High Quality Content by WIKIPEDIA articles! Condensing boilers are water heaters in which a high efficiency (typically greater than 90%) is achieved by using the waste heat in the flue gases to pre-heat the cold water entering the boiler. They may be fuelled by gas or oil and are called condensing boilers because the water vapour produced during combustion is condensed into water, which leaves the system via a drain. In many countries their use is compulsory or encouraged with financial incentives.