Darkroom Ventilation advice, please

[David Wolf]

I’d appreciate some help, please, to clarify a couple things about properly ventilating a darkroom. Thanks in advance to all ~

The space:

11’ x 13’ ground floor room opening onto 5’ x 7’ alcove. The doorway from outside is along the 11’ wall, which has an adjacent eye-level, sliding window, 14” x 34”. (Because of the sliding action, the actual window opening is about 15” wide.)
The alcove, where the darkroom will be, opens into the opposite corner away from the front wall. There’s a second, sliding window at eye-level here that’s approx 22” square (8” wide opening). The sink will face this window.

So, I’m thinking to put a Doran 400 CFM fan in the window in the larger room to draw fresh air in, and a passive air outlet through a light louver in the window above the sink in the alcove to release the fumes outside while maintaining a positive pressure. (I've been told positive air pressure is important to reduce the amount of dust in the darkroom.)

Or should it be the other way around? with the passive, louvered inlet of fresh air flowing into the larger room and the fan serving to exhaust (extract) the fumes above the sink and out the alcove window?

Which is correct?

I think what’s confusing me is the idea of positive pressure necessarily being created by a stronger in-flow of air relative to the fumes leaving the space.

Also appreciated would be tips on how to keep the fan area facing the outside dry, as well as alternates to the Doran fan and louvers as needed. And is some type of additional air filter on the intake side needed as well?

Many thanks for your kind advice and shared experience!

[Vaughn]

I have heard told and seems to be true, that fans have an easier time pulling air out of places than fans trying to push air into places (air being able to compress, etc). You will want to have the air pulled over your trays and away from your face, then exhausted. The passive exhaust system via positive air pressure as you describe might work in a nicely balanced system, but would be interrupted by any significant air movement caused by you moving and the opening of doors, etc. If you can make a box for the exhaust fan that can hang in the window, or sit outside the window, that will keep the fan noise down.

You could couple this with a fan and filter in the far window to bring in the fresh filtered air. Might take a little to get them nicely balanced.

[Pere Casals]

You will want to have the air pulled over your trays and away from your face, then exhausted.

+1


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In a ventilation you can blow in or blow out.

Your priority is making fumes from the wet area go directly to the extraction, if you have turbulence then fumes mix in the whole room air and this is not ideal.

So better having a fan extracting over the wet area because if you blow air in then you generate turbulence inside the room.

The input vent may have a filter, this is perfect to eliminate dust, but IMHO best choice is to also placing a fan with mild power on that filter, or to close the extraction fan when openning the door, because if not unfiltered air enters when openning door, this is YMMV, but I find very convenient to eliminate dust. Ideally you may use an HEPA spare filter (home air purifier) for the input, with a foam before it to protect it from most of the coarse dust. You can also use an spare anti-particle for the habitacle of a car.

I made flow simulations, but a good way to check if you have turbulences is using a cuban cigar (Cohiba Behike if possible ) to see how smoke flows to the extraction vent without turbulences.

Turbulences generated by blowing in, bad situation:




Good flow generated by an extraction system placed in the right place, trays are vented without spreading fumes in the rest of the room:

[Louie Powell]

Any discussion of ventilation must also take into consideration two important factors. First, dust is a problem that must be addressed in the design of the ventilation. Second, it's not possible to practically seal a darkroom so that you have total control over all paths for air flow.

To me, that says that a negative pressure design is going to have problems with dust - since you can't control/filter all paths through which air enters the darkroom, there will be a uncontrolled flow of dust. So I opted to design my darkroom to have positive pressure - that is, air is taken from outside the darkroom, passed through a filter, and forced to flow into the darkroom near the enlarging station. There is a passive vent that allows air to exit the darkroom at the wet sink so that chemical fumes aren't circulated throughout the room. It also means that the flow of air through those uncontrollable channels will be outward, thereby significantly reducing the risk of dust entry via those paths.

Also, my darkroom is wired so that the main power switch at the door simultaneously turns on power to both lighting and ventilation. There are additional switches at the enlarging station to turn off the white light, and turn on the safelights. As a result, ventilation is automatically on whenever I'm in the darkroom.

The other important point is that ventilation systems can be noisy, and high ambient noise isn't really conducive to creative work. I designed the ventilation system in my darkroom to place the blower totally outside the darkroom, and to have the filtered air flow through a duct (corrugated plastic shower vent piping) into the darkroom. As a result, I can only barely hear the blower inside the darkroom.

Final thought - the darkroom in our previous home didn't have ventilation, and my experience was that I tired far more rapidly while working in that space compared with my newer darkroom. There are obviously many factors that contribute to that observation, but I believe that the improved ventilation is a major consideration.

[Pere Casals]

Second, it's not possible to practically seal a darkroom so that you have total control over all paths for air flow...

To me, that says that a negative pressure design is going to have problems with dust - since you can't control/filter all paths through which air enters the darkroom,

With negative pressure, the single uncontroled path is the slits in the door boundary, but we can use soft rubber/foam seals like those for thermal efficiency in window frames. Anyway that flow can very small to cause dust problems.

The positive pressure is very prone to create turbulences that mix the fumes in the whole room, then it requires a much higher air flow, and this may make difficult to have 20.0ºC in the lab for total consistency, and filter is clogged sooner.

Yes, the negative pressure may allow some little unfiltered flow incomming, but positive pressure requires a too high air volume for a good ventilation, so IMHO it's not the right one, to me best is negative pressure and addressing its drawback for dust control, this is providing an aceptable sealing for the room door or to place a low power fan in the input filter to have zero pressure difference. In my experience a very simple seal in the door makes the negative pressure way the right one, no odor, no fumes around, smaller vents, less (or no) noise, easier temperature control and (easy to obtain) also perfect dust control with less dirt in the filters.

IMHO positive pressure works but it cannot be recommended, what it can be recommended it's sealing a bit the door for negative pressure. Simply use light seals that will also seal (or filter, foam) air.


Do the cigar test with positive pressure, you will see how smoke is dispersed in the whole room, then test it by reversing the fan, you will see how smoke goes direcly to the output in a laminar flow without spreading.

[Bruce Watson]

So, I’m thinking to put a Doran 400 CFM fan in the window in the larger room to draw fresh air in, and a passive air outlet through a light louver in the window above the sink in the alcove to release the fumes outside while maintaining a positive pressure. (I've been told positive air pressure is important to reduce the amount of dust in the darkroom.)

Or should it be the other way around? with the passive, louvered inlet of fresh air flowing into the larger room and the fan serving to exhaust (extract) the fumes above the sink and out the alcove window?

Which is correct?

Air is compressible. As such it's much easier to pull air than to push it. Air pushes about as well as pushing a rope. Much better to put the fan on the wall exhausting to the outside. Much better to pull air out of the darkroom from above where the noxious chemicals are (typically the wall above the darkroom sink) and let the air flow past you, over the trays, and out. If needed, pull the air from above the sink through a duct to that fan on the outside wall. Let "make up air" into the building through a vent that's pretty far away from the exhaust of the fan (or you'll just end up pulling the noxious fumes right back into the darkroom).

[David Wolf]

Thanks to all for sharing their experience ~

How important is the height of the incoming air louver/fan? It would be much simpler in this room to locate the inlet louver (with or without fan) in the already existing, shoulder-level sliding window (across the room from the sink), rather than cutting a hole into the bottom of the adjacent exterior door. With the louver in the window, however, the fresh air would be incoming at about the same height as the the window above the sink on the opposite side of the room, where the fumes will exit. Would this matter much? Some recommend the air intake to be lower than the exit ~

Also, what brand/model do you use for fans, whether intake or exhaust? HEPA filter type?

Thanks again!

[Drew Wiley]

Pulling air is far more efficient than pushing it. I use a big externally-mounted squirrel cage fan on the exterior of the building. This also isolates the noise. It has a variable power control switch. An industrial unit made by Broan, not at all like an ordinary residential fan. If you use a residential type, I strongly recommend a Panasonic in-line fans with speed control. Around $400. Also squirrel-cage design and very quiet. Propeller-type fans are noisy. Of course, you need commensurate intake vents.

[Graham Patterson]

I put my extraction vent in the ceiling. It was a pragmatic choice, but less than ideal. I clears the fumes and ventilates OK, but in the winter it also removes the warm air! Since my intake is from the outside, that can be significant. Extraction behind the wet area and below head height might be better, but you also need to avoid long extraction ducts and bends in the them. That causes turbulence and reduces efficiency.

Oh, and make sure your ducts do not rattle!

My space is 8'x9'x7'6" high.

[Leszek Vogt]

In my case I have a window on the opposite side, which allows the air to enter and the amount can be controlled. There are 100s of ways to set up ducting over the chemicals (dev. area). Quiet fans would work best and there are plenty to choose from. Also, there are inline fans that help the air to push out of the duct.....but I'd never buy one of those $18 ones from homey depot or Lowes. Yes, the quality ones cost and I happen to have one on clothes drier outlet line....it's been in use for over 4yrs w/o issues.

Here is a sketch (adaptation ?) from Chris's darkroom, who is on this forum....and you can install as many fans as you desire....preferably quiet ones.



Les