Based on the situation that thermal insulation hot water tank and oil/gas fired hot-water atmospheric boiler keeps pressure-less; and oil/gas fired hot water atmospheric boiler is often installed on the roof disposing corresponding thermal insulation hot water tank, this paper proposes a kind of heat source device — large-volume oil/gas fired hot water atmospheric boiler which is especially suitable for hot water supply system with the boiler installed on the roof. The water volume of this kind of boiler is equivalent to the thermal insulation hot water tank, and can be used for hot water supply without thermal insulation hot water tank cold-water pump and circulation pipes. Besides, compared with the hot water supply system using common hot water boiler, hot water supply system using large-volume oil/gas fired hot water atmospheric boiler has such advantages as small area of land, low cost for installation, simple system, easy operation, high utilization of heat source, low operation consumption. Large-volume oil/gas fired hot water atmospheric boiler can be used widely in hot water supply system with boiler installed on the roof to replace the traditional hot water supply system using “oil/gas fired hot water atmospheric boiler +hot water tank +circulation pump”, and obtain obvious economic benefit and environment benefit. It is worth pointing out that this kind of boiler can be used not only in sanitary hot water supply but also in heating and air-conditioning system.