Condensing boiler

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Condensing boilers are water heaters in which a high efficiency (typically greater than 90% on the higher heating value) 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.

Principles of work[edit]

In a conventional boiler, fuel is burned and the hot gases produced are passed through a heat exchanger where much of their heat is transferred to water, thus raising the water's temperature.

One of the hot gases produced in the combustion process is water vapour (steam), which arises from burning the hydrogen content of the fuel. A condensing boiler extracts additional heat from the waste gases by condensing this water vapour to liquid water, thus recovering its latent heat of vaporization. A typical increase of efficiency can be as much as 10-12%.[citation needed] The effectiveness of this condensing process varies, it depends upon the temperature of the water returning to the boiler, but for the same conditions, it is always at least as efficient as a non-condensing boiler.

The condensate produced is slightly acidic, 3-5 pH, so the choice of materials used in the areas where liquid is present have to be suitable. At high temperature most commonly used are aluminium alloys and stainless steel, in the low temperature areas plastics are most cost effective, for example uPVC and polypropylene.[1] The production of condensate also requires the installation of a heat exchanger condensate drainage system. For a basic installation this is the only difference required compared to a non-condensing boiler.

For the heat exchanger within a condensing boiler to be economical to manufacture, and for the appliance to be manageable at installation, the smallest practical size for its output is preferred. This has produced heat exchangers with very high combustion side resistance and so the use of a combustion fan to move the products through narrow passageways has been adopted. This also has had the benefit of providing the energy for the flue system as the expelled combustion gases are usually below 100°C (212°F) and as such have a density close to air, with little buoyancy. The combustion fan helps to pump exhaust gas to the outside.


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 Netherlands was the first country to adopt them broadly.[2][3] 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 applies to oil-fired boilers from 1 April 2007 (warm air central heating systems are exempt from these regulations). In the United States, there is a Federal tax credit for the installation of condensing boilers and additional rebates from power companies in some states. In Western Canada, energy suppliers now offer energy rebates when these systems are installed in multi-unit dwellings. The decrease in natural gas prices in North America has not hindered[citation needed] the retrofit of existing boiler installations with condensing equipment.


Condensing boiler manufacturers claim that up to 98% thermal efficiency can be achieved,[4] 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)[5] Band A efficiency rating, while in North America they typically receive an Eco Logo and/or Energy Star Certification.

Boiler performance is based on the efficiency of heat transfer and highly dependent on boiler size/output and emitter size/output. System design and installation are critical. Matching the radiation to the Btu/Hr output of the boiler and consideration of the emitter/radiator design temperatures determines the overall efficiency of the space and domestic water heating system.

One reason for an efficiency drop is because the design and/or implementation of the heating system gives return water (heat transfer fluid) temperatures at the boiler of over 55°C (131°F), which prevents significant condensation in the heat exchanger.[6] Better education of both installers and owners could be expected to raise efficiency towards the reported laboratory values. Natural Resources Canada[7] also suggests ways to make better use of these boilers, such as combining the space and water heating systems. Some boilers (e.g. Potterton) can be switched between two flow temperatures such as 63°C (145°F) and 84°C (183°F), only the former being "fully condensing". However, the boilers are normally installed with the higher flow temperature by default because a domestic hot water cylinder is generally heated to 60°C (140°F), and this takes too long to achieve with a flow temperature only three degrees higher. Nevertheless, even partial condensing is more efficient than a traditional boiler.

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 due to the corrosive nature of the condensate, and is the reason why most condensing boiler heat-exchangers are made from stainless steel or aluminum/silicon alloy. External stainless steel economizers can be retrofitted to non-condensing boilers to allow them to achieve condensing efficiencies. Temperature control valves are used to blend hot supply water into the return to avoid thermal shock or condensation inside of the boiler.

The lower the return temperature to the boiler the more likely it will be in condensing mode. If the return temperature is kept below approximately 55°C (131°F) the boiler should still be in condensing mode making low temperature applications such as radiant floors and even old cast iron radiators a good match for the technology.

Most manufacturers of the new domestic condensing boilers produce a very basic "fits all" in-built control system that ends up with the boiler running in condensing mode only on initial heat-up, after which the efficiency drops off, although it should still exceed that of older models (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 Application Manual AM3 1989: Condensing Boilers by Chartered Institute of Building Services Engineers).


The control of the domestic condensing boiler is crucial to ensuring that it operates in the most economic and fuel efficient way.

Almost all have modulating burners. The burners are usually controlled by an embedded system with built in logic to control the output of the burner to match the load and give best performance.


Condensing boilers are claimed to have a reputation for being less reliable, requiring professional installation and regular service, and may also suffer if worked on by installers and plumbers who may not understand their operation.[8] Claims of unreliability have been contradicted by research carried out by the UK-based Building Research Establishment (see "Myths" below.)

Heat exchangers in condensing boilers are primarily manufactured using stainless steel and aluminum. Initial testing and annual monitoring of the heat transfer fluid in condensing boilers with aluminum or stainless steel heat exchangers is highly recommended. 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 some professionals in the field that the condensate produced on the combustion side of the heat exchanger may corrode an aluminum heat exchanger and shorten the boiler's life. Statistical evidence is not yet available since condensing boilers with aluminum heat exchangers have not been in use long enough.

To ensure reliability, condensing boilers must be properly installed and maintained.


The Building Research Establishment, which is the UK's major research body for the building industry, has produced guidance on domestic condensing boilers. This was originally published in 2003 as General Information Leaflet 74 (GIL74), entitled "Domestic Condensing Boilers: the benefits and the myths". The publication is based upon the BRE's long experience with installed condensing boilers since the 1980s, and is now also published by the Energy Saving Trust as document CE52. It seeks to highlight the many myths and misconceptions about condensing boilers, and to explain the known benefits. This guidance can be downloaded from the web site:

In particular, the BRE's research has found the following:

  • condensing boilers are nowadays just as reliable as standard boilers
  • condensing boilers are no more difficult to service, nor do they require more frequent servicing
  • servicing is not expensive; the only (minor) additional task is to check the correct function of the condensate drain
  • condensing boilers are not difficult to install
  • condensing boilers are always more efficient than standard boilers, under all operating conditions


The condensate expelled from a condensing boiler is acidic, with a pH between 3 and 4. Condensing boilers require a drainpipe for the condensate produced during operation. This consists of a short length of polymer pipe with a vapour trap to prevent exhaust gases from being expelled into the building. The acidic nature of the condensate may be corrosive to cast iron plumbing, waste pipes and concrete floors but poses no health risk to occupants. A neutralizer, typically consisting of a plastic container filled with marble or limestone aggregate or "chips" (alkaline) can be installed to raise the pH to acceptable levels. If a gravity drain is not available, then a small condensate pump must also be installed to lift it to a proper drain.

The primary and secondary heat exchangers are 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 an atmospheric boiler 38°C (100°F) vs. 204°C (400°F) a mechanical 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 conventional chimney requirements. Indeed, the use of conventional masonry chimney, or metal flue is specifically prohibited due to the corrosive nature of the flue products, with the notable exception of specially rated stainless steel and aluminum in certain models. The preferred/common vent material for most the condensing boilers available in North America is PVC, followed by ABS and CPVC. Polymer venting allows for the added benefit of flexibility of installation location including sidewall venting saving unnecessary penetrations of the roof.


Condensing boilers are up to 50% more expensive to buy and install than conventional types in the UK and the US. However, as of 2006, at UK prices the 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[citation needed] at US prices. Obviously the exact figure will depend on the efficiency of the original boiler installation, boiler utilisation patterns, the costs associated with the new boiler installation, and how much 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 they are slightly 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)
  • addition of fan assisted flue (as cooler flue gases have less buoyancy), but many non-condensing boilers also have this feature
  • 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 said sarcastically 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.[8] However, the same article points out that the Heating and Hotwater Information Council, together with some installers, have found that condensing boilers are nowadays just as reliable as standard boilers.


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