When your furnace sits idle through the warm months of the year, dust and debris can accumulate on the heater coils. The first time you turn on the the furnace after months of not using it, the dust will “burn off” but, depending on the amount of dust on the system, instead of burning off, the dust could catch fire.
This why it is recommended to have you furnace cleaned at least once a year and, preferably, before turning it on for the first time each year.
Additionally, gas furnaces need proper ventilation to function as intended. They must be installed with sufficient clearances from walls and ceiling and other obstructions. Shelving, boxes, or other items stored too close to a furnace will impede the air flow and create a fire hazard.
Combustible materials stored near a furnace create the possibility of flash-fire or explosion. Gasoline, solvents, and other volatile substances emit vapors that can be ignited by the heat or flame inside a gas furnace. Paper, cloth, or other flammable items left too close to a furnace could catch fire.