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What are the three types of boilers?

The traditional gas boiler has discreetly been at the center of most British homes for half a decade, if the kitchen is the heart of a home then most certainly the gas boiler is the engine that makes it all work. 

They’re often neglected, many times are taken for granted but always missed when they breakdown, the gas boiler, Heatsy salutes you.

For 50 years you’ve kept us and our families warm, protecting us from the cold winters, and gifted us with warm water to bathe and relax after a hard day at the office.

There are generally three main types of gas boiler: the Heat Only boiler, the System Boiler, and the Combi Boiler.

Heat only boiler

The Heat Only boiler is the oldest type of boiler, it’s a very simple appliance and generally consists of little more than a gas valve, a burner, and a heat exchanger.

The heating boiler will be installed with a hot water cylinder and you’ll usually have two water cisterns in the loft.

Heating from campfires to heat pumps

System boiler

The system boiler is a natural progression from the heat only boiler. It’s a lot more sophisticated incorporating many more safety features, as well as many of the external parts from the heating system – hence the name, system boiler.

Combi boiler

The combi boiler is the most popular boiler in the UK, and with good reason.

Being a small island space has always been a point of contention in big cities and big bulky hot water cylinders took up far too much of the limited space available in many flats. 

Enter the combi boiler, with the ability to provide heat for your central heating and instant hot water for your taps, there was no longer a need for the hot water cylinder and we’ve loved them for the last 25 years.

If you’d like a more in depth explanation of the different types of gas boilers and how they work, read our blog post – What type of boiler do I have?

How does a boiler work?

Now we have a basic understanding of the different types of boilers, we can take a look at how the magic happens and see how boilers work.

The basic chemistry of how the gas boiler works is very simple:


In our case the fuel is natural gas, the heat comes from an ignition source the oxygen that starts the combustion process comes from the air and the stored energy in the fuel is released as heat.

The heat is transferred through the boiler’s heat exchanger to water which is then distributed by either a pump or gravity throughout the heating system and passed to heat emitters.

For a more scientific analysis of combustion and the complete process of heating your home, take a look at our blog post – How Your Gas Boiler Works.

Boiler servicing

Given the importance of the gas boiler in our everyday lives and the potential dangers from gas and CO leaks, it’s not surprising that both the HSE and boiler manufacturers suggest that your gas boiler should be serviced at least once every twelve months.

An annual boiler service is essential maintenance, and vital to ensure your trusted gas boiler continues to operate at its optimum efficiency and to prevent any dangerous CO leaks or costly boiler breakdowns.

To see the checks we do and the process we follow when servicing your gas boiler visit our Annual Boiler Service page.

Boiler repair

There is a common misconception that modern condensing gas boilers are unreliable and don’t last as long as older boilers.

The truth is that the majority of gas boilers breakdown because of poor installation and maintenance, given that your boiler regularly endures flames burning up to 1950 degrees Celsius, we think it manages just fine.

Of course, boilers do breakdown even when they’re maintained perfectly and looked after lovingly.

If you need your boiler repaired or you’d like to read more about boiler repairs and how we can help, visit our Boiler Repairs page.

Boiler Installation

Unfortunately, they don’t last forever and the day will come when you’ll need a new boiler installation.

Installing a gas boiler is not like fitting a washing machine, it’s a complex piece of building engineering and should only be completed by a competent professional who’s registered on the Gas Safe Register.

A properly installed gas boiler and heating system require complex calculations such as heat loss, pump sizing, and pipe sizing to make sure the installation is correctly specified for your home. This will in turn save you money on your energy bills and reduce the risk of unnecessary boiler breakdowns.

To see which appliances we recommend and how we install them visit our Boiler Installation page

Boiler Fault Codes

If your Vaillant boiler shows an error, you might find a solution how to fix it on one of the following pages:

Vaillant F22 Fault Code – How to fix?
Vaillant F61 Fault Code – How to fix?
Vaillant F62 Fault Code – How to fix?

If you have a gas boiler and your heating system is not getting hot, you can find the possible reasons and solutions here: Why aren’t my radiators hot?
System boiler

Thank you for your interest

We hope you enjoyed reading our brief overview of what a gas boiler is and how it works and that you’re intrigued enough to read our more in depth pieces that go into far more detail.

But for now, we’re happy that hopefully, next time your trusted boiler breaks down or your annual service reminder pops up in your inbox you’ll be a little more forgiving.

As standard, our heating & boiler services come with:

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12 Month Guarantee

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5 Star Reviews

Genuine Manufacturer Boiler Materials

Genuine Manufacturer Boiler Materials

Gas Safe Registered Engineers

Gas Safe Registered Engineers

Heatsy promise to deliver excellent boiler repair services

The Heatsy Promise

If we can't diagnose the fault with your boiler, we'll pay you double the diagnostics fee.

Heatsy your reliable heating engineers London and nearby places

Key to a reliable heating system is proper maintenance

At Heatsy we don’t just service your boiler and leave in 20 minutes like so many in our industry. We check the water quality, balance the heating system, check the water tanks, controls and radiators and we leave you a report with all of our findings and any recommendations so you can make an informed decision in your own time and prevent any major problems before it’s too late.

An emergency boiler repair in London

At Heatsy we specialise in repairing old and new boilers by all manufacturers, we offer a 24/7 same day response service. All our work is guaranteed and if we can’t diagnose the fault with your boiler or heating system we’ll pay you DOUBLE the diagnostics fee. 

Watch the video and see why we insist on powerflushing 90% of our installations, and how we can help if you’re boiler brakes down.

Our boiler repair services in includes:

Tried, Trusted, Reviewed

The Heatsy promise:

“We promise to always give 100% in our efforts to help you in your time of need, to never take advantage, overcharge or give you unnecessary advice.

To keep our charges fair, competitive and 100% transparent while keeping our service above and beyond by using only fully vetted, qualified, experienced and skilled tradespeople.

Should you ever have a query about the service you have received, the price you were quoted, or the work we have completed, we promise to treat the situation with the seriousness it warrants and give you the dignity, respect, and honesty you deserve." 

Mr Lee Moloney

Heatsy are trained and accredited by Vaillant and Viessmann

At Heatsy we’re trained and accredited by the two leading names in heating, Vaillant, and Viessmann, and can offer up to a 12-year guarantee on our installations.

These accreditations mean that we maintain very high standards for all of our services. You can rest assured that all Heatsy engineers have expert knowledge of the products we install which ensures our boiler installation services are some of the best in London.

Please contact us to arrange an exceptional survey you can rely on.

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Our Google Reviews

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Heatsy Ltd, 4 Selsdon Crescent, South Croydon

4.9 67 reviews

  • Avatar Sheshebens Algonquin ★★★★★ 3 months ago
    Heatsy completed our boiler repair quickly and efficiently.
    We called at 12pm and were offered two appointments, 4-6pm that evening or 8-10am the following morning at a lower rate.
    … More As it’s winter and mom was visiting from the US we decided to get the emergency appointment.
    The engineer called and text before he arrived and kept me updated throughout the work.
    Coming from the states we’re used to great customer service and I must say Heatsy made me feel right at home.
    Your slogan should be ‘British class with American service’
    I’ll be telling all the other moms at the next PTA meeting 😊
  • Avatar Zacharie Paimboeuf ★★★★★ 3 months ago
    Great boiler service, the engineer took everything apart and cleaned the inside and changed the seals.
    He also took a water sample from the boiler which showed the system was dirty
    … More and he was able to book me in to get my system cleaned without me needing to do anything.
    Very helpful and professional.
    Thank you
  • Avatar Sarah Luetchford ★★★★★ a month ago
    Was extremely helpful and explained what needed doing and was honest about what I needed to consider. Once he had finished and I messaged with an enquiry my engineer offered to come … More back and have a look. They sent a follow up message to ensure everything was still working and to check it was ok. Would definitely use again.
  • Avatar Martin Gilbert ★★★★ 3 months ago
    I had a combined boiler service with landlords gas safety certificate.
    The company were very professional and my tenant was very happy with the engineer.
    All paperwork and certificates
    … More were emailed in real time and I could track the engineer which is a nice touch.
    Four stars as the appointment had to be rescheduled due to over running work.

Other services we offer

System boiler

New Boiler Installation

If you are interested in having a new boiler, we can inform you about the best and affordable solution. we offer a variety of boiler installation packages to suit most budgets.

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Annual Boiler Service

We service gas boilers everyday and have tons of 5 star reviews to vouch for us. We guarantee to leave you’re boiler performing better than when we got there.

Boiler & Heating Services for Landlords

Services for Landlords

With automated service reminders, real time job reports, engineer tracking and a cloud based storage system we offer hassle-free compliance for all your rental properties.