June 15, 2023


Demystifying Gas Boiler Systems: Understanding Their Components and Functionality

Nearly every home or workplace in the UK has a gas boiler. Your boiler is the wall-mounted or floor-standing white box you tend to only look at when it’s not working. We rely so heavily on our gas boilers to heat our homes and provide us with hot water, especially during colder months which we have many of here in the UK. But apart from glancing at them when they’re not working, let us explain how they work and what their purpose is?

Gas boilers and heating systems in general play a vital role in providing heating and hot water in countless homes and buildings in the UK. Whether you’re a homeowner or a curious individual seeking knowledge, it’s very beneficial and essential to understand how boilers work and the role of their components.

In this guide, we will delve into the basics of gas boilers, explore their key components, discuss their operation, and touch up on maintenance and safety considerations. By the end, you’ll have a comprehensive understanding of different types of gas boilers, how gas boiler systems work and the crucial elements involved.

To begin, let’s establish what gas boilers are and their fundamental purpose.

A gas boiler is the core component of a heating system that uses natural gas as its fuel source to generate heat. The heat produced by the gas burner is then transferred to the surrounding water through a heat exchanger, resulting in heated water for central heating or domestic hot water purposes.

There are various types of gas boilers available, each with its own advantages and considerations. There is no right or wrong when it comes to choosing a type of gas boiler as it depends on your lifestyle and needs. Sometimes we inherit a boiler as part of a house purchase and by understanding what the options are you can ensure you choose a replacement boiler that is suited to your needs.

Conventional boilers

Conventional boilers, also known as regular boilers,  traditional boilers or heat only boilers are an essential part of a heating system. They work by heating water to provide warmth throughout a building. With the help of numerous electrical components like a central heating pump, motorised valves, and tanks. Conventional boilers circulate water through a network of pipes to radiators throughout the property but also heats the hot water in the hot water cylinder, providing a comfortable cosy environment and flowing hot water.

Heat only boilers heat up your hot water cylinder, delivering hot water straight to your taps and showerheads whenever you need it. With the help of a  Grundfos central heating pump, the heated water circulates through your radiators and to your hot water cylinder keeping every corner of your home warm and toasty.

Generally, you’ll have a 50-gallon cold water storage tank in your loft adjacent to your feed and expansion tank. The cold storage tank will feed cold water to your toilets, baths, and basins. It also feeds cold water to your hot water cylinder ready to be heated via the conventional boiler.

As part of your conventional boiler system, you will have a feed and expansion tank in the loft which will fill water into your heating system ( boiler, radiators, pipework etc). You also have an expansion pipe that goes over the top of the feed and expansion tank allowing the system to breathe.

When the water in your heating system heats up, the water will expand thus needing a breather pipe on the heating system. The expansion pipe is a vital part of your heating system that you can not comprise. The expansion pipe must be a minimum of 22 mm in diameter and needs to be a certain distance above the feed and expansion tank.

Both the feed and expansion tank and cold storage tank are filled up with mains water via ball valves. A common issue is when the ball valves get stuck or fail causing the tanks to run dry with no mains water refilling. Symptoms of this are typically, boiler breakdown and no running water out of any taps or toilets except your kitchen tap which is always run of mains-fed cold water providing you with drinking water.

Discover the world of heat-only boilers and experience the comfort they can provide. Watch our lead engineer talking about the right way of installing a heat-only boiler here https://youtu.be/t_jolviFVV0

Who should opt for a conventional heat-only boiler:

Heat-only boilers are well-suited for properties that have existing regular boilers with old existing heating pipework and radiators. It is particularly recommended for homes in areas with low water pressure as alternative boilers/systems relying on mains water pressure require high incoming water pressure for them to work as they should. With heat only boilers you can also add a shower pump to boost your shower pressure.  Additionally, heat only boilers are also compatible with solar water heating systems, offering a sustainable and cost-effective solution that reduces carbon footprint.

System boiler

Many homeowners may not be aware that there are two types of boilers that work with stored hot water cylinders. In addition to the conventional boiler, there is another type called: the system boiler.

System boilers and regular boilers share many similarities in their function, but there are also some key differences between the two which you should be aware of.

Regular boilers rely on a header tank in the loft to maintain pressure, while system boilers use a pressurised heating circuit that is topped up by mains water. Additionally, system boilers have an internal expansion vessel to regulate water pressure and an integral central heating pump while regular boilers use an external expansion vessel and external central heating pump.

Since the expansion vessel and central pump are built into the system boiler, they are covered by the manufacturer’s parts and labour guarantee as they form part of the boiler. This can provide homeowners with added peace of mind and potentially save money on repair costs in the long run.

In contrast to a conventional boiler, a system boiler operates without the need for a feed and expansion tank to facilitate the filling of the heating system. Instead, it enables direct filling of the system using water from the mains water supply. This feature not only enhances operational efficiency but also saves space. Additionally, it reduces the probability of corrosion and leakage, thereby prolonging the lifespan of the system.

System boilers are often preferred for larger properties with multiple bathrooms. Usually installed with an unvented hot water cylinder however this is not a must. Understanding the differences between these two types of boilers can help you as the homeowner to make an informed decision about which one is best for your needs.

Combination boilers

A combination boiler also known as a combi boiler is a highly popular choice of boiler type. Well-suited and designed for small to medium-sized properties with lower hot water and heating demand than larger properties. It is recommended for properties with 1 bathroom but don’t let this put you off as you can still have a combination boiler if you have more than 1 bathroom but it is not guaranteed you can run multiple outlets at the same time. This is because a combination boiler is fed off your cold water main pie, as you run one outlet you will benefit from 100% of the supplied water – when you turn 2 outlets simultaneously the water pressure will be divided as its fed off 1 incoming pipe.

Your heating and hot water is produced by 1 unit which is usually wall-hung with compact models available for example Vaillant Ecofit Pure and Worcester Greenstar range which are designed to fit inside a standard kitchen cupboard with a depth of 30 cm.

Combination boilers are fed from your cold incoming water mains and do not preheat hot water which means it delivers hot water on demand. Turning your hot water tap on will trigger the boiler to heat water instantly via a secondary plate heat exchanger built into the boiler. This means that you benefit from not having to schedule your hot water to come on, no waste of excess hot water from a preheated cylinder, and you only heat and consume the required amount of hot water that is needed at that moment in time.

When we visit a customer for a combination boiler replacement quote we always carry out a flow rate test to establish the available pressure/flow on their incoming mains cold water supply.

This enables us to match the available pressure/flow rate to a suitable combination boiler model. Combi boilers range between 25kw – 55kw,  each model will specify their performance for heating and hot water flow rate separately. We always ensure that the specification of the combi boiler is in line with the customers available flow rate to prevent customers from overspending on a model that is suited for higher flow rates.

A combi boiler can only heat whatever is available at the source, so if you have 14 liters per minute at your incoming main and you invest in a 45 kw Worcester Bosch boiler that is able to heat 16 liters per minute, you’ll still only get 14 liters as that is the flow rate entering the boiler.

How are hot water and heating heated by a combination boiler?

A combination boiler has 2 heat exchangers inside of it:

Primary heat exchanger – for your central heating radiators/underfloor heating

Secondary plate heat exchanger – for the hot water that goes to your taps/showers/baths

The role of the primary heat exchanger

The radiators in your property are heated by the boiler’s primary heat exchanger. The heated water will leave the boiler and travel to your radiators all around the house and then back to the boiler. As long as the boiler keeps firing and your heating is set to be on, the boiler will continue to heat the water and circulate it around the radiator loop at a temperature of 60 degrees. The water in your heating system does get dirty due to rust, debris and sludge from radiators and pipework.

The role of a secondary heat exchanger

As you can imagine, the last thing you want is to bathe in the dirty central heating water. This is where the secondary heat exchanger comes in.

Every time you turn on a hot water tap in your home, your combi boiler will pause your central heating and open a diverter valve which allows the dirty central heating water to reach the secondary plate heat exchanger which then heats sealed pipes that carry clean water that’s then delivered to your hot water taps and showers.

Types of hot water cylinders:

System boilers and conventional boilers are both compatible with vented and unvented hot water cylinders. Vented cylinders store and heat water sourced from a cold storage tank located in the loft. Vented cylinders rely on gravity which is why the cold storage tank must be located above it to gravitate down to the vented hot water cylinder which enables the supply of hot water to all tap outlets in the property.

If you are looking to experience really great hot water pressure then an unvented hot water cylinder is for you. An unvented hot water cylinder is unique in its ability to provide pressurised hot water directly from the cold mains. The cold water is drawn directly from the main water supply and heated, providing you with an abundance of hot water, the ability to run multiple taps at the same time, and delivering great pressure to your hot water outlets.

Upgrading to a pressurised unvented hot water cylinder, in place of a vented cylinder, means the homeowner no longer requires a cold storage tank in their loft and can instead rely solely on a hot water cylinder filled with mains cold water.

For homeowners seeking to free up their loft space for a possible loft conversion or any other reasons, a popular option is to install a system boiler in conjunction with a pressurised unvented hot water cylinder. This is because neither the system boiler or unvented pressurised hot water cylinder requires any additional tanks in the loft.

What are the types of fuel in boilers?

Here at The Boiler Installation Specialists, our top priority is to provide exceptional service to our customers. When customers contact us about a new heating system or boiler installation we ensure we establish what fuel your existing boiler is running on. Oil and natural gas are the 2 most popular and easily accessible types of fuel used by homeowners. Both types are able to serve all boiler types so there is no limitation on what type of boiler you can have. In most cases, homeowners using oil heating live in an area where there is no readily available natural gas connection.

How does fuel turn into heating?

The fuel enters the boiler either through a  gas pipe or oil line into the burner, which ignites and combusts. The combustion process takes place within the combustion chamber, where the fuel and air mix to generate heat. The heat is then distributed into the heating system via a heat exchanger.

Heat Exchanger

The heat exchanger plays a critical role in transferring the heat produced by the burner to water. Gas and oil boilers typically have two types of heat exchangers: a primary heat exchanger and a secondary heat exchanger. The primary heat exchanger extracts heat from the combustion gases, while the secondary heat exchanger further utilises any remaining heat to heat water.

Flue and Ventilation System

Gas and oil boilers require a flue to safely exhaust the combustion gases outside. The flue is a  pipe or duct that removes the byproducts of combustion, such as carbon dioxide and water vapor. Proper ventilation is crucial to ensure the correct operation of the boiler and prevent the accumulation of harmful gases. The flue system is designed to either exit through the wall to the outside or it can run vertically up into a loft and through the roof. Oil boilers also have the additional ability to have a flexible flue liner running through a chimney to the outside.

Controls and Sensors

Various controls and sensors are used with gas and oil boilers to regulate their operation and to meet regulations. In order to reduce the carbon footprint produced by boilers, building control regulations require new boiler installations to operate with a thermostat which is designed to cut off the boiler when the desired room temperature is reached. Combination boilers use a programmable room thermostat which enables homeowners to program their heating schedule and set the desired room temperature, all from 1 control unit ( programmable room thermostat). Conventional and system boilers use a room thermostat to set their desired room temperature and a separate programmer where they set the heating and hot water schedule which can be set to work together or heating and hot water completely independently. The benefit of using these controls to work with your boiler is that there should be no waste of gas resulting in lower heating bills and be able to operate your heating and hot water with the benefit of signals being sent to the boiler to switch off when the desired temperature has been met.

Pressure relief valve

A pressure relief valve acts like a safety guard for a boiler. It helps to release pressure when it gets too high. The presence of a pressure relief valve is really important because if the pressure gets too high and it can not escape anywhere it can essentially cause an explosion and leaks.

The pressure relief valve operates by opening up when the water pressure gets too high and then closes again when the water pressure goes down to a safe level. There are different types of pressure relief valves but the most common type used for domestic homes are usually spring-loaded.

Central heating pumps

The central heating pump ensures hot water circulates throughout the system. The central heating pump moves the hot water around the heating system to each of the radiators or underfloor heating in a continuous motion doing a full circle back to the boiler and then out to the radiators/underfloor heating again. When your central heating pump fails you will notice your radiators will not heat up.

Expansion Vessel and Pressure Gauge

To accommodate the expansion of water when heated, gas and oil boiler systems incorporate an expansion vessel. This vessel helps maintain consistent pressure within the system, preventing damage from excessive pressure. The pressure gauge allows users or engineers to monitor the pressure levels and ensure they remain within the recommended range.

Understanding how gas and oil boiler heating systems work provides insights into how these components work together to provide heating and hot water and how important each of their roles is.

Start-up Process

When heating or hot water is required, the gas or oil boiler undergoes a start-up process. This involves igniting the gas burner, initiating a pre-purge to remove any residual gases, and initiating the combustion cycle to generate heat.

Heating and Hot Water Demand

During operation, the gas or oil boiler adjusts its output based on the heating or hot water demand. Modern boilers often feature modulating burners that can vary their heat output, providing greater energy efficiency. In larger homes, you will find that zoned heating is used which means that different areas of the house are controlled independently. For systems with multiple heating zones, zone valves are used to control the flow of heated water to different areas of the building on demand.

Shutdown Process

Once the heating or hot water demand is met, the gas or oil boiler enters the shutdown process. This typically involves a post-purge to remove any remaining combustion gases and safely shut off the gas or oil supply.

Maintenance and Safety Considerations

To ensure the optimal performance and safety of your gas or oil boiler system, an annual service is vital. Here are some key reasons and safety considerations:

During an annual boiler service:

Cleaning and inspecting the heat exchanger helps prevent the buildup of debris and ensures efficient heat transfer. Flue maintenance and clearance should be checked to guarantee proper ventilation and prevent blockages. Additionally, filters should be inspected, cleaned or replaced as necessary to maintain efficient operation. For oil boilers, the burner nozzle gets replaced on an annual basis.

Gas and oil boiler safety precautions

Safety is of utmost importance when it comes to gas and oil boilers. Carbon monoxide detection and prevention measures, such as installing CO detectors, should be in place. Proper ventilation and adequate air supply are crucial to ensure the safe operation of the boiler. Monitoring pressure and temperature through the pressure relief valve and pressure gauge helps prevent potential hazards.

Troubleshooting Common Issues

Gas and oil boiler systems may encounter occasional issues. Here are some common problems and potential solutions:

Lack of Heat or Hot Water:

If your boiler is not igniting then always check your gas supply as sometimes customers with pre pre-payment plans forget to top up their gas which causes the boiler not to work.

If you have no heating then check your room thermostat to see if there are any unusual symbols displayed as sometimes wireless thermostats lose connection to the wiring center connected to the boiler or batteries might need replacing.

For wireless room thermostats, check the thermostat receiver to ensure the appropriate lights are on the receiver.

For combination boilers, if you have heating but no hot water then it is possible the diverter valve in the boiler or the secondary heat exchanger has failed.

For conventional or system boilers, if you have no heating or hot water at all then it is pointing towards a boiler fault.

If you have heating but no hot water then it is possible that the hot water motorised valve has failed.

If you have hot water but no heating then it is possible that it is either the central heating pump or heating motorised valve which has failed.

Customers with stored hot water, if you experience really hot water all of a sudden then there could be an issue with either your cylinder thermostat or the hot water motorised valve.

Pressure and water leakage Issues: Low or high-pressure readings, as well as water leakage, could indicate problems with the expansion vessel, pressure relief valve, or other components. These issues require immediate attention from a qualified engineer.


In conclusion, understanding the components of a gas and oil boiler system is vital for homeowners and anyone interested in heating systems. Here we have covered the basics of gas and oil boilers, types of boilers and systems, discussed their key components, examined their operation, and highlighted maintenance and safety considerations. By understanding how your system or boiler works you can make informed decisions, ensure optimal performance, and understand the importance of an annual boiler service.

Always consult qualified engineers for installation, maintenance, and repairs, as their expertise guarantees a well-functioning and safe gas and oil boiler system. Engineers working on your gas boiler system must be qualified with Gas Safe or if you have an oil boiler system then they should be qualified with OFTEC.

As we move forward, advancements in gas and oil boiler technology will continue to improve efficiency, eco-friendliness, and user comfort, and remember that new green heating solutions that are developed and advertised are not suited for existing homes.

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