Category - Boilers
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Will Scholfield, Engineer

19 Jun : Updated 21 Mar ● 11 min read

How does a boiler work? Learn how your home is heated

If you’re new to the world of boilers, you may not have thought about how one actually works. Considering boilers provide vital heating to our homes, how are they able to do this?

Different types of boilers operate in various ways and in this guide, we go through the intricacies of how each of them work. From different fuel types to the size and placement of the boiler, we’ll give you everything you need to know about boilers.

How do boilers work?

The main purpose of a boiler is to provide central heating to the home by heating up and pumping hot water around your heating system pipework and radiators as well as giving you hot water to bathe and wash in.

Boilers work with either gas, electricity, oil, or biomass to generate heat. Additionally, renewable energy fuels such as solar are beginning to make their way through as potential replacements for fossil fuels.

How do different types of gas boilers work?

Gas is the most common type of fuel for heating in the UK, meaning that there are a few different systems you can use. The following are the different gas boiler types and how each of them operate:

Combi boilers

A combination boiler, or combi boiler, is a single boiler unit that produces hot water for your central heating system as well as your hot water taps. Less space is required with this boiler type as it has no requirement for a hot water storage cylinder or water tanks to fill it; an advantage if you’re short on space!

These boiler types are quick to install, cost-effective, and energy-efficient. A disadvantage of this type, however, is their size, and hot water delivery capability. They are not always suitable to heat up larger properties and will not satisfy large hot water demands or properties wanting to run multiple hot water outlets/showers at once.

Discover which boilers are best for large houses.

System boilers

A system boiler uses a separate hot water storage cylinder. Although these types of boilers take up a bit more space due to their size, they can supply a lot of hot water. This boiler is great for larger properties where hot water consumption is higher and multiple hot water outlets/showers are being used at once.

System boilers are mostly filled directly from the water mains. Although they are bigger than combi boilers, they don’t require a cold water tank to take up further space.

Standard boilers

Standard boilers are also called "regular boilers" “conventional boilers” or "heat-only boilers". These boilers are well suited for very large properties. They take up the largest amount of space, the boilers themselves are of a similar size to the types previously discussed however in addition to the boiler they also require a hot water storage cylinder and associated feed and expansion tanks these are usually stored in an airing cupboard and/or loft space. The boiler itself and all radiators including the primary hot water pipes are filled by the feed and expansion tank normally located in the loft. Then, in most cases, the hot water storage cylinder is fed under gravity by the header tank which is also normally situated in the loft. 

If at any point your boiler breaks down as a heat source, then your hot water storage cylinder could have an immersion heated fitted, which is operated electrically. This will give you an alternate source of hot water to bathe and shower in. However this would not heat up your radiators for the central heating system. All hot water storage cylinders and pipework within a meter of them must be insulated as not to lose their heat to satisfy current regulation.

Natural gas boilers - The pros and cons

Gas boilers work by either releasing gas or forcing a premixed gas and air combination into a burner via a valve. At this point the gas passes through the burner, igniting within the sealed combustion chamber and then transfers heat to the system or primary water within the heat exchanger. Simultaneously, a pump, either situated on the system or in the boiler itself, will be running water through the heat exchanger. As the water is heated by the burner and heat exchanger it is moved by the pump around the heating system to the radiators and also to the hot water storage cylinder, if the boiler is of the standard or system type. Combination boilers heat the hot water instantaneously within the boiler itself.


  • More affordable than other boiler fuels. Natural gas is much more affordable and economical than electricity as the price of gas is currently about 4 times less than electricity per kWh.
  • Gas is a clean fossil fuel: gas is considered to be the cleanest fossil fuel available. Burning gas produces less carbon monoxide and carbon dioxide than oil or coal burning.
  • Gas is dependable and is available readily and safely.
  • There is a large range of gas boilers compared to other fuel types, meaning you can find the right boiler for your needs. This also means that boiler replacements are a lot faster and simpler.


  • Though it is the cleanest fossil fuel, that doesn’t mean it has no impact on climate change.
  • Installation might be more expensive than electric - this is due to the skill and knowledge required to fit gas heating safely and compliantly.
  • Your home must be connected to the grid. If you are not already connected to the gas network, it can be a costly exercise to get connected.
  • Annual servicing of gas burning appliances is recommended and has a cost attached to it.

How do other boiler types work?

Oil boilers

Similar to a gas boiler, this type of boiler burns oil in the combustion chamber. The heat exchanger then warms up the water and supplies heat to a combi system or conventional system. 


  • It doesn't need gas. Of course, this makes sense, but if your house is not connected to the gas network, then the option of an oil boiler is the cost-effective alternative.
  • Easy service and maintenance
  • Oil can be burned hotter than gas


  • Your oil supply can become depleted. Unlike gas and electric boilers, an oil boiler does not have the fuel source constantly supplied to it. Oil must be ordered and stored, and it is up to the property owner to monitor the amount of oil available and order more before running out.
  • Oil can be a messy fuel and will build up soot easily
  • Lump sum payments. Unlike gas or electricity, most oil suppliers will not send a monthly bill, and you will have to pay a large bulk sum when you order your oil.
  • An oil boiler requires an additional storage tank. The oil tank will take up additional space and requires a flat area with a solid base. You can also opt to store the oil tank in the ground to save some space.

Liquified Petroleum Gas (LPG) boilers

An LPG boiler burns liquefied petroleum gas to heat water and produce energy. It operates similarly to gas and oil boilers.


  • LPG boilers are more affordable than oil boilers. If you are not connected to the gas grid, then an LPG boiler is an affordable option as the boiler itself will cost about £350, while an oil boiler retails for around £800.
  • There are plenty of LPG boiler options available. Many popular boiler brands produce high-quality LPG boilers, meaning there are various brand options to choose from.


  • It is costly to run. The price of LPG is much higher than natural gas, oil, and electricity. Although the original cost of the boiler is affordable, the fuel cost per unit is typically higher.
  • You have to monitor the fuel tank levels. Similar to oil heaters, you will have to keep an eye on your fuel tank to make sure you do not run out of fuel.
  • Require additional LPG fuel tanks. These tanks will have to be purchased or rented separately from the boiler and will require safe storage space.

Electric boilers

These boilers use electricity in order to heat up water much like you would when boiling a kettle, only on a much larger scale. Water is then passed through the metal piping and transfers to radiators and taps. 


  • Electric boilers have very high efficiency. These types of boilers are about 99% efficient and don’t lose heat like other boiler types.
  • Compact size and easy to install. These boilers are pretty straightforward and make use of simple technology. Their installation costs can be lower, as there is no need for a flue or additional storage tank. You do not require a Gas Safe Registered Engineer for the installation however, a certified electrician is always recommended.
  • Does not require a gas supply. If your property does not have access to the gas main, an electric boiler is a great alternative for effectively heating a home.
  • Solar energy supplies. With the ever-increasing use of solar panels installed onto new build properties, electric boilers are a popular choice for those generating their own electricity. This could help offset any electricity drawn from the national grid, and become a more affordable solution to heat your home.


  • Electricity is expensive. Although they are incredibly efficient, electric boilers are expensive because of the high cost of electricity per kWh. Gas boilers are much more affordable to run, so if you have the option of gas, rather consider this type of fuel.
  • Electric boilers are not great for very big properties. These boilers are only able to heat smaller volumes of water at a time. Therefore a property with a lot of hot water outlets that are used at the same time is not compatible with these types of boilers.
  • You require a steady supply of electricity. If your area is prone to power cuts, then an electric boiler may not be a reliable solution for your property.

Biomass boilers

Biomass boilers use and burn wood pellets, logs and chips to generate heat for the boiler. These are usually found to heat a single room or provide enough heat for a stove. The biomass boiler can pass the heat through a heat exchanger and heat up the water.


  • Tend to be more environmentally friendly than other fuel types. Biomass boilers are considered carbon-neutral if the fuel is sourced relatively locally so they are much more eco-friendly than other boiler types. This is because wood produces as much carbon when being burned as it absorbs while growing. Try to find locally grown and produced biomass fuel to cut down carbon costs even more.
  • Don't have to rely on a supply of gas or electricity. This means you will not be impacted by price increases from the suppliers, and you will also not be affected by any outages.
  • You could qualify for the government's Renewable Heat Incentive. This includes a series of payments over a few years.


  • The cost of installation is high. This will depend on the brand you go for, but biomass boilers, on average, cost more to install than other boiler fuel types.
  • You can run out of fuel. You will have to make sure you have a large supply of fuel available for the colder months so that you are not left out in the cold. Only certain types of biomass are suitable (for example, you cannot just burn garden waste), so it is up to you to make sure there is enough appropriate fuel.
  • Fuel reserves will require storage space. During the winter months, you will require a lot of wood or other appropriate biomass, and you will need a space to store the fuel reserves.

The importance of boiler maintenance

It’s important that you can make the most of your boiler. Having regular maintenance checks and boiler servicing from a registered engineer can ensure that your boiler is functioning correctly and safely so both you and your home are getting the most out of it.

If your boiler is still under warranty, you must have an annual service check, otherwise you risk invalidating your warranty.

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Final thoughts

Most properties have a central heating system that we often don’t need to worry about when they’re operating correctly. We rely on our boilers to provide our properties with hot water in our taps and warm rooms and floors throughout the home, so having a good understanding on how boilers work is useful to help notice if there’s anything wrong.

For more information on how long it can take to fit a new boiler or if you need further assistance from BOXT, contact our specialist team.

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