Waste heat recovery system for gas-fired boiler

The utility model discloses a waste heat recovery system for a gas-fired boiler. A main flue is connected in parallel with a bypass flue; the bypass flue is sequentially connected in series with a bypass flue butterfly valve, a boiler economizer, an air preheater and a condensing heat exchanger; the bypass flue is communicated with a bypass chimney; a water inlet of a heat exchange pipe of the boiler economizer is connected with a water tank by a water supply pipeline; a water outlet of the heat exchange pipe of the boiler economizer is communicated with the boiler by a pipeline; an air inlet of a heat exchange pipe of the air preheater is communicated with a fan by a pipeline; an air outlet of the heat exchange pipe of the air preheater is communicated with a boiler combustion air inlet pipe by a pipeline; a water inlet of a heat exchange pipe of the condensing heat exchanger is communicated with the water tank by a downcomer; and a water outlet of the heat exchange pipe of the condensing heat exchanger is c