The blaze that killed a firefighter in a Harlem building that was being used as a film set was caused by a boiler that triggered flammable materials, fire marshalls said Tuesday.

In a statement, FDNY officials cited “heat from a boiler ventilation flue pipe ignited nearby combustible materials” in the cellar of 773 St. Nicholas Ave, where Michael Davidson, 37 died the night of March 23.

Marshalls also determined the building’s sprinkler system was turned off when the flames broke out and fire resistant materials were not present.

At the time, the building was being used to film Edward Norton and Bruce Willis’ upcoming flick “Motherless Brooklyn”.

Davidson, from Engine Company 69, ran out of air and died of smoke inhalation after becoming separated from his unit.

Thousands attended the funeral of the fallen firefighter, who was posthumously promoted to lieutenant.

He left behind a wife and four children.