanced technology such as weather compensation and smart boiler controls, it's possible to achieve optimum performance and reduced energy bills. These features not only benefit homeowners in the long run but also contribute to a greener environment.

Secondly, Ideal boilers are known for their ease of installation and maintenance, a factor that is valued by both homeowners and professional engineers. This ease translates into lower costs, as the installation process is quicker and requires fewer resources. Furthermore, the simplicity extends to the service and maintenance of their products, ensuring that they remain efficient and reliable over time.

In addition to product innovation, Ideal offers comprehensive warranties to back up their boilers. These warranty periods can range from 2 to 12 years, depending on the make and model, and show the brand's confidence in the longevity and reliability of their products.

Lastly, Ideal has a good reputation for customer support. With their UK-based customer service team, they provide dependable assistance and technical expertise when needed. This strong support network instils trust and confidence in Ideal products, further enhancing the overall user experience.

In conclusion, Ideal's commitment to efficiency, ease of use, comprehensive warranty coverage, and excellent customer support make it a reliable and popular choice among British homeowners and heating engineers alike.


Vaillant is a well-known boiler brand that has earned a reputation for reliability and quality. With a history of innovation and engineering excellence, Vaillant has become one of the top-rated gas boiler brands in the UK. In fact, their entire range of domestic boilers was awarded 'Best Buy' status by Which? in 2020.

A key selling point for Vaillant boilers is their energy efficiency. By using advanced technology and high-quality components, these boilers manage to achieve lower emissions and reduced energy consumption. This not only makes them a more environmentally friendly option but also helps homeowners save on their energy bills.

In terms of performance, Vaillant offers a wide range of boilers to suit different needs and preferences. Their combi boilers are highly regarded for their compact design and user-friendly features. These systems provide both heating and hot water from one unit, making them an ideal choice for smaller homes with limited space.

For larger homes or properties with multiple bathrooms, Vaillant also offers heat-only and system boilers. These types of boilers are designed to work in conjunction with a separate hot water cylinder, allowing for a more consistent supply of hot water to multiple outlets.

When it comes to ease of installation and maintenance, Vaillant boilers are highly appreciated by heating engineers and installers. With straightforward installation procedures and accessible components, they make for a hassle-free experience.

Despite their top-notch performance and reliability, it's essential to choose the right Vaillant boiler for your specific needs and requirements. To do so, it's recommended to consult with a professional heating engineer, who can assess your home's heating and hot water demands, and recommend the most suitable Vaillant boiler model.

In summary, Vaillant is a dependable choice among boiler brands, offering energy efficiency, performance, and user-friendly features. With a diverse range of boiler models to cater to various household needs, it's clear why Vaillant remains a popular option for homeowners in the UK.

Worcester Bosch

Worcester Bosch stands as a reputable boiler brand in the UK market, offering reliable and efficient products for many homeowners and businesses. Founded in 1962, this company has gained its position in the heating industry by consistently manufacturing top-of-the-range boilers.

The Worcester Bosch range comprises heat-only and combi boilers, catering to various customer needs and preferences. Boilers by this brand are known for their energy efficiency and eco-friendliness, contributing to a reduction in emissions and fuel bills. Moreover, Worcester Bosch boilers boast innovative technology that enables users to adjust the settings based on their specific requirements.

Not only do these boilers offer reliability, but their compact designs also make them an ideal choice for smaller spaces. This characteristic allows the Worcester Bosch range to cater to apartment dwellers and homeowners with limited room for installation. These boilers are also user-friendly, featuring intuitive controls that make navigating through various settings a breeze.

Furthermore, Worcester Bosch is highly regarded in the heating industry for its after-sales service and support. The company provides a comprehensive warranty for its boilers, instilling trust among customers who require reliability and longevity. In addition, the brand's robust network of accredited installers ensures that customers receive top-notch service and timely assistance when needed.

In surveys and customer reviews, Worcester Bosch boilers consistently receive high praise for their reliability and performance. These factors, combined with the wide range of solutions offered by the brand, make Worcester Bosch an ideal choice for homeowners searching for a dependable and efficient boiler system.

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Frequently Asked Questions

What are the parts of the boilers?

Boilers consist of various components that work together to ensure efficiency and safety. Some common parts include the burner, combustion chamber, heat exchanger, aquastats, circulator pumps, and controls. Additionally, you may find safety equipment such as deaerators, expansion tanks, backflow valves, and safety valves.

What are the parts of a central heating boiler?

Central heating boilers share many components with regular boilers, such as the burner, combustion chamber, and heat exchanger. Other parts specific to central heating systems include supply lines, return lines, and a manhole for pipe inspection and cleaning. Some systems may also have an economizer to improve energy efficiency.

How much are boiler parts?

The cost of boiler parts varies widely depending on the specific component, manufacturer, and system requirements. Smaller components like valves or thermostats might cost anywhere from £10 to £100, while more significant parts like heat exchangers could range from £200 to over £500. It is essential to consult with a professional to determine the exact costs for your system.

How much is a heat exchanger for a gas boiler?

Heat exchangers for gas boilers come in varying sizes and types, leading to a broad price range. On average, a heat exchanger for a gas boiler can cost between £200 and £500. However, some specialised or high-quality models may exceed this range. It's crucial to choose the right heat exchanger that fits your boiler system and budget.

How much does it cost to replace a heat exchanger in the UK?

In the UK, replacing a heat exchanger typically costs between £500 and £1,000, which includes both the part and labour charges. The specific expense may vary depending on factors like your boiler's make and model, the complexity of the installation process, and local labour rates. Always obtain multiple quotes from professional boiler engineers to find the best deal for your needs.

How do I know if my boiler heat exchanger is broken?

Signs of a broken boiler heat exchanger include inconsistent heating, water leakage around the boiler, or unusual noises like banging or rattling. In some cases, you might even notice a lingering smell of gas or a heightened carbon monoxide alarm. If you suspect your heat exchanger is damaged, you should contact a qualified engineer immediately to diagnose and resolve the issue.

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Article by
Stephen Day | Co-founder
Gas Safe registered and FGAS certified engineer with over 20 years experience in the heating and cooling industry.