Source: Beijing Bosch wall-hung boiler after-sales service phone number http://blog.tianya.cn/blogger/blog_main.asp?BlogID=3778593. Bosch wall-hung boilers classification method according to the working fluid flow pattern can be divided into two major categories: natural circulation wall-hung boiler and forced circulation wall-hung boiler according to the working fluid flow pattern in the wall-hung boiler. In the natural circulation wall-hung boiler, the flow of the working fluid inside the evaporative heating surface is carried out by the cycle pressure head formed by the density difference of the working fluid in the water-cooled wall and the downcomer. Forced circulation wall-hung boilers are further divided into three types: controlled circulation wall-hung boiler, direct current wall-hung boiler, and compound circulation wall-hung boiler. In the controlled circulation wall-hung boiler, the flow of the working fluid within the evaporative heating surface mainly relies on the pressure head provided by the furnace water circulation pump in the downcomer. In the direct current wall-hung boiler, the flow of the working fluid in the heating surface is carried out by the pressure head provided by the feedwater pump, completing heating, evaporation, and overheating in one go. The compound circulation wall-hung boiler is formed by a combination of direct current wall-hung boiler and furnace water circulation pump, Beijing Bosch wall-hung boiler repair phone number.