The clothes dryer vent (the copper hooded structure on the right side of the photograph) was located too close to the air intake for the boiler. Lint, which was expelled through the dryer vent, was sucked into the boiler’s air intake; it clogged the burner screen, which starved the boiler of oxygen. Combustion of the propane fuel was incomplete, which created carbon monoxide as a dangerous and lethal byproduct.