Darkroon Ventilation Requirements

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I am in the process of completing a new darkroom in the basement and wanted to solicit some additional opinions on the subject of insuring adequate ventilation. The complete darkroom will be 2,400 ft3 and the front of it will be wet and is also where the 8ft sink will be located. A manual I got for darkroom design says that I should cycle the air in it every 6-8 minutes. That calculation would require a 400 CFM fan. Is that adequate? I need to go 9 feet along the ceiling from where the sink will be located to where the wall is that I can get acess to the outside. Above me I have floor joyce support members that are 14" wide and if it is possible I would like to put the vent motor(s) and the ducting in this ceiling access to maintain as much ceiling height as possible. Secondly, for a exhaust fan should I look at manufacturers that have products specifically designed for the photo industry or will any fan that fits inside the joyce space work? Should I place the vent motor at the wall and use custom ducting to place the negative pressure inlets over the sink area? How about louvers that maintain a light tight work area? Are they required and where is the best place to put them? Many Thanks.

-- Michael Kadillak (m.kadillak@attbi.com), March 25, 2002


Michael, A fan's rating is generally for conditions of no restriction at the inlet or outlet. In other words if the fan is rated at 400 CFM it will not produce that flow with 6 feet of duct on the inlet and outlet due to the pressure loss. My darkroom is about 8 x 8 feet. I have two inexpensive bathroom fans mounted on the wall above the sink. They both feed into a 4" round duct (even though the fan body is 3") and then run about 10 feet to the outside with 2 90 degree bends. Ventillation is adequate but I wouldn't want any less.

I decided against mounting the fans in the wall between studs or in the ceiling joists. The reason was that I wanted to have access to them in the event a motor burns out. If they are in the wall or ceiling it may be difficult to get to them without destroying the light proof integrity. I also thought it would be easier to keep the fans inside the darkroom and then only deal with lightproofing the exit of my duct as opposed to lightproofing around each fan cut out.

Pay some attention to fresh air intakes. I have two 8x16 inch louvers through the wall. They have filters to prevent dust and a baffle arrangement for lightproofing. This may be too small since I can tell that the air flow increases when the door is opened which indicates that the fresh air intakes are restricting the flow.

-- Dave Schneider (dschneider@arjaynet.com), March 25, 2002.

I know that most home darkrooms have the exhaust motors either in the room, or just above it - and I know that nearly everyone reading this will say that is the best place to put them, for a variety of reasons.
HOWEVER, that is not the best place to put them from an environmental engineering standpoint (i.e. the health of you and your family). Any commercial operation having to deal with worker-safety and OSHA requirements will have a system designed specifically to provide a NEGATIVE exhaust. This means that the entire exhaust system, from point of collection to exhaust, is under a constant negative pressure while operating.
Any duct connected after the fan will be under positive pressure - meaning that any leaks in the duct or joints will "bleed" exhaust into the surrounding space..your basement.
If you can arrange it, the best place to put the motor is at the exhaust point (at the wall outside). This provides a negative exhaust duct throughout the installation. Any leaks in the system will draw air from the surrounding space INTO the duct, eliminating the possibility of contamination of the surrounding space.
It can be more expensive as the motor needs to be either built for external mounting, or be protected from the elements (under a patio roof, or have an enclosure built around it) and it will need to be of a higher capacity (for reasons stated by the previous poster).

As you have the opportunity to do it right, because you are now building the darkroom, you should seriously consider the long term effects of even minor exposure levels. Spend a few dollars more now, and you will be very glad you did.

-- Matt O. (mojo@moscow.com), March 26, 2002.

From a dust control standpoint, it is desirable to maintain a slight positive pressure in the darkroom. This prevents unfiltered air being drawn into the room through electrical outlets, around the door, etc. You might want an exhaust fan located at the outside wall, and a supply fan on the darkroom wall, balanced for a slight positive pressure in the darkroom.

Locate the exhaust vent below nose level near the sink, and the supply vent on the opposite side of the room, to produce an airflow pattern that will draw chemical vapors away from your face.

To maximize airflow, use a supply air filter much larger than the fan input. I built a frame around the fan that holds a high performance furnace filter. (The filter gets used in the furnace after the darkroom.)

Since your room is new construction, you can use stud cavities for light baffles. Be sure that they are clean, and paint the interior flat black. I have a secondary filter on the inside of the supply vent to catch any dust that accumulates in the cavity.

Unless you are going to operate the fan continuously, use a 12 hour timer switch to control it, to dry out the room after a darkroom session.

-- Chris Ellinger (chris@ellingerphoto.com), March 26, 2002.

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