Abstract: In a multi-unit installation system of fluid heating units such as hot water boilers, an outstandingly superior responsivity to load variations is obtained, and fluid of stable temperature is supplied by preventing useless starts and stops. In a units count control method, a plurality of fluid heating units (1) are arranged in parallel, these fluid heating units (1) and a load (2) are connected to each other through a fluid supply passage (3) and a fluid return passage (4), and the operated-units count out of the fluid heating units (1) is controlled in response to the load condition, wherein if a fluid supply temperature To in the fluid supply passage (3) is within a preset range, then a current operated-units count is maintained; if the supply temperature To is below the preset range, then the operated-units count is increased; and if the supply temperature To is above the preset range, then the operated-units count is decreased.