the output of the burner to match the load and give best performance. Almost all have modulating burners. These allow the power to be reduced to match the demand. Boilers have a turndown ratio which is the ratio of the maximum power output to the minimum power output for which combustion can be maintained. If the control system determines that the demand falls below the minimum power output, then the boiler will cycle off until the water temperature has fallen, and then will reignite and heat the water. Reliability Condensing boilers are claimed to have a reputation for being less reliable and may also suffer if worked on by installers and plumbers who may not understand their operation. Claims of unreliability have been contradicted by research carried out by the UK-based Building Research Establishment (see Building Research Establishment). In particular, the problem of 'pluming' arose with early installations of condensing boilers, in which a white plume of condensed vapour (as minuscule droplets) becomes visible at the outlet flue. Although unimportant to boiler operation, visible pluming was an aesthetic issue that caused much opposition to condensing boilers. 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. Building Research Establishment The