Assuming the blower is connected directly to the hood that should
remove all the fumes generated from soldering & annealing.
However all the air that is removed by the blower must be replaced by
air from someplace. Typically the furnace, water heater and laundry
equipment are installed in many basements. If these devices are fired
by natural gas, propane or oil, they generate exhaust gases, one of
which is carbon monoxide. The exhaust gases are generally vented
through a chimney. There are no check valves (reverse flow preventers)
The 600 cfm removed by the blower must be replaced with air from
somewhere. The easiest way for air to get into a relatively sealed
basement is via the chimney or any other outside vent. This will cause
the exhaust gases to reverse their flow & fill the basement until
they’re expelled by the 600 cfm blower.
In addition, the force of the air coming down the chimney may blow
out pilot lights in gas operated equipment. If there are any
improperly operating valves in the equipment, this could result in gas
collecting in the basement & an explosion when conditions are right.
One way to avoid the back drafts when the exhaust blower is on is to
put the soldering/annealing hood in a room by itself. Then provide a
separate fresh air intake to that room. If the fresh air intake is
gravity fed ( no fan) the opening should be about twice the size of
the blower outlet. A barometric damper could be added to the fresh air
intake to close it off when the blower isn’t operating.