One of the hot gases produced in the combustion process is water vapour (steam), which arises from burning the hydrogen content of the fuel.
To economically manufacture a condensing boiler's heat exchanger (and for the appliance to be manageable at installation), the smallest practical size for its output is preferred.
This approach has resulted in heat exchangers with high combustion-side resistance, often requiring the use of a combustion fan to move the products through narrow passageways.
This has also 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 that of air, with little buoyancy.
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.
[2] Condensing boiler manufacturers claim that up to 98% thermal efficiency can be achieved,[3] compared to 70%–80% with conventional designs (based on the higher heating value of fuels).
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.
Natural Resources Canada[6] also suggests ways to make better use of these boilers, such as combining space and water heating systems.
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.
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.
Older boilers may also have used thick cast heat exchangers, rather than sheet, which had slower time constants for their response but were also resistant, by their sheer mass, to any corrosion.
Maintenance of a slightly alkaline (pH 8 to 9) liquid with anti-corrosion and buffering agents reduces corrosion of the aluminium heat exchanger.
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.
The primary and secondary heat exchangers are constructed of materials that will withstand this acidity, typically aluminum or stainless steel.
However, as of 2006[update], at UK prices the extra cost of installing a condensing instead of conventional boiler should be recovered in around two to five 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 Institution of Building Services Engineers), and two to five years[citation needed] at US prices.