extra cost of installing a condensing instead of conventional boiler should be recovered in around 2–5 years through lower fuel use (for verification, see the following three documents published by the Building Research Establishment: Information Papers 10-88 and 19-94; General Information Leaflet 74; Digest 339; see also Case studies in Application Manual AM3 1989: Condensing Boilers by Chartered Institute of Building Services Engineers), and 2–5 years at US prices. Exact figures will depend on the efficiency of the original boiler installation, boiler utilisation patterns, the costs associated with the new boiler installation, and how frequently the system is used. The cost of these boilers is dropping as the mass takeup enforced by government takes effect and the manufacturers withdraw older, less efficient models, but production cost is higher than older types as condensing boilers are more complex. The increased complexity of condensing boilers is as follows: increased size of heat exchanger, or the addition of a second heat exchanger (it is important that the heat exchangers are designed to be resistant to acid attack from the "wet" flue gases) The necessity of a fan-assisted flue (as cooler flue gases have less buoyancy). Many non-condensing boilers also have this feature, however as the cooler flue gases produce condensate, this needs to be drained, and so the boilers are plumbed into a waste or drain With respect to modern boilers, there are no other differences between condensing and non-condensing boilers. Reliability, as well as initial cost and efficiency, affects total cost of ownership. One major independent UK firm of plumbers stated in 2005 that it had made thousands of call-outs to mend condensing boilers, and that the greenhouse gas emissions from its vans were probably greater than the savings made by the shift to eco-conscious boilers. However, the same article points out that the Heating and Hotwater Information Council, together with some installers, have found that modern condensing boilers are just as reliable as standard boilers. Gallery See also Energy efficiency in British housing OpenTherm References External links European Patent Office Patent for drain free elimination of condensate. Condensing boilers page on the website of the UK National Energy Foundation Comparisons of different boiler systems available in North America on the Natural Resources Canada website Exploded diagram of condensing boiler (Malvern Boilers 30) (pdf file - 71KB) On the design of residential condensing gas boilers Boilers Residential heating appliances