Dark Room Ventilation With Numbers

Qebs · May 17, 2021

Hey Everyone,

I'm working on my second darkroom and need to make a better ventilation system as I will be doing the RA4 process.

My down stairs bathroom is 54"x106"x81" = 269 Cubic Feet

The exhaust fan I'm interested in does 205 CFM.

If I did the calculations right, that's 45.7 room changes of air per hour.

From my research tonight, it is recommended to have changes of 10-15 per hour.

So I think I'm okay for exhaust.

1. Now my question is did I do the math right?

2. Can I use this louver for the air input? Or can someone recommend a louver that has a fan for positive pressure?

https://www.bhphotovideo.com/c/prod...1212_L_1212_Light_Tight_Darkroom.html/reviews

Thanks for reading this.
And thanks for any advice shared

Be well
Best Regards,
Kevin

gone · May 18, 2021

In the reviews on your link, someone says "I needed a air intake that was light proof for my indoor garden. Couldn't ask for more."
That sounds like it can be used for intake. If your math is right, you're getting twice the air changes per hour as a hospital operating room gets w/ positive pressure, so you're probably pretty safe. Operating rooms need a high rate of air exchange for a reason most people wouldn't guess. Seems that occasionally a valve or hose will leak on the anesthesiologist's equipment, and before they went to these high exchange rates, when that leak developed it would put everyone in the room asleep along w/ the patient.

MattKing · May 18, 2021

If possible, position the intake for the exhaust near the chemicals and on the other side of those chemicals from where you will be standing.
And position the intake louver on the opposite side of the room, preferably at a different height than the exhaust intake. If it is lower, the dust drawn in may be less of a problem.

wiltw · May 18, 2021

momus said:
In the reviews on your link, someone says "I needed a air intake that was light proof for my indoor garden. Couldn't ask for more."
That sounds like it can be used for intake. If your math is right, you're getting twice the air changes per hour as a hospital operating room gets w/ positive pressure, so you're probably pretty safe. Operating rooms need a high rate of air exchange for a reason most people wouldn't guess. Seems that occasionally a valve or hose will leak on the anesthesiologist's equipment, and before they went to these high exchange rates, when that leak developed it would put everyone in the room asleep along w/ the patient.

Sounds like something for a script of Grey's Anatomy or New Amsterdam or Chicago Med

runswithsizzers · May 18, 2021

Given your specs, I get the same numbers, so your math checks OK.

Logically, the intake opening must be some minimum number of square inches in area to allow the exhaust fan to work at optimum volumes. If the intake vent is undersized, then the exhaust fan will work harder, but remove less air. However, I do not know how to calculate the minimum intake area for a given exhaust fan.

MattKing · May 18, 2021

Unless your darkroom is otherwise air-tight, the louvered air input might not even be necessary. It is still a good idea though.

Qebs · May 18, 2021

Hey momus, Oh wow, I guess that's a lot of movement of air. Thanks!
~~~
Thanks Matt, yep, I think I read the intake should be lower than the exhaust.
And the user should be in between.

Sorry, I'm confused Matt, if it's air tight how does fresh air get in easily?
~~~
Funny comment wiltw
~~~
Hey runswithsizzers, Thanks for confirming the numbers, much appreciated!
I wonder what would happen if the louver is too big?

runswithsizzers · May 18, 2021

Qebs said:
[...]
Hey runswithsizzers, Thanks for confirming the numbers, much appreciated!
I wonder what would happen if the louver is too big?

If the intake vent is too big, I would not anticipate any problems at all, except possibly temperature control if the louver is to an outside space. Most of the articles I have read suggest it is better to have too much intake area rather than not enough. If the intake vent is from the outside, you will probably want some kind of screen to block insects, and that will reduce it's effective size a little bit. But as others have suggested, there will probably be some amount of air infiltration into the room from other openings unless you have taken extraordinary precautions to prevent leaks around doors, pipes, electrical fittings etc.

DREW WILEY · May 18, 2021

It's not quite so simple. Your hypothetical fan air exchange rating will be significantly diminished by two things : air duct friction, and the resistance of outdoor hydrostatic pressure in damp weather. Expect about half of the rating on the box. But 200 to 250 rated CFM should be adequate for your relatively small darkroom. And yes, you'd need equal makeup air just to achieve that. B &H probably still sells Doran light-proof air-intake vents, which I see has already been linked. They're not perfectly light-proof in direct sun, so you might need to place a little shade or visor above them outdoors.

With respect to fans, you get what you pay for. I recommend Panasonic is you want something quiet and reliable. One problem in either overly hot or cold weather is that you might be exchanging more air on that day than actually necessary, making life inside the darkroom uncomfortable. That's why I additionally recommend purchasing a model of fan which will work with a dedicated accessory speed control.

MattKing · May 18, 2021

Qebs said:
Sorry, I'm confused Matt, if it's air tight how does fresh air get in easily?

I was referring to a darkroom that is otherwise air tight - very few are.
In most cases, amateur darkrooms are not air tight.
If yours is the exception, than an intake vent is critical.

resummerfield · May 18, 2021

MattKing said:
If possible, position the intake for the exhaust near the chemicals and on the other side of those chemicals from where you will be standing.
And position the intake louver on the opposite side of the room, preferably at a different height than the exhaust intake. If it is lower, the dust drawn in may be less of a problem.

Excellent suggestions, especially about the positioning of the exhaust.

DREW WILEY · May 18, 2021

Yes, the whole point is to gently draw the air across the room toward the back of the sink. But I personally like a serious power reserve in the fan in case there's ever a spill of something noxious, like glacial acetic acid or concentrated color chemistry. The company I worked for sold thousands of fans, and I consulted on all kinds of darkrooms as well as other industrial applications. It's all basic HVAC knowledge if someone is really interested. A simple efficiency test for a room is a smoke test.

I personally have a huge industrial externally-mounted squirrel-cage fan, with all the noise isolated outside, and lots of extra pull power if needed (pulling air is more efficient than pushing it). But it serves my several darkrooms at once. The only downside is that wild bees mistake it for an ideal hive site. That happens about once a year; and I've had to keep running the fan on high as long as three days at a time to discourage them. They're sure determined litter critters! Once in awhile a live bee gets through and lands in the darkroom sink, stumbling around dazed. A few others get chopped up in the process, with little bits of them landing in the sink below the fume hood. Most are unable to even land on the fan itself due to the force of opposing air. Maybe I should go into the honey business instead.

gordrob · May 18, 2021

I would think that a 205 CFM fan would be a bit large for your space of 269 cu'. The calculation I did for your space would be 269 divided by 60 minutes = 4.48. Your max air exchanges of 15 times 5 (4.48 rounded up) would suggest a fan with a CFM of (15x5) 75. As Drew mentioned you have to take in to consideration the friction of the air being exhausted. If you have a short straight run from the fan to the outside of the building then there will not be much of a drop in the fans efficiency If you have a lengthy run of ducting and any.turns in the run then you have to take those into your calculations. If you have a short run a smaller fan with a lower sones rating of 1 or less may work better. Also ensure that your exhaust to the outside is at least 3 feet from any openings back into the house like dorrs or windows.

Qebs · May 18, 2021

Hey runswithsizzers, great, I'll keep looking into louvers then.
The intake will be on a washroom door.
Thanks
~~~
Hey Drew, thanks for your reply.

Okay, so, a couple of things I should bring up.
1. I was thinking of doing loops to block light from getting in to the tube that will be stuck outside
through a makeshift window.
2. The duct piping will be around 32 feet from exhaust hood to outdoors.
Would I need to get "bigger" fan?
Should I build a baffle/light proof vent.

Oh, so, the Doran light proof intake vent would be in door which on one side is a bedroom and the inside is a bathroom.

Drew, could you recommend a model of the Panasonic fans?

I found this one, but no speed control listed.
https://www.amazon.com/Panasonic-FV-10NLF1E-WhisperLine-Line-White/dp/B000EDUIX2?th=1

Since you mentioned having a reserve "power" is good: Do you think this one with Airflow of 402 CFM will work better? - The only issue is it's louder than the 4" model I re-listed bellow.

https://www.amazon.ca/AC-Infinity-C...1621388548&sprefix=AC+Infinity,aps,201&sr=8-2

The one I was looking at does have the speed controller:

https://www.amazon.ca/AC-Infinity-C...&keywords=AC+Infinity+4"&qid=1621307083&sr=8-

~~~

Hey MattKing, ohhh okay, sorry for my confusion.
Thanks!

~~~

Hello gordrob, thank you so much for trying to also try to help.

In terms of friction, the duct will need to be around 32-33 feet.
I might do some bends to block light if i don't build a light proof vent.
Okay, looked up sones. The two AC Infinity fans I listed above have not sones but decibels of 28-32 dBA.
I would be mounting the fans in the room outside of the darkroom.

Okay, thank so much for the tip about mounting the exhaust away from doors or windows.

~~~

Thanks again everyone for your continued sharing of knowledge and time!

DREW WILEY · May 18, 2021

The bigger diameter the ducting, the more air will get through efficiently. And corrugated aluminum ducting is easier to use, but introduces more air friction, especially in darkroom applications where a certain amount of bends might be necessary to keep all light blocked off. I'm not familiar with the brand on Amazon you linked, but yes, an inline fan somewhere in between will move air more efficiently than a ceiling or wall-mounted ordinary one, so approx 200 CFI nominal rating would seem sufficient UNLESS you live in an especially humid or damp region, and EXCEPT for the fact you plan on 32 feet of relatively small 4 inch ducting, which is quite a distance. So if possible, it would be wise to acquire one of the higher rated 350 or 400 CFM versions. Even 30 DB's is fairly quiet. And most of the time you'd probably be using it at low speed anyway, so very little detectable noise. I use a 350CFM inline Panasonic fan installed in the attic for my home itself, vented to the roof, and it's easily capable of handling both bathrooms in this routinely foggy coastal climate. I don't hear it at all. It has variable speed control, and is matched to 6 inch ducting; but these cost nearly $350 here. The Amazon price is lower, but you'd have to add about 30 more dollars for a dedicated VC wall control. More CFM requires a larger duct of course; but 6 inch ducting is only a little more expensive than 4 inch if expandable corrugated aluminum is involved.

Qebs · May 19, 2021

Hey Drew Wiley,

Awesome, okay, thanks so much for your detailed reply.
I'll go with the 6" 400CFM unit, with a door louver and a light proof vent for the exhaust to outside.
~~~

Oh, what do you all use for the vent hood?
How well does the PVC pipe with holes in them work?
~~~

And thanks again everyone!

AgX · May 19, 2021

DREW WILEY said:
Your hypothetical fan air exchange rating will be significantly diminished by ... the resistance of outdoor hydrostatic pressure in damp weather.

Could you explain on this?

DREW WILEY · May 19, 2021

Air pushing against not just air, but air holding water mass - moist air representing resistance. The greater the moisture or humidity, the greater the resistance. There are mathematical formulas for that; but there's no need to go into that degree of complication for our present purposes.

DREW WILEY · May 19, 2021

I'd opt for corrugated aluminum ducting over flexible corrugated ABS drainage pipe (PVC drainage is generally rigid). For one thing, residential building codes might not allow it. And perforations or holes aren't going to do you any good. All the air needs to be vented outdoors to a roof vent cap, or if a wall, a dryer type vent and flap works OK.

Qebs · May 19, 2021

Hello Drew, Thanks for the advice/suggestion!

When you say, corrugated aluminum ducting, that's for the ducting but may I ask what you think I should make the vent hood out of?

Could one just use the PVC pipe and drill holes.
I've attached an image I found, (sorry I didn't note the original poster)

Thank you!!

DREW WILEY · May 19, 2021

Oh, down there near the sink. Yeah, gray PVC or black ABS as depicted in the jpeg would be fine. I keep on hand special "steeple-point" drill bits for plastic, which work much better for that purpose than ordinary drill bits. Instead of large plastic pipe, for one processing station I personally used a section of PVC roof gutter upside-down, then connected that to the hose - in my case a flexible high-quality rubber ducting which can be transferred to other spots if needed. For example, I suspend the hose with a square to round plastic adapter over the end to create a very localized exhaust duct directly over the graduates I mix RA4 color chemisty in, since I'm allergic to that. Then that portable hose feeds via a booster fan into the main fume hood itself.

Qebs · May 19, 2021

Hello Drew,

Woah, very interesting setup, it sounds so safe!

Thanks again for your help in this thread and through my private messages!

I'm hoping to be set in the coming months but I think I have the info I need now to get to that goal.

Be well

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