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. Usage 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. In Europe, their installation is strongly advocated by pressure groups and government bodies concerned with reducing energy use. In the United Kingdom, for example, all new gas central-heating boilers fitted in England and Wales since 2005 must be high-efficiency condensing boilers unless there are exceptional circumstances ; the same regulations apply to oil-fired boilers from 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 resulted in increased retrofitting of existing boiler installations with condensing equipment. Efficiency 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. 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. Better education of both installers and owners could be expected to