Condensing boiler

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A condensing boiler is a kind of hydronic boiler which achieves enhanced efficiency by incorporating an additional heat exchanger. This uses the heat in the exhaust gases from the boiler to preheat the water as it enters the boiler, and so recapturing energy that would otherwise be lost. When a condensing boiler is working at peak efficiency the water vapor produced by the consumption of gas or oil in the boiler condenses back into liquid water - hence the name "condensing boiler". The condensation of exhaust gases releases the latent heat of vaporization of the water, a more significant source of energy than the transfer of heat by cooling the vapor. Essentially this design captures the difference between the higher heating value and lower heating value of the fuel; hence the improvement is greatest with fuels such as natural gas though still of practical value with oil.

Condensing boilers are now largely replacing earlier, "conventional" designs in powering domestic central heating systems in Europe and North America. The Netherlands was probably the first country to take them up in a large way. In Europe, their installation is strongly advocated by pressure groups and government bodies concerned with reducing energy use. In the United Kingdom, for example, since 2005 all new gas central-heating boilers fitted in England and Wales must be high-efficiency condensing boilers unless there are exceptional circumstances, and the same will apply to oil-fired boilers from 1st April 2007 (warm air central heating systems are exempt from these regulations). In North America; specifically Western Canada energy suppliers now offer energy rebates when these systems are installed in multi-unit dwellings.

Condensing boilers claim that up to 95% efficiency of fuel conversion can be achieved in normal domestic use, compared to 70%-80% with a conventional design. Typical efficiencies are around 90%, which brings most brands of condensing gas boiler in to the "SEDBUK Band A" energy efficiency rating. In North America these boilers typically receive an Eco Logo and/or Energy Star Certification. Condensing boilers are up to 50% more expensive to buy and install than conventional types, and they have a reputation for being more troublesome, though this reflects a lack of familiarity with them among installers and plumbers. In reality, gas condensing boilers have only one extra element to be installed - the drain pipe for the condensate collected during operation. This comprises a short length of inexpensive plastic waste pipe. The water that condenses out tends to be acidic because of the impurities in the fuel, chiefly sulphur and nitrogen, so it has to be piped into a neutralizing tank; also, the relevant parts of the boiler have to be constructed of materials that will withstand this acidity, typically stainless Steel.

Since the final exhaust from a condensing boiler has a lower temperature than the exhaust from a conventional boiler a fan is sometimes required to expel it, and it produces a visible "plume" of condensing water vapor.

As of 2006, at UK prices the extra cost of installing a condensing boiler should be recovered in around 2-3 years through lower fuel use; although this figure will depend on the relative boiler and fuel prices, and weather conditions, it is not likely to vary greatly between countries.

Typical condensing boilers in North America cost on average 2-3 times that of an atmospheric boiler, with an average payback of 2-5 years depending on the efficiency of the original boiler installation and costs associated with the new boiler installation.

North America has begun to discover the benefits of condensing boiler systems; the increase in natural gas prices has encouraged the retrofit of existing boiler installations as well as the creation of centralized plants for both heating and domestic hot water services. One such Plant is that of Lonsdale Energy Corp or LEC of North Vancouver, British Columbia, Canada.