A method and an arrangement for recovering heat from flue gas of a boiler (10). The method comprises passing the flue gas (G) of the boiler though a flue gas cooling unit (1), cooling the flue gas (G) by transferring heat from the flue gas (G) into a circulation (3) of a flue gas cooling liquid (CL), transferring heat energy of said flue gas cooling liquid (CL) into a heat pump (2), and arranging the heat pump (2) for receiving heat energy also from a circulation arrangement (8) of a district cooling system. The heat pump (2) is coupled to a circulation arrangement (6) of a district heating system, wherein the method further comprises transferring in the heat pump (2) heat energy (H) received from said cooling liquid (CL) and from said circulation arrangement (8) of district cooling system into said circulation arrangement (6) of district heating system, for lowering the temperature of said flue gas cooling liquid (CL) and cooling fluid of said district cooling system, and raising the temperature of heating fluid of said district heating system.

A forced convection fin tube condensing heat exchanger for supplying heat includes a shell, burner and fin tubes bundle. The burner is at the top of the shell, circular fin tubes bundle installed around the burner tightly, circularly and coaxially. The flue channel which is formed by the shell and a row of fin tubes bundle is below the burner. The flue flows along the flue channel to flue outlet. There are front and rear water manifolds at the two ends of the fin tubes. Water baffles inside the rear water manifold divide the rear water manifold into water inlet and outlet areas. The water enters the small portion of circular fin tubes bundle from water inlet area, through the front water manifold, back to the water outlet area via the main portion of the circular fin tubes bundle. The use of bent fin tube to improves efficiency.

A forced convection fin tube condensing heat exchanger for supplying heat includes a shell, burner and fin tubes bundle. The burner is at the top of the shell, circular fin tubes bundle installed around the burner tightly, circularly and coaxially. The flue channel which is formed by the shell and a row of fin tubes bundle is below the burner. The flue flows along the flue channel to flue outlet. There are front and rear water manifolds at the two ends of the fin tubes. Water baffles inside the rear water manifold divide the rear water manifold into water inlet and outlet areas. The water enters the small portion of circular fin tubes bundle from water inlet area, through the front water manifold, back to the water outlet area via the main portion of the circular fin tubes bundle. The use of bent fin tube to improves efficiency.