Flue Gas Volume

P₁V₁/T₁ = P₂V₂/T₂.

(1)(510,000)(273+20) = (0.5)(V₂)(273+400) [Assuming bar absolute.]; or

(1+1)(510)(293) = (1+0.5)(V₂)(673) [Assuming barg.]

(Said the fly, let us flee; said the flea, let us fly; so they flew through the flaw in the flue.)

(Said the fly, let us flee; said the flea, let us fly; so they flew through the flaw in the flue.)........she sells sea shells along the sea shore . Tounge twister

If you want to calculate the gas volume of the exhaust gases, at an earlier stage (400 deg C and 0.5 bar(g), could describe the burner exit), and you want to base your calculation on the gas volume at 20 deg C and 1 bar(Abs), then you should consider also the volume of the water vapour that has been condensated.