Condensing Combi Boilers: A Full Guide

Since 2005, all new gas central heating boilers installed in the UK need to be high efficiency condensing boilers unless there are exceptional circumstances. In many countries, the use of condensing boilers is compulsory or encouraged with financial incentives due to their efficiency and green credentials. Therefore, if you’re interested in getting a new boiler, you should be looking for condensing models.

Condensing boilers are water heaters fueled by gas or oil that reach higher efficiency levels by using waste heat in flue gases to heat cold water entering the boiler. The water steam that is produced during combustion is condensed into liquid form, which leaves the system through a drain.

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There are two different types of condensing boilers available. System or conventional boilers work by heating water contained in a cylinder. This choice is suitable for larger households with more than 2 bathrooms or when mains pressure is low. The second type is the combi boiler, which does not require a water tank since it heats water directly from the mains. This is the best solution for small houses and apartments since it is a compact, space-efficient solution that can fit into any room.

Condensing boilers include a special heat exchanger that also acts as a condenser. They can achieve greater efficiency levels because they capture more heat from the exhaust gases: they have a larger heat exchange surface to enable them to transfer as much heat as possible from the boiler's burner into usable heat, thereby allowing less heat to escape.

Because of this, condensing boilers are currently the most energy efficient systems on the market, with a typical increase of efficiency around 10-12% compared to regular ones. This operating mode reduces nitrogen oxide and carbon monoxide emissions thanks to the use of particularly advanced combustion technologies, which makes them a more eco-friendly choice.

There are different variations of combi boilers, but they all function the same way. When the temperature falls below the set level, the room thermostat signals the boiler to activate, and the heat will then be transferred to the central heating system. Once the required temperature has been reached, the thermostat reacts again to turn off the combi boiler.

Before the hot water is delivered through your tap, the cold mains water travels through a highly efficient heat exchanger. The heat exchanger transfers the vast majority of the heat from the burnt gas (approx. 90%) to the cold water and then distributes it to the taps as needed.

Control valves inside the combi boiler proceed in different directions, either directing the water flow through central heating or leading it to a hot water tap – never both at the same time. A condensing combi boiler also has an additional pipe to drain the condensed vapours while the boiler is in function.

A combination boiler produces a continuous flow of hot water; however, water pressure can slightly decline if more than one appliance is using the boiler at the same time. This makes combination boilers more suitable for smaller households or apartments.

For best results, buying a brand new combi boiler from a respected retailer or manufacturer is always recommended. Although new boilers are more expensive than second-hand equipment, they are guaranteed to last longer and often include repairments in their warranty.

The cost of a combi boiler depends on two main elements:

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