James Elston

Published April 8, 2024

A boiler is a heating appliance used for both central heating systems and to provide hot water to households.

Boilers use natural gas or electricity to convert cold water into warmth using a heat exchanger. There are different types of boilers, each suited to different-sized homes and heating needs. These are:

  • Combi boiler
  • System boiler
  • Conventional boiler

These types of boilers come in different sizes – measured in kilowatts (kW) – to meet different heating and hot water needs.

Different boilers also use different fuel sources, such as gas (the most common fuel), electricity, oil, biomass and LPG.

In this article, we’ll explore how boilers work and different boiler types, from fire tube boilers that use combustion gases to water tube boilers with water tanks. We’ll also discuss the benefits of boilers and how to choose the right boiler for your home. 

Why Homeowners Need Boilers 

Most homes use heating and hot water daily, even more so during the colder winter months. Boilers have a direct impact on homeowners’ comfort and form part of our daily routines.

A central heating boiler, be it a gas or electric boiler, isn’t a mere appliance; it serves as an integral element (no pun intended) that provides warmth and hot water needed for various household tasks. Furthermore, high energy-efficient boilers offer cost savings and environmental benefits too.

Understanding the role boilers play in everyday life empowers homeowners to appreciate their significance beyond their functionality. Boilers offer home comfort, ensuring efficiency, economy, and an inviting ambience in every corner of our homes.

How Do Boilers Work

Boilers operate by burning fuel or using heating elements to heat water. 

Cold water enters the boiler, either directly from the mains or a cold water storage tank (depending on the boiler type). The boiler then creates hot water, which is directed to your central heating system and radiators, a hot water tap (if opened), or stored in a hot water tank. Again, the exact way in which a boiler heats and stores hot water depends on the type. 

Most British home boilers work by generating heat from gas, which is supplied by the national gas grid. The majority of gas boilers are combination boilers – combi boilers for short.

Boilers used to rely on older methods, such as wood fire-based heating. Today, they’re the epitome of heating sophistication. This evolution is a testament to technological advancements like condensing technology. Modern boilers ooze efficiency and convenience, ensuring consistent warmth and instant hot water in our homes.

So, what fuels do boilers use if they no longer rely on wood fires?

The different types of fuel boilers use include:

  • Gas
  • Liquid Petroleum Gas (LPG) 
  • Oil 
  • Electricity 
  • Biomass 

Components Of A Boiler System

Let’s discuss the components that bring a central heating boiler to life:

  • Heat exchangers are responsible for burning fuel, like natural gas or electricity, to produce heat.
  • The combustion chamber (also called a firebox) is where the air mixes with boiler fuel to ignite.
  • Diverter valves redirect hot water from radiators to your taps, when needed. (Make sure to read our guide on how to free a stuck diverter valve if you suspect yours is malfunctioning. 
  • The expansion tank is a little tank next to the main boiler unit that protects the system from excess pressure.
  • Pressure gauges indicate the pressure levels within boiler systems.
  • Supply lines transport and circulate hot water through the boiler’s pipes.
  • Return lines are the channels that bring cooled water or steam back to the boiler for reheating.

Types of Boilers

There are three main types of boilers. These are:

Combi boiler

A combi boiler is a highly efficient water heater and central heating boiler combined into a single unit.

Combi boilers heat water directly from the mains supply whenever hot water is needed. This eliminates the need for a separate hot water storage tank or cylinder and ensures a constant supply of hot water on demand. 

Since there is no need for a separate hot water tank or cylinder, a combi boiler is particularly beneficial for homes with limited space. 

Here’s how a combi boiler works:

  1. Cold water is heated by the heat exchanger, which burns fuel to produce heat that is circulated through our homes.
  2. As byproducts, carbon dioxide (CO2) and water vapour (steam) are released.
  3. Combi boilers capture these exhaust gases (flue gases) to heat cooler pipes in the boiler system.
  4. The excess wastewater (condensate) is expelled from the system to the sewer via the condensate pipe.

System boiler

System boilers, unlike combi boilers, require a separate hot water cylinder for storage. They are ideal for larger homes where extra space is available. System boilers perform well in homes with multiple bathrooms or higher hot water demands, as the separate tank allows them to supply hot water to many taps simultaneously.

Conventional boiler

A conventional boiler is also known as a standard boiler and is often found in older homes. They are the largest boiler type of the three as they require space for a hot water cylinder and a cold water tank. If you’re looking for something smaller, why not change from a conventional boiler to a combi boiler?

The Benefits Of Boilers

Now that we’ve covered the three most common types of domestic boilers, let’s look at their benefits.

Combi boiler

  • Modern energy-efficient gas boilers, like combi boilers, release three times less exhaust gases than traditional boiler systems.
  • They have an energy efficient rating of 90%, whereas traditional models are only 70% – 80% efficient.
  • They lower energy bills and a household’s carbon emissions.
  • They are compact units – something to bear in mind when choosing a boiler for a small home.
  • Combi boilers provide on-demand hot water, reducing waiting times for systems to heat up.
  • Due to their compact size, these boilers can fit inside kitchen cupboards, utility rooms or air cupboards, which is particularly beneficial in renovations and new builds.

Are you interested in investing in a combi boiler? To help you make the right choice, read our guide on the best small combi boilers.

Condensing boiler

Condensing boilers are closely related to combi boilers in terms of functionality and energy saving. They are equipped with condensing technology that converts CO2 and steam production back into water to heat cooler pipes. Their benefits include:

  • Less fuel consumption
  • Ideal for small to medium-sized homes
  • Instant hot water without needing storage tanks
  • Safe and reliable boiler units
  • Reduces energy bills, potentially saving you up to £300 per year (we’ve even compiled a guide on how to save on your electric bill to reduce your bills even further).
  • Can reduce carbon emissions by 1,200 kg of CO2 per year.
  • Well-maintained systems can last 15 years.

Make sure to check out our guide on the best condensing boilers!

System boiler

  • Provide on-demand hot water.
  • Ideal for larger homes with multiple bathrooms and higher hot water demands.
  • It has better water pressure and flow rate than combi boilers.
  • System boilers do not need cold water storage tanks.
  • They produce fewer carbon emissions as opposed to a conventional boiler.
  • System boilers are eco-friendly units that can help reduce energy costs.

Check out our guide on the best system boilers for your home.

Conventional boiler 

Conventional boilers are used in older households and do not offer many benefits when compared to the above. These systems heat water the “traditional” way, where a hot water cylinder and a cold water tank are needed. They consume more energy to generate warmth and heat water, release more carbon emissions, and are not very energy efficient, losing up to 30% of heat during the combustion process.

Choosing The Right Boiler

When faced with choosing a boiler for your home, bear these things in mind:

Home size

Home size directly impacts boiler functionality and efficiency. Choosing a boiler that is too small can result in a limited hot water supply, while opting for larger boilers may lead to wasted heat and money. For example, a three-bedroom home generally requires a condensing electric or gas boiler with 20-30 kW of power. The Viessmann Vitodens 100 w (26 kW) is a good option to go for.

Boiler type

The type of boiler you choose is always very important. If you have limited space, a condensing combi boiler may be your best bet. If your home has excessive hot water demands, a system boiler is most likely the smart choice. Still confused? Eco Happy’s guide on finding the right type of boiler for your home is sure to help.

Fuel type

Similar to the boiler type, the fuel type is also important. Consider and weigh up various fuel types, such as gas, LPG, or electricity.

Energy efficiency

Energy efficiency is vital, especially for eco-conscious homeowners. A higher energy efficiency rating indicates that your central heating boiler produces the maximum amount of heat per volume of fuel. For instance, an electric boiler with a 90% efficiency rating generates 90p of heat for every £1. These amounts impact your monthly energy bills drastically, giving you more bang for your buck.

Boiler costs

Consider both the upfront and installation costs for new boilers. But, how much does a new boiler cost?

Mid-range combi boilers cost between £900 and £1,200, whereas large system boilers can cost up to £4,000! Professional plumbing and heating engineer services must also be accounted for, usually costing between £120 and £140 per hour.

Boiler brands

Check top boiler brands and manufacturers and their warranties, including cover for a damaged combustion chamber, heating surface, condensate pipe, heat pipes and gas leaks. Usually, warranties range between 7 and 12 years (Valliant is a reliable boiler brand with a 10-year warranty as opposed to Alpha’s 7 years).

Future-Ready Home Heating

Efficient home heating has made a significant shift towards sustainability ever since the building regulations came into effect in 2005. This regulation states that all new boiler installations must be condensing boilers, eliminating older systems such as oil or back boilers. This is to align with the UK Government’s ambitious plan to reach net zero carbon emissions by 2050.

A few notable future enhancements for home heating include:

  • Heat pumps are emerging as key technologies in the realms of boilers and central heating. They’re designed to extract energy from the outside air or ground and convert it into heat.
  • Hybrid systems are also making an appearance, especially for properties where heat pumps are viable. Hybrids combine combi boilers and heat pumps to provide an eco-friendly heating alternative.
  • Government plans to install heat pumps in new builds by 2025 are paving the way for a greener future.
  • Natural gas boilers are slowly being phased out to preserve precious resources. Hydrogen boilers are a new technology that will supposedly replace these systems. However, 100% hydrogen is still too expensive for many households, making hydrogen-natural gas blends the alternative – for now.
  • Most average three-bedroom homes spend over £1,100 on heating each year. With a more energy-efficient mindset and installing eco-friendly boilers, homeowners can shave off between £300 and £350 on their annual energy bills.

Caring for your home boiler

Regular service and maintenance are crucial if you want your boiler to produce steam, generate hot water and minimise heat exchanger faults. It also makes sure they are kept in tip-top condition. For boilers to create heat energy, they need to be properly cared for – much like servicing your car to keep it running.

Here are a few home boiler maintenance tips you can start implementing today:

  • Whether you have a gas or an electric boiler, regular maintenance is important. This should be done at least once a year to keep your system in good working order. Heating professionals will check the heat exchangers, hot water pipes, cylinders, flue flow, steam drum, storage tank and electrical components.
  • Periodically inspect your boiler unit for signs of corrosion (rust), leaking pipes or faulty switches.
  • For condensing boilers, installing auxiliary syphon units to condensate pipes can prevent your boiler system from freezing.
  • Install a carbon monoxide detector to alert you when a gas leak occurs.
  • Set combi boiler thermostats at 60 °C to ensure they produce hot water at maximum efficiency. If thermostats confuse you or you just can’t get it right, our guide on how to connect a wireless thermostat to a boiler will put you on the right track.

There is also no shame in calling out the experts, like Eco Happy. They can help you maintain your boiler by performing checks and recommending minor repairs. If there is debris in your boiler that’s causing issues, a power flush can do the trick.


What fuel is used in boilers?

There are two main fuels that modern boilers use: gas (natural gas or LPG) and electricity.

What is boiler capacity?

Boiler capacity refers to the size of the water tank. It is how much water the boiler tank can hold.


Boilers are essential for a cosy, warm home. If you’re looking to upgrade or maintain your heating system, Eco Happy is your go-to source for expert advice and service. Contact us for a free quote and let us help you choose the best energy-efficient home heating system that meets your needs.

Let’s make your home heating efficient, sustainable, and eco-happy!

James Elston

Boiler Expert

James Elston is the top boiler replacement and heating expert at Eco Happy. He has over 20 years of experience in the industry, focusing on Gas Safe boiler installations and offering home-heating and energy-saving solutions to homeowners across the UK. From sourcing the most energy-efficient combi boiler to providing specialist heating advice, James ensures that Eco Happy maintains the highest standards and best customer service.

Leave a Reply

Your email address will not be published. Required fields are marked *