Condensing boilers can offer a significant increase in efficiency compared to traditional models. While a typical non-condensing boiler operates at around 75% efficiency, condensing boilers can exceed 87%.

The principles of a condensing boiler

This impressive boost in efficiency is achieved through the extraction of heat from what would otherwise be wasted flue gases. Most boilers feature a single combustion chamber enclosed by the waterways of the heat exchanger, allowing hot gases to pass through. These gases are then expelled through the flue, located at the top of the boiler, at a temperature of approximately 180°C.

On the other hand, condensing boilers are designed first to allow the heat to rise upwards through the primary heat exchanger; when at the top, the gases are rerouted and diverted over a secondary heat exchanger. These can reduce the flue gas temperature to about 55°C. This temperature reduction causes the water vapour (formed during the combustion process) to condense and, as the droplets of water form, fall by gravity to collect at the base of the flue manifold. A fan-assisted balanced flue expels the remaining gases to the outside environment. The condensation produced within the appliance should be drained into the waste discharge pipework or externally into a purpose-made soakaway.

Condensing boilers in practice

For a condensing boiler to operate at these impressive efficiencies, it's crucial to maintain the flow and return pipework below 55°C. This temperature range is necessary for effective heat transference from the flue to the water, following the principle of heat transference from hotter to cooler materials.

Many consumers are installing condensing boilers in homes which are fitted with radiators and a primary flow and return to the hot water. Low flow and return connections are essential to make savings. For a central heating system to work with radiators and hot water primary circuits, flow temperatures need to be around 82°C, so actually may only slightly improve efficiency over the more traditional boiler. The appliance only works in its condensing mode during the initial heat-up.

To achieve a system that will function in its condensing mode, the installer needs to consider a suitable heating emitter system.

Improving an EPC rating by upgrading to a condensing boiler

Upgrading to a condensing boiler can improve an Energy Performance Certificate for domestic and non-domestic buildings. Regulations have been introduced that will prohibit the use of fossil fuel boilers in the future.