If you are talking about dilute formic acid in water at 1 atm pressure. The dilute solution will boil away long before reaching 300 C. Regards

In general, the flammability of formic acid increases with higher concentrations. If the formic acid is diluted to a very low concentration, it may not be highly flammable. However, at elevated temperatures, even dilute solutions of formic acid can pose a fire hazard due to the release of flammable vapors. it again depends on the concentration it was diluted to.