A more significant issue is the slight (pH 3-4) acidity of the condensate liquid. Where this is in direct contact with the boiler's heat exchanger, particularly for thin aluminium sheet, it may give rise to more rapid corrosion than for traditional non-condensing boilers. 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. The acidity of the condensate means that only some materials may be used: stainless steel and aluminium are suitable, mild steel, copper or cast iron are not. Poor design or construction standards may have made the heat exchangers of some early condensing boilers less long-lived. Initial testing and annual monitoring of the heat transfer fluid in condensing boilers with aluminium 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 aluminium heat exchanger. Some professionals believe that the condensate produced on the combustion side of the heat exchanger may corrode an aluminium heat exchanger and shorten boiler life. Statistical evidence is not yet available since condensing boilers with aluminium heat exchangers have not been in use long enough. The Building Research Establishment, which is the UK's major research body for the building industry, produced a leaflet on domestic condensing boilers. According to the Building Research Establishment:  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.