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A condensing boiler or Co Boiler is a water heating device designed to recover energy normally discharged to the atmosphere through the flue. It can do this through the use of a secondary heat exchanger which most commonly uses residual heat in the flue gas to heat the cooler returning water stream or by having a primary heat exchanger with sufficient surface for condensing to easily take place. The best term for boilers designed to condense on the primary heat exchanger may be "fully condensing."

How condensing boilers work

In a conventional non-condensing boiler, hot combustion gases from the burning of fuel heat water contained in a heat exchanger. The waste gases are still quite hot (180 °C-200 °C) and significant heat is lost to the atmosphere.

In a condensing boiler working at peak efficiency, the water vapour produced by the burning fuel in the boiler is condensed back into liquid water. Provided the returning water is sufficiently cool, the steam condenses to liquid water, hence the name condensing boiler. Some of the extra efficiency of the condensing boiler is due to the cooling of the exhaust gases, but the majority of the energy recovered is from the condensation of the water vapour in the exhaust gases. This releases the latent heat of vaporisation of the water (2,260 J/g, or 970 BTU/lb, of condensate) into the heat exchanger.

The actual operating efficiency of a condensing boiler depends on the temperature of the return water stream: if it is too warm then little condensation takes place and little extra energy is extracted. Because of this, a newer generation of condensing boilers (so-called modulating control boilers) have microprocessor-controlled combustion that modulates the quantity of gas/air fuel mixture which is supplied to the burner using a configurable embedded algorithm that considers outdoor air temperature, water temperatures supplied and returned to the boiler, and time at a specific temperature. The most sophisticated algorithms learn the building requirements at specific outdoor air temperatures, more successfully returning cool water that condenses the vented exhaust gases and recovering the heat of vaporisation. Modulating control units also minimize on-off cycling to increase efficiency. They attempt to supply only the amount of heat to the building that the building loses at a specific outdoor air temperature.


Condensing boilers are now largely replacing earlier, conventional designs in powering domestic central heating systems in Europe and, to a lesser degree, in North America. The Netherlandsmarker 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 Kingdommarker, 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 applies to oil-fired boilers from 1 April 2007 (warm air central heating systems are exempt from these regulations). In the United Statesmarker, there is a Federal tax rebate for the installation of condensing boilers and additional rebates in some States. In Western Canadamarker, energy suppliers now offer energy rebates when these systems are installed in multi-unit dwellings. The increase in natural gas prices in North America has encouraged the retrofit of existing boiler installations with condensing equipment.


Condensing boiler manufacturers claim that up to 98% thermal efficiency can be achieved, compared to 70%-80% with conventional designs (based on the higher heating value of fuels). Typical models offer efficiencies around 90%, which brings most brands of condensing gas boiler in to the highest available categories for energy efficiency. In the UK, this is a SEDBUK (Seasonal Efficiency of Domestic Boilers in the UK) Band A efficiency rating, while in North America they typically receive an Eco Logo and/or Energy Star Certification.

When installed in real houses, the performance of condensing boilers is typically 4-5% lower than in laboratory tests by groups such as SEDBUK. This gives typical seasonal efficiencies of 82-89% in the UK (HHV). Part of the efficiency drop is because the design and / or implementation of the rest of the heating system gives return temperatures at the boiler over 50°C, which prevents significant condensation in the heat exchanger. Better education of both installers and owners could be expected to raise efficiency towards the reported laboratory values. Natural Resources Canada also suggests ways to make better use of these boilers, such as combining the space and water heating systems.

Most non-condensing boilers could be forced to condense through simple control changes. Doing so would reduce fuel consumption considerably but would quickly destroy any mild steel or cast-iron components of a conventional high-temperature boiler. This is due to the corrosive nature of the condensate and the reason why most condensing boiler heat-exchangers are made mostly from stainless steel or aluminium/silicon alloy.


Condensing boilers have a reputation for being less reliable, requiring professional installation and regular service, and may suffer because installers and plumbers may not understand their operation.

Heat exchangers in condensing boilers are made of stainless steel, a combination of cast iron with a secondary heat exchanger made of stainless steel, or aluminum. Regular monitoring of the circulated liquid in condensing boilers with aluminum heat exchangers is vital. Maintenance of a slightly alkaline (pH 8 to 9) liquid with anti-corrosion and buffering agents reduces corrosion of the aluminum heat exchanger. There is a feeling among professionals in this field that the condensed liquid produced at the exhaust, which has a pH between 3 and 4, may corrode the aluminum heat exchanger and shorten the boiler's life. Empirical evidence proving this feeling has not been available to date, since condensing boilers with aluminum heat exchangers have not been in use long enough.

Condensing boiler technology is evolving rapidly. A fully condensing boiler with a cast iron heat exchanger has been introduced in the United States. It deals with the corrosive nature of the condensate produced in it by increasing the wall thickness of the heat exchanger sufficient to deliver an expected residential life of four to six decades and by discouraging the conditions that accelerate corrosion.


The condensate expelled from a condensing boiler is acidic, with a (pH between 3 and 4). Gas condensing boilers require a drain pipe for the condensate produced during operation. This consists of a short length of inexpensive polymer pipe with a vapor trap to prevent exhaust gases from being expelled into the building. The vapor trap is sometimes configured as a drum trap and filled with limestone chips to alter the pH of the acidic condensate to avoid damage to iron or concrete components downstream. If a gravity drain is not available, then a small condensate pump must be installed to lift it to a proper drain.

The relevant parts of the boiler have to be constructed of materials that will withstand this acidity, typically aluminum or stainless steel. Since the final exhaust from a condensing boiler has a lower temperature than the exhaust from a conventional boiler a fan is always required to expel it, with the additional benefit of allowing the use of low-temperature exhaust piping (typically PVC in domestic applications) without insulation or chimney requirements. Indeed, it is specifically forbidden for retrofit installations in existing housing with conventional masonry chimneys to use that masonry vent for condensing boilers, due to their characteristic wet, acidic exhaust which would quickly corrode conventional brick and mortar. Polymer venting allows for the added benefit of flexibility of installation location. One UK company, Atmos Heating Systems, has patented a drain-free system which negates the need for a drain pipe.


Condensing boilers are up to 50% more expensive to buy and install than conventional types in the UK and the US. However, , at UK prices the extra cost of installing a condensing boiler should be recovered in around 2–3 years through lower fuel use, and 2–5 years at US prices. Obviously the exact figure will depend on the efficiency of the original boiler installation, boiler utilisation patterns, and costs associated with the new boiler installation.


Image:DSCN5370.JPG|Viessmann Vertomat Condensing BoilerImage:condensevapor.JPG|Condensing Boiler Exhaust VapourImage:DSCN4452.JPG|Viessmann Vitodens Condensing BoilerFile:Condensing boiler.JPG|Stainless Steel Exaust w/ Condensate

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Typical manufacturers of residential condensing boilers:

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