everything you need to know about a combi boiler
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Everything you need to know about a combi boiler

A Combi Boiler is a short form of Combination boilers! And have been the fastest growing sector for the past ten years. These very efficient boilers give instant and continuous hot water without needing a storage tank, making it one of the most efficient boiler, especially for homeowners.

Combi boilers, as they're commonly known, provide central heating and instantaneous hot water at the turn of a tap, and have no need for header tanks and a hot water storage cylinder.

As a result, many homeowners are attracted to Combi boilers as they take least amount of space, therefore being suitable for smaller homes. It’s particularly suitable for an average three bedroom, one bathroom property and especially suits a busy family who do not want to wait for the water to warm up before taking a bath. In this article, we will discuss everything about a Combi Boiler from installation to its advantages and disadvantages. Let’s get started:

Definition of a Combi Boiler

A Combi Boiler or Combination Boiler doesn’t require hot water cylinder or cold water cistern because it instantly warm up the water passing through the supply line, allowing you to enjoy without even waiting for a large cylinder to heat up, making it perfect for small house owners.

What makes a Combi Boiler Different?

a) No Hot Water Cylinder Needed

For those who have no idea what a combination boiler is, it serves as an effective water heater as well as a central heating boiler without needing a hot water cylinder or cold water cisterns.

If you have opted for a Combi boiler the instant hot water is provided by the Combi boiler itself. It takes cold water directly from the mains and heats it delivering it to the hot water tap.

b) Instant Hot Water without Waiting

These boilers provide both instant hot water and central heating, but not at the same time. They are “hot water priority” which means when hot water is being run there is no heat output to the radiators. These boilers are ideal in smaller homes where space is at a premium or where the demand for hot water is not too great. 

These are not recommended for houses with more than one bathroom due to the flow rate which can only feed one tap at a time. Basically, it’s a double threat and one of the reasons why a lot of UK residents can’t live without Combi boilers in their homes.

c) Can be used to grid & off-grid Homes

The interesting feature of a Combi Boiler is that it can run on different fuels, such as natural gas, oil, LPG, and even on electric, making it perfect for grid and off-grid homes.

Popular Variant of Combi Boiler

  1. Storage Combi Boiler
  2. Condensing Boiler

It should be noted that Combi Boilers provide maximum pressure through only one tap at a time.  If two taps are used at the same time the pressure is substantially reduced.  Large houses with larger families should consider a Combination Storage Boiler.

a) Storage Combi Boiler

Storage Combi boilers provide both instantaneous hot water and full central heating but it has a hot water cylinder built in which assures an even better hot water service.

It is the best solution for supplying a household with more than one bathroom. A storage Combi offers you several savings and advantages:

  • No more hot water cylinder
  • No tanks in the loft, which means a lot less of pipework and a quick and easy installation
  • No delay in the domestic hot water delivery thanks to the integrated hot water storage, which means a definite annual saving on the water bill.

b) Condensing Boiler

Don’t get confuse because condensing boiler is not a type of boiler – Actually, it’s a technology within the boiler, which makes the boiler more efficient.

Condensing boilers are the best solution to suit the customers' environmental concerns: In a condensing appliance the heat that normally goes to waste out of the flue is harnessed and used to improve the thermal efficiency. It can easily be installed as a direct replacement for an existing combination model. Condensing boilers combine two major benefits:

  • Energy savings – With 98% full efficiency while the next best type of combi is around 80% and 70% for older boiler. This can mean savings up to 30% on a typical full bill.
  • Environmental gains – 98% energy efficiency means less gas burnt, less carbon dioxide produced and less heat wasted. The overall result is a clean combustion.

How Does Combi Boiler Works?

Gas Combi Boilers are great for giving you central heating and hot water throughout your home. They supply an endless amount of hot water as the water is heated directly from the mains supply.

When the tap is turned on, the heat exchanger of the combination boiler transfers the pre-heat energy to the water, heating it up and making it ready for use.

Moreover, due to its no to small water reservoir, it’s particularly easy to maintain the temperature via thermostat, meaning one can enjoy instant water when they turn on the tap and making it environmentally friendly because of its energy-saving feature.

Advantages & Disadvantages of a Combi Boiler

Advantages of a Combi Boiler

  • Space Efficient – A Combi boiler is very convenient, because –unlike the traditional boilers– it does not require a hot water storage tank, or a cold water feed tank. Those tanks are bulky and take a lot of space in the loft or basement.  Installing a Combi boilers also reduces the amount of pipes and components.
  • Energy Efficient – The Combi boilers take the cold water directly from the cold mains supply and instantly heat it for you. This is a big advantage, saving energy, money and space.   As a result, Combi boilers are more energy efficient and money saving than the regular boilers.
  • Least Pipe Work – With its ingenious design, it gives user the freedom to save cost and energy on least pipework, making it perfect for small home use.
  • Incredible Flow Rate – With Combi Boiler, one can enjoy an incredible flow rate up to 18 liters per minute, making it perfect for apartments and smaller homes.
  • Adjustable Temperature – With its built-in thermostat feature, you’re allowed to control the temperature and enjoy hot water bath anytime.
  • Eco-friendly – As compared to other conventional methods, combination boilers cause negligible harm to the environment. Thus, these boilers work in an environment friendly way.

Disadvantages of a Combi Boiler

  • Complex Maintenance – The compact design and advanced electronics mean that Combi boilers are more complex than the average or traditional boiler and this can lead to problems and difficulty for the issue to be diagnosed.
  • Reduced Water Pressure – It should be noted that Combi Boilers provide maximum pressure through only one tap at a time.  If two taps are used at the same time the pressure is substantially reduced. 

Things to Consider When Buying a Combi Boiler

If you are looking to buy a Combi boiler, you should definitely spend some time looking the best Combi boiler and things to consider before buying for the amount you are willing to pay.

  • Water Output – Some combination boilers supply hot water quicker than others. This is mainly because some store a small amount of hot water, so when the hot tap is run the stored water is used first. Others have to start up from cold which could mean running a lot of cold water before the hot water gets to the tap. It is recommended to get a 28KW output as a lower output affects the hot water temperature.
  • Available Installation Space – Careful consideration as to the location of a combination boiler is essential. It should be fitted as close to the taps as possible. (halfway between the kitchen & bathroom would be ideal) If a combination boiler is fitted a long way from the taps, a lot of water will be wasted waiting for the hot water to reach them.
  • Size of Property – Don’t forget to consider the size of the property because large boiler with smaller family will end up increasing your bills.

Top 3 Best Combination Boiler to Purchase

If there is a need in your home or office to conserve space and energy then a Combi boiler might be just what you need.  These are a combination, high efficiency heating unit that can produce radiant heat for a conventional radiator system as well as hot water for your home or office. Below are top 3 best combination boiler, which you can consider to purchase:

ideal logic 24kw combi boiler

Ideal Logic 24KW Boiler

This fantastic 24KW Combi Gas Boiler offers endless water heating, making it eco-friendly so you don’t need to waste water just for the sake of waiting.

  • Lightweight construction, weighing only 28.9KG
  • Large display with easy control makes it user-friendly.
  • Ideal for homes up to 15 radiators.
  • It comes with built-in frost protection and Honeywell gas valve.
  • It comes with 8-year Warranty
worcester bosch 25kw boiler

Worcester Bosch 25KW Boiler

This ingeniously designed Combi Boiler can be installed anywhere in the house, which blend seamlessly with the interior and offers continuous yet instant hot water.

  • Ideal for home up to 15 radiators
  • Lightweight construction, weighing 36KG only.
  • Large display with easy controls makes it user-friendly.
  • Noiseless boiler design doesn’t hinder your lifestyle
  • It comes with 5-years Warranty
worcester bosch 8000 30kw boiler

Worcester Bosch 8000 30KW Boiler

Waiting for hot water in bathrooms or kitchen located far from heating source can be frustrating – Not Anymore! Because this fantastic 30KW boiler offers instant hot water when you turned on a tap.

  • It’s offers 10-year longest warranty
  • Unique intelligent water filling system
  • Built-in Wi-Fi for easy monitoring
  • Ideal for home up to 15 radiators
  • Easy to use controlling features

Some common FAQ’s regarding a Combi Boiler

Is it easy to install a Combi Boiler?

Yes it’s easy to install Combi Gas boiler, but you’re not allowed to install on your own.

Before you buy a combination boiler you should have an installation audit to ensure it can be fitted in your home. The heating fitter will ensure that your proposed installation complies with the latest building regulations.

Just because you had a conventional gas boiler install does not mean that you can do a direct switch for a condensing heating boiler as they have unique installation requirements.

Moreover, only certified Gas Safe Engineer, like an individual or company is allowed to install safely on your premises.

Is it true that Combi Boiler Doesn’t Last Long?

Absolutely not! Usually, a good brand yet well-maintained Combi boiler can last up to 10 to 15 years, making it one of the most efficient yet cost-effective solution.

What size KW Boiler Do I need?

Yeah that’s the most important question to answer because choosing appropriate boiler can help you save energy and allows you to enjoy instant hot water. We answer this question in the simplest way:

  • 02 bedroom house with 01 bathroom need 26KW whereas 29KW for 02 bathrooms.
  • 03 bedroom house with 01 bathroom need 29KW whereas 35KW for 02 bathrooms.
  • 04 bedroom house with 01 bathroom need 29KW whereas 35KW for 02 bathrooms.
  • 05 bedroom house with 35 bathroom need 29KW whereas 35KW for 02 bathrooms.


We hope that this comprehensive guide about Combi Boiler helps you a lot, and at the end you’ll be able to know why it’s best as compared to conventional boilers.

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David Roberts
David is an extremely passionate heating engineer. He particularly enjoys the technology aspect of the industry especially how new heating controls work to help people save money and keep warm.
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