How to split a single fan's pressure evenly between unequal sized areas

Howdy everyone. I hope y'all don't mind me dropping this trippy problem on you right off the bat. But, I've searched all over and a couple posts here were the closest anyone got to an answer.

https://www.rollitup.org/t/exhaust-2-tents-1-fan-filter.439683/
https://www.rollitup.org/t/cooling-two-grow-rooms-from-one-vent-fan.826096/
https://www.rollitup.org/t/one-fan-two-tents-and-a-carbon-filter-can-it-work-rep-for-help.187076/

I think those posts would have solved it if the areas were equal size or even close. I realize that putting adjustable dampers on each line might be a solution. I plan on doing that also if I can find the parts. But, I hope to get the ratios as close as I can so less adjustment is required.

In this case one area is 50 cu. ft. and the other is 175 cubic feet. The smaller area contains a 4" carbon filter so I'm adding 25% to get 66.5 wich I turn straight into CFM to get my desired avrage CFM.

The larger area is easy at 175 CFM, though I wanted to add some kind of bug screen to the intake and I have no idea how much air resisitance panty hose or air conditioner filter might cause. If it's close to the same as a carbon filter then adding another 25% makes it 232.75 CFM. This larger area is the mother and clone area and doesn't need odor control.

I'll be using a 440ish CFM 6" inline fan. I'm hoping to be able to run it somewhere near medium speed with a controller and equally ventilate two very different sized areas.

So, (and please let me know if you know of a better way. The HVAC crowd online seems hostile to DIYers so I didn't even try at any of those places.) it seems to figure this out I need to get the ratio between the room volumes. Using some online tools I got the following.

50 is 28.57% of 175 - no filters
66.5 is also 28.57% of 232.75 - both areas filtered equally
66.5 is 38% of 175 - small area filtered only

It looks like I need to split the stream somewhere between 60/40 and 70/30, or make the smaller stream 29-38% the size of the larger.

Now the ratio between round duct sizes.

3 is 37.5% of 8
3 is 33% of 9
3 is 30% of 10
4 is 40% of 10
4 is 33% of 12


It looks like, if I want to keep the existing 4" hole and carbon filter in the small area and not have to use adapters, I'd have to use a 12" duct for the larger area.

Okay, but the 12" insulated ducting is huge! I would like keep this whole thing as small as possible.

If I try to go with the 3" size for the smaller area it seems like I might run into problems finding supplies as not many stores seem to carry the odd sizes. .

What do you guys recommend for a situation like this? I'm open to different ideas.


Oh, wait! I almost forgot the trippy part that is sending my brain into spirals.

The smaller area is INSIDE the larger area. Yeah, Doh! How does that change things? Those HVAC guys must be near rocket scientist geniuses to figure this stuff out.

And, the (flowering) smaller area uses the suction, or pressure, created to seal the area with back pressure.

So, will opening the door to the larger area (which has some back pressure also) unseal the door of the smaller area?

Lastly, would what's known as a "return plenum" in HVAC help in this situation or make it worse? I can't quite figure out how they work when pressures change on different ducts. I just read they equalize pressure in multiple lines. What happens to the other duct's pressure when pressure drops in a duct?

If a return plenum would solve it, I got a great idea from the last thread on my list above. Combining the plenum box with the sound proofing box I was already planning on making for this fan would be awesome.

I'm making a sketchup model of the project so once I get closer to what duct sizes I'll be using I'll post some pics.

I know this is a head scratcher. Thanks in advance for any advice.

You are way overthinking this. What are your room dimensions? Are you using tents? If the real problem is you only have one fan, then buy another one.

Banana444 said:

You are way overthinking this. What are your room dimensions? Are you using tents? If the real problem is you only have one fan, then buy another one.

Click to expand...

Overthinking? (Yet you offer no simple solution?) I hope so. But, I doubt it. Anyway, for me this is fun. I've been half-assing this ventilation thing for far too long. It's time to get it dialed in once and for all. I have plenty of time to "measure twice, cut once".

Dimensions:

Outer: 5x5x9 minus inner
Inner: 5x2x5

No, it's not tents, drywall and wood mostly.

As for adding another fan, sorry no. It puzzles me why so few people consider the amount of electricity they use, or the amount of space they waste. But, both of those are considerations for us. So, just adding another fan is wasteful in both areas, especially considering the total ventilated space is only around 250 cu. ft. and one fan should be able to handle it just fine, and more quietly than just stacking the suckers.

Lots of people use a single fan for two areas. They just normally have two equally sized areas, oh and they're usually separate instead of inside one another.

I'd rather keep them (the ventilation ducts for each area) separate too, instead of venting one into another. It's a thorny one. Sorry if I broke your brain.

Vape some more and mull it over. Maybe something will come to you. That's what I'm doing.

Here's the beginning of a sketchup model of this thing. That "bench" you can kind of see inside is the inner area. The ducting will be going through the wall on the other side in this pic where I have the framing.

it is possible to use the same fan and filter for 2 tents, run duct from the exhaust sock of the small tent to the intake sock of the big tent, the big tent has the exhaust system, then its just a case of opening and closing passive intakes on both tents until both tents have the desired airflow.

although not specifically what you asked you can see my example holds some bearing, the adjustable damper is an option. But when you decrease the airflow moving through one duct the air pressure will increase in the other to the point where it will get tricky to control both environments simultaneously.

when the smaller space is a veg room it helps cos for the most part you want a higher temperature and humidity in that area anyways

run the duct from the smaller space exhausting out, exhaust the larger space passively from the smaller spaces passive intakes. Mind fuck! But it works

Thanks, but the smaller area is for flowering so between it needing negative pressure and putting out the most heat, I'd rather not vent it into the larger area if possible.

Blue brother said:

run the duct from the smaller space exhausting out, exhaust the larger space passively from the smaller spaces passive intakes. Mind fuck! But it works

Click to expand...

That sounds like if I understood that it might be a solution... but, uh whaaaaaa? Please elaborate if that was a real idea. I'm intrigued.

Here's what I have so far. Making this model I realized that anything larger than 6" duct is getting too big for this little area. So, it looks like I'll just have to use dampers to adjust it. (The green parts are the adjustable dampers.)

Side view:

So, that's a 6 in. fan and 6 in. duct parts. It turns into wall stack then a 4" carbon filter on one end. The other will probably be an open 6 in duct with some window screen or pantyhose strapped around it.

I'm leaning towards rigid duct since this will be a fairly permanent structure and it should help cut down on noise. Plus, it's the only thing that does wall stack shape, right?

I guess I was overthinking it considering I don't have enough space to do anything exactly right. Hopefully this will work to provide more ventilation and also cut down on noise.

I wanted to build a soffit around the whole thing but now my wife wants to make it look steampunk. So, now I'm looking into how to build a steampunk looking soundproof fan box. I hope I can make MDF look the part. ha

it is easy once you think of air as flowing through the easiest path. you can simply make 2 holes for exhaust with a Y connection, if one hole is sucking faster than the other, simply use a shutter to fine tune it, the harder it is for the air to come through this hole, the more air will be passing through the other.

im imagining a small tent inside a big tent, hang your carbon filter and fan inside the small tent, run duct from that fan, through the extraction port on the small tent, the duct runs through the larger tent and out through the extraction port on that tent. This means the air will flow passively through the intakes of the big tent, then into through the intakes of the small tent then it will be extracted from the small tent via the fan, filter and ducting, to an area outside of both tents air masses. If you don't understand let me know and il draw a pic when I get home

when you're done with this, can you help me split an atom?..............just kidding.

super skunk collider

zem said:

it is easy once you think of air as flowing through the easiest path. you can simply make 2 holes for exhaust with a Y connection, if one hole is sucking faster than the other, simply use a shutter to fine tune it, the harder it is for the air to come through this hole, the more air will be passing through the other.

Click to expand...

I hope it's as simple as adjusting the dampers. After reading about HVAC for weeks I finally found a pdf describing how the engineers do it for buildings. I taught myself a handful of coding languages so I figured I could handle a little math. But, after looking over all that, they say themselves, right there in the book about it, that all that engineering only gets things in the ballpark!

Then the builders come in and do stuff with their "rule of thumb" knowledge. And, then if you actually want your ventilation system balanced you have to call in another expert with a slew of expensive gadgets to professionally balance the system.

So... uh... yeah I guess it looks like dampers are the answer, even for the pros. I just wanted things to be in the ballpark and not rely solely on dampers because the run is very short and part of the reason for this job is to get rid of all the noise. A mostly closed damper might whistle or moan or something, no?

The only way I could figure to get in the ballpark was to compare the relative sizes of the ducts and the areas.

There is stuff online that promises to help a person calculate duct size based on CFM but none of the numbers coming out of those tools seems to make sense. Also, every tool or chart or graph seems to only be for the positive pressure side, not the return. Hardly anyone else seems to have ventilation like this except maybe the woodworkers and their sawdust collection systems. But, even their explanations turn into trade gibberish I can't decipher.

Hopefully this helps you some...

https://skunkpharmresearch.com/ventilation-system-design/

Blue brother said:

im imagining a small tent inside a big tent, hang your carbon filter and fan inside the small tent, run duct from that fan, through the extraction port on the small tent, the duct runs through the larger tent and out through the extraction port on that tent. This means the air will flow passively through the intakes of the big tent, then into through the intakes of the small tent then it will be extracted from the small tent via the fan, filter and ducting, to an area outside of both tents air masses. If you don't understand let me know and il draw a pic when I get home

Click to expand...

I love this idea. I wish I could make it simple like that. I love simple, elegant solutions. I think I understand now. But, I've been trying to figure out how to make it work for me and I haven't yet.

Unfortunately, the two intakes are currently a couple feet from each other, down low, on the near side of the room. The air goes out up high on the other side.

I could move them. I'd rather not. But, I would if I could get this simpler plan to work.

But...
Say the smaller area held the main intake, then vented into the larger area:
The smaller area is the hottest area (flowering) and it relies on negative pressure to keep the doors sealed. So, making the hot exhaust from the smaller area the intake of the larger area poses a problem especially because summer temps area already sometimes an issue.

Plus, the air would stop flowing through the smaller area whenever the door to the larger area (room) was opened.

And if it were reversed?
The air flows into the larger area ( up high I guess so it can cross the room to the smaller area's intake down low? ) then into the smaller, then finally out the ducts and through the fan.

This would take already heated air and then heat it again in the large area before flowing through the smaller area. This doesn't seem like a good idea.


Am I missing something?

a senile fungus said:

Hopefully this helps you some...

https://skunkpharmresearch.com/ventilation-system-design/

Click to expand...

Yes! Thanks. This looks like it could be it. It does seem to be figures for positive pressure though, but maybe it works the same both directions?

I found out that I can get 3" round duct and fittings online so I hoped to just use that instead of the wall stack. It should fit in the wall.

If I do the numbers with this new info:
The fan is 6".

6x6x.7854 = 28.2744 sq. in.
3x3x.7854 = 7.0686 sq. in.

28 - 7 = 21

(In the article, he seems to make an error at this point, in rounding up to 48, instead of 38. If that was intentional he didn't explain why he would do such a thing.)

The square root of 21 is roughly 4.6

So, it looks like if I have one 3" line, the other one should be only 4-5" to evenly split the volume of the 6" fan.

Hmmmm. Man, that's way different from what I got doing it my way. I'm glad you chimed in. This is probably the right way to figure it. I was concerned that having a 7-8" intake split with a 3" would just bypass the smaller one.

Here's the latest version of project Budffleupagus. From the filter it's three inch duct (can't avoid that one 90) then up to a gradual turn with two 45s. The gold parts are where the dampers may be. The wye is just some 45 degree 6" wye from the overrated and seldom useful 3d warehouse. I plan on using a 30 degree wye or maybe even one of those ideal-air evenly split ones.

This model is just to get a general idea of the parts I may need. Closer than a drawing on a napkin at least since I can guestimate distances (and get some from specs online). I realize that I'll need to actually go get most of these duct parts and physically measure them to get the actual instqaled dimensions.

why cant you use two fans?

Buds4Fun said:

why cant you use two fans?

Click to expand...

Its too simple, by the time he buys all these ducting parts, he probably could of bought another fan. I know if he spent all the time he has been thinking and reading he could have worked at a job for a day and bought a fan and been on to the actually growing.