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What Is A Combi Boiler?

Person pressing button on a Vaillant boiler
New Vaillant Combi Boiler

A combi boiler is short for a combination boiler. It is a combination of a water heater and a central heating boiler system in a single unit. As soon as you turn on the hot water tap, it heats hot water on demand. This eliminates the need for a storage tank. Storage tank required in the case of a conventional boiler.

How Does Combi Boiler Work?

A Combi boiler is one compact system without any external cold-water tank or hot water cylinder. Combi boiler heat water and warm home through radiators. Combi boilers provide instant hot water, by constantly being on standby. As soon as you turn the hot water tap, the boiler will start and the hot water will be available immediately.

When some open the hot water tab and request hot water, this will signal the boiler to start heating water inside of the system and the hot water supply begins quickly. The heat exchanger inside the combi boiler will transfer the majority of the heat from the burnt gas inside the boiler to the cold water, make it hot, and then deliver the hot water to taps as required.

Inside the combi boilers, the control valves operate in different directions. These valves either allow water to flow through the central heating system or divert it to the hot water tap as required. The Combi boiler can’t do both at the same time. You can also pre-set your desired temperature and the boiler will work with the room thermostat to ensure your home will heat, as you like.

Pros of Installing Combi Boilers

Combi boilers are the most popular choice of modern households in the UK because they offer the following advantages:

High Efficiency: Combi boilers are highly efficient. You can save money on your heating bills by switching from a conventional boiler to a combi boiler.

More Affordable: The installation cost of a combi boiler is low as compared to a traditional heating system. Combi boiler parts are easy to find as well. The whole combi boiler system is less complex, and few man-hours are required to install them.

Faster & Quick Operation: Combi boilers are faster than the convention boiler. There is no need to wait to get hot water. You will get an unlimited supply of hot water on demand.

Less Space Requirement: Combi boilers are compact in size. They don’t have a separate cold & hot water storage tank. This makes them ideal for small homes.

Clean Water Supply: As the water is directly supplied from the mains and it is not stored in a tank, you don’t have to worry about built-up sludge. The water that runs through the taps will be much cleaner.

Safe to Drink Water: The cold water supplied by the combi boiler is safer to drink. There is no storage tank involved and the water comes directly from the mains supply makes it safer for drinking.

Fewer Pressure Issues: You won’t have to worry about air locks or low pressure when using a combi boiler system. The hot water comes from the mains directly.


Combi boilers are compact, quick to install and provide hot water on demand. They don’t have storage tanks or cylinders, which makes them ideal for small homes.


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