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Combi Boiler: Everything You Need to Know

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There are three types of boilers which all operate in different ways, namely:


1.    Combi boilers

2.    System boilers

3.    Regular boilers (Heat only)


What is a Combi Boiler?

Combi boilers, or combination boilers, are considered to be the United Kingdom’s most popular type of boiler.

Usually installed as a single wall-hung appliance, commercial combi boilers are very compact and highly efficient, not only supplying all the heating requirements, but also hot water for sanitary use, such as bathing or showering, washing up, food preparation, hand washing etc,

They ensure not only significant savings on installation costs, but also guarantee lower running costs too, making them the best choice for larger domestic properties or commercial applications, where both heating and hot water are required.

Unlike traditional systems, the combi will instantaneously heat water when turning on the hot tap, so there is no need for a hot water storage cylinder, making this product the ideal choice for properties with limited space.


How a Combi Boiler Operates.

A combi boiler integrates a central heating boiler and a water heater, heating water via an integral heat exchanger directly from the cold mains. The boiler fires up if the room thermostat gives a signal to do so when the temperature falls below a set point, allowing heat to be injected into the central heating system. Additionally, the boiler may ‘modulate’ or turn itself down once you reach the thermostat set point, maintaining the heating at the proper temperature, which saves energy.


When a hot water tap is switched on, a valve inside the combi boiler redirects the heat into the hot water supply, producing hot water until you turn the tap off, at which point the valve resets to the ‘central heating’ position. Heat may be delivered to either the hot water taps or central heating system, but never both simultaneously.


Due to the much higher efficiencies, it is now a legal requirement to have a condensing boiler instead of a non-condensing boiler when replacing a boiler or fitting a new one for the first time.


A non-condensing boiler only has one heat exchanger, so the waste gases that exit the boiler flue could reach up to 250°C. This is wasted heat that could be recycled and used in the system to reach maximum efficiency. A condensing gas boiler is able to recycle this heat from the waste gases and use it to heat the water.

A combi boiler, unlike an open vent boiler, operates on a closed system. Any air that gets into the stream has an impact on the heating, boiler, and pump. If the boiler loses any more pressure, it shuts down and displays an error code. A loose or rusted connection might result in a sudden loss of pressure.


To restore a combi boiler’s pressure to the appropriate levels, one uses a filling loop. A pressure gauge is normally located next to the display on each boiler. The combination boiler’s pressure should never be permitted to dip below 1 bar or rise above 3 bar. The safest range is between 11/2 and 2 bar.


How Do You Fuel a Combi Boiler?

As most UK Commercial properties are linked to mains gas via an underground network, gas is often used to fuel a gas combi boiler. However, this is not the only available method. Combi systems can also be powered by LPG, oil, or electricity. LPG is normally delivered in bottles that must be replenished on a regular basis. Similarly, you normally store oil in a tank that must be replaced on a regular basis.


The future of Combi Boilers

In an effort to reduce our greenhouse gas emissions by 80% by 2050, the UK government is seeking to change the way in which we heat our homes. According to recent figures, heating constitutes around 40% of the UK’s energy consumption. The introduction of fuel sources such as hydrogen could help us reduce our carbon emissions.


Hydrogen would be used as an alternative to Natural Gas, LPG and oil, which all produce carbon dioxide as a by-product of the combustion that takes place. There have been many suggestions as to how hydrogen can be produced as a fuel source, including an electrochemical process where hydrogen is separated from the source fuel. In this case, the carbon emissions would have to be captured and permanently contained.

Should You Purchase a Combi Boiler for your Commercial Application?

A combi boiler needs enough mains water pressure to give a reasonable water flow rate; if the mains water pressure is too low, hot water only trickles out the tap. System boilers may be a better alternative for you if your mains water pressure is low or if your Commercial premises have more than one washroom facility.


It is also crucial to make sure the boiler’s heat output is adequate for your requirements.

Combi boilers produce two types of heat: one for domestic hot water and another for central heating. Producing hot water requires more effort and thus more heat than heating radiators; as a result, the hot water output is generally what defines your boiler option. A Gas Safe Registered installer can assist you in determining what boiler you need to meet all your Commercial heating & hot water needs.

What are the advantages of a Combi Boiler?

Combi boilers offer the following benefits:

  • They are easy to use – The majority of combi boilers contain simple controls. You can program the system’s heating to turn on when you need it due to the temperature and time controls. The ability to choose your desired temperature, at any given time, enhances the comfort of your premises, which reduces your fuel bills and gives you more control.


  • The boiler itself contains everything needed to supply heat to the radiators or hot water to the taps, eliminating the need for a central heating pump or hot water storage cylinder – ideal for commercial premises with limited space.


  • As a combi boiler does not require a water tank or a hot water storage cylinder, it requires less plumbing, which saves time and money during the installation process.


  • Lower maintenance costs – the integral heating and hot water functions are covered by the boiler manufacturer’s warranty/guarantee.


  • No long wait times for hot water



What are the disadvantages of a Combi Boiler?


  • They do not work well in properties with poor flow rates and/or poor incoming water pressure.


  • Hot water flow rates are reduced when providing hot water to two or more outlets simultaneously, although this is really only a problem in properties with two or more washrooms.


  • A combi boiler system does not have backup immersion heaters to heat any hot water in the case of a breakdown.

Other common questions about Combi Boilers

Do You Need a Separate Hot Water Cylinder?

No, you do not. A combi boiler only heats the water when you turn the tap on, which means there is no need to store any hot water.


Can You Install a Combination Boiler Yourself?

No – Gas appliances must be installed by a  Gas Safe Registered  installer.


What Accessories or Controls Do You Need Regarding Combi Boilers?

Building Regulations state that every boiler installation now requires a timer and controls, as well as additional energy efficiency measures regarding combination boilers.


1.    The seasonal space heating efficiency must be at least 92% (A rated on the ERP label).

2.    All installations require temperature and time management.

3.    One of the following has to be included in every combi boiler installation:

·      Compensation for the weather

·      Compensation for load

·      Heat recovery from flue gas

·      Automation and optimisation with smart control



Which Combi Boilers Are the Best?

Factory Heaters only supply ATAG Hydrogen Ready combination boilers (blended) which are energy efficient and trusted as a premium brand by both the industry and customers alike. With a wide range of boilers available, you can be sure to find the most popular choice and ideal system for your commercial application.

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