Last edited 29 Jun 2021

Condensing boiler

According to the Non-domestic building services compliance guide, 2013 edition, published by HM Government, a condensing boiler is: '… a boiler that offers a high energy efficiency by recovering heat from the flue gases. This is achieved by increasing the heat exchanger surface area, which recovers extra sensible heat whenever the boiler fires. The boiler becomes even more efficient when system water temperatures are low because the larger heat exchanger area promotes condensation, allowing much of the latent heat to be recaptured. Standard losses (when the boiler is not firing) are low, and part load performance is very good. In multiple-boiler systems, condensing boilers can be used as the lead boiler.'

The Illustrated Guide to Mechanical Building Services, Third Edition (BG 31/2017), by David Bleicher, published by BSRIA in 2017, states in relation to condensing boilers: ‘The flue gases (otherwise known as products of combustion) from a conventional boiler consist primarily of carbon dioxide (CO2) and water vapour, plus other gases such as oxides of nitrogen. The water vapour holds a significant amount of energy, known as latent heat. In a condensing boiler, the flue gases are cooled before being discharged, so the water vapour condenses (turns into liquid water). The latent heat released is used to preheat water returning to the boiler, and so condensing boilers are more energy-efficient than conventional ones. In order for this condensing process to work, the return water must be below a certain temperature. For this reason, and also that the flue gases are cooler and less buoyant, condensing boilers can’t always directly replace non-condensing boilers in existing buildings.’