Darkroom ventilation

brian steinberger · Mar 13, 2015

so I just got my new darkroom up and running. I've been developing tons of films so far but today I did my first serious printing session. With four 16x20 trays, two of which were fixer I felt a bit groggy after 4 hours in there and my throat is irritated. Obviously my ventilation isn't adequate.

My darkroom is 11x11' with 7' ceilings (basement). I cornered off the room with studded walls and the room is pretty air tight as I had to do so to make it light tight. I put a bathroom exhaust fan (70cfm) above and to the back of my counter on the wet side. Opposite this fan low on the wall is a 12x12" louver to allow fresh air in. This is the normal setup correct?

My question is do I simply need a more powerful fan? My room is 120 sq feet and I'm using a fan for a 65 sq foot room (at least according to the specs). The fan was used in my previous darkroom with no problems. The other thing I'm wonder about is the "positive pressure" ventilation I keep reading about. Not sure how I could turn my setup in a positive pressure set up or if it would even be worth it.

Thoughts? Suggestions? Thanks

paul_c5x4 · Mar 13, 2015

From the numbers you give, you would have a maximum of five changes of air per hour - Possibly not the best ratio for a darkroom. Axia suggest a minimum of ten changes per hour, so you might want to consider a bigger fan.

gone · Mar 13, 2015

If you just had a fan running in the darkroom that pulls filtered (HEPA) air in, and you had a small vent to the outside some distance from this, this would give you positive pressure. Instead of sucking the air in, you would be pushing it out. Seems like a more effective way to vent it. This can be a little tricky to dial in, as venting placement is key (room air is usually lighter than your chemical fumes, so a low vent is good). If it were me, I'd use a low powered vent fan to get the air out, and a high powered filtered fan to bring the air in.

Also, if you used citric acid for stop bath, and an odorless fixer (see link below) most of your problems would be solved. For what its worth, my "darkroom" is either a tiny bathroom, or a bedroom, depending on what is going on in the house at the time. Even in the tiny bathroom I have no issues because I use odorless fixer and citric acid for stop. But, I'm not in there for 4 hours either.

http://www.freestylephoto.biz/6200-Arista-Premium-Odorless-Liquid-Fixer-32-oz.

brian steinberger · Mar 13, 2015

paul_c5x4 said:
From the numbers you give, you would have a maximum of five changes of air per hour - Possibly not the best ratio for a darkroom. Axia suggest a minimum of ten changes per hour, so you might want to consider a bigger fan.

Thanks Paul, can you tell me the math of how you figured that out?

brian steinberger · Mar 13, 2015

momus said:
If you just had a fan running in the darkroom that pulls filtered (HEPA) air in, and you had a small vent to the outside some distance from this, this would give you positive pressure. Instead of sucking the air in, you would be pushing it out. Seems like a more effective way to vent it.

Also, if you used citric acid for stop bath, and an odorless fixer (see link below) most of your problems would be solved. I would still use positive pressure venting though.

http://www.freestylephoto.biz/6200-Arista-Premium-Odorless-Liquid-Fixer-32-oz.

Thanks momus, I'm not sure how I could turn my setup into a positive pressure setup. Could I put a fan behind my filtered louver in the wall to blow air into the darkroom? There is a hole in the wall to the outside that has a dryer vent now. Would the air coming in push that damper open and force air out?

mgb74 · Mar 13, 2015

It's not just the cfm of the fan, but also the characteristics of the exhaust vent. The type of venting (smooth or pleated), diameter, length, and number of bends. So, as an example, you used 4" pleated venting and have 3 90 deg bends, you've substantially reduced the airflow going out.

As to placement of fan and inlet, you've got that right.

brian steinberger · Mar 13, 2015

mgb74 said:
It's not just the cfm of the fan, but also the characteristics of the exhaust vent. The type of venting (smooth or pleated), diameter, length, and number of bends. So, as an example, you used 4" pleated venting and have 3 90 deg bends, you've substantially reduced the airflow going out.

As to placement of fan and inlet, you've got that right.

Ok thanks. My exhaust fan is 4" and literally travels a straight line about 3 feet and it's out the wall. So I'm not reducing any airflow that way. Think I just need a 120-140 CFM fan?

Dr Croubie · Mar 13, 2015

Also think about where your air is coming in.
If you've got an outlet fan in one corner and the rest of the room is well sealed, how's the air going to come in to replace what's being taken out? Under the door, around windows cracks etc, or it just doesn't and the fan does nothing.
If you do have an inlet vent, and it's on the same wall as the outlet, you're not going to get much bad-air out either, it'll just replace that side of the room unless you've got a ceiling fan or something mixing the air up.
Best practice is to put an inlet vent (or fan) down low, and an outlet fan (or vent) up high diagonally opposite each other, such that your biggest source of bad-air (like fixer in a tray) is pretty much on a straight-line between those two.

MattKing · Mar 13, 2015

Check to see that air is actually flowing through the inlet, and out the exhaust.

brian steinberger · Mar 13, 2015

Dr Croubie said:
Also think about where your air is coming in.
If you've got an outlet fan in one corner and the rest of the room is well sealed, how's the air going to come in to replace what's being taken out? Under the door, around windows cracks etc, or it just doesn't and the fan does nothing.
If you do have an inlet vent, and it's on the same wall as the outlet, you're not going to get much bad-air out either, it'll just replace that side of the room unless you've got a ceiling fan or something mixing the air up.
Best practice is to put an inlet vent (or fan) down low, and an outlet fan (or vent) up high diagonally opposite each other, such that your biggest source of bad-air (like fixer in a tray) is pretty much on a straight-line between those two.

I have an exhaust fan above the fixer trays and the inlet down low on the wall opposite the exhaust fan. Now I'm thinking maybe add a fan behind my inlet? Or in front? The I'd have a fan blowing air in and a fan pulling air out. Obviously after I get a stronger exhaust fan.

brian steinberger · Mar 13, 2015

MattKing said:
Check to see that air is actually flowing through the inlet, and out the exhaust.

Indeed it is. But not very strongly.

Ken Nadvornick · Mar 14, 2015

On the topic of positive pressure, keep in mind that in such an arrangement you would not only be forcing dust out of the room, but also forcing fumes out into the surrounding house and/or living areas.

Now I happen to like the smell of an acid fixer. But my other half does not. So in my basement darkroom I opted for neutral pressure. A matched set of high-velocity blower fans on the same circuit, wired to the same PWM speed controller.

One fan inputs clean filtered air from outside the house and forces it down low. The other outputs stale filtered air (for neutral pressure) to the outside removed from up high. Turned all the way up in an emergency they can change the room air in about a minute.

Where I live for most of the year I have an inexhaustible pool of cool to cold outside air from which to draw. The fans are actually mounted outside to keep noise and vibrations down. The duct work passes through a permanently boarded window frame and is double right-angled and painted Krylon flat black on the inside. There are no light leaks.

Ken

paul_c5x4 · Mar 14, 2015

brian steinberger said:
can you tell me the math of how you figured that out?

Sure. Quite simple really....

brian steinberger said:
My darkroom is 11x11' with 7' ceilings (basement).[...] I put a bathroom exhaust fan (70cfm) above

11x11x7 gives us 847 cubic feet. The fan is rated at 70cfm, so times this by 60 to get the volume shifted per hour (70*60=4200) under ideal conditions. Divide this by the volume of the room (4200/847) gives a result of near-as-damit, 5 complete changes of air per hour.

Factors that will affect this include (as has already been mentioned) ducting away from the fan and the type of material used. The size of the inlet vent and also the airflow available on the other side of that wall - If the air source is from a short corridor, and there are closed doors either end, you will not get maximum airflow.

Darkroom ventilation

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