The utility model discloses an utilize low -grade drawing gas to provide fan heater of heat source system, including no. 5 heaters, no. 6 heaters, no. 7 heaters, fan heater, air preheater, boiler and flue gas processing system, 7 the circulating water export of number heater is linked together with the circulating water entry of no. 6 heaters, and the circulating water export of no. 6 heaters is linked together with the circulating water entry of no. 5 heaters, and the circulating water export of no. 5 heaters is linked together with the water inlet of fan heater, and the delivery port of fan heater enters the mouth with the circulating water of no. 7 heaters and is linked together, the gas outlet of fan heater is linked together with the air inlet of air preheater, and the air outlet of air preheater is linked together with the income gas port of boiler, and the exhanst gas outlet of boiler is linked together with the flue gas of air preheater entry, and the exhanst gas outlet of air preheater is linked together with flue gas processing system's entry. The utility model discloses can realize regarding low -grade drawing gas as the heat source of fan heater, energy -concerving and environment -protective, the security is high.