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QUESTION:I'm just completing the drywall on a new darkroom about 16'X10' with a 8'
ceiling ~1300 cubic feet. How much ventilation do I need? How do I calc.
CFMs/air exchange. How big a pipe do I need. I'm doing B&W, Color and maybe
Cibachrome work. The room is in a basement and I was thinking of tying into
an existing bathroom fan pipe (4" I'm guessing). Would I need to put a
backflow type of value to protect the bathroom? As I mentioned the bathroom
is adjacent to the darkroom. I've debated putting a vent in the wall between
the two rooms and keeping both doors closed and turning on the bathroom fan
while working too drawing the stale air into the bathroom and then out. Is
this advisable? (It is after all a basement bathroom)
ANSWER: A few ideas concerning fans:
Fans are often rated for "free air" airflow, which is much greater
than the actual installed airflow due to resistance in filters, ducts,
and vents. Use a fan with a "free air" CFM rating at least 50%
greater than your calculated required airflow.
Large slow fans are generally quieter than small fast fans. Many fans
are rated in "sones". The lower the sones value, the quieter the fan.
To minimize dust, pressurize the room and filter the supply air. I
built a frame to hold a "high performance" furnace filter over the fan
intake. This provides a filter area much larger than the fan input to
minimize resistance to airflow.
A timer switch for the fan will make it convenient to run the fan for
several hours after a darkroom session to dry out the room.
Locate the supply vent high on a wall to avoid blowing dust up from
the floor, and as far from the sink as possible to avoid blowing
chemical vapors around the room. Locate the exhaust vent near the
sink below nose level to draw chemical vapors away from your face.
In new construction, the cavity between wall studs can make a good
light trap. Locate one vent high and the other low, and paint the
interior of the stud cavity flat black. Be sure the supply air cavity
is clean (and filter the supply vent just to be sure.)
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