No Growmie, I’m saying the inline fan is drawing the cooler air from outside the tent nothing was stated about that adding to your exhaust fan the lower one is pulling in fresh cool air and the upper is exhausting the hot air. Do what you like, I was merely offering you an option to cool your tent down to a range that your plants can tolerate. It works…I do it often 
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This illustration may help. Where it’s reads intake, I use an 6” inline booster fan with a hose piped into the lower tent port.
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The problem with running the fans in series is that it restricts the area necessary for the fans to reach their full rated CFM. If the fan needs (for example) 4x its own intake port area to reach max cfm, & you reduce the intake area to 1x in order to install an identical fan in series, you’re down by 3x on your intake area. So (for example) running the one fan with 1x intake area reduces its flow by 75%. So lets say you double your flow by installing the intake fan. So now you’re at 200% of the one fan with 1x intake area, but you still could have flowed more than that with one fan at 4x intake area. And you can flow twice as much as the one fan with 4x intake area, if you run the second fan in parallel ( ’ side by side ', not in line) as a second exhaust fan & go to 8x intake area. Air in is air out, never more or less, so the flow rate does matter. However you are correct in thinking that if the ambient air is too hot to begin with, flowing more of that air won’t help you reach your target temps.
Also, even with cool ambient air, you can easily have a scenario where the initial exhaust fan is too small, or choked out to flowing almost nothing for whatever reason, & then adding an intake fan would probably show more of an improvement, but still very inefficient & also probably put the tent into positive pressure, which is usually not wanted. Really in that situation the intake fan is an inefficient band aid & the real problem of why is the initial fan not flowing enough should be addressed.
@venturi have you taken any bench-test pressure readings yet? I’d be surprised if that carbon filter is stealing only 25% of your flow, I’d guess closer to twice as much. But I’d be interested to know what the pressure numbers are on the exhaust side with an open intake, then half-blocked intake, then unblocked again but instead with the carbon filter installed to get a better idea.
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That’s obviously correct and is why I’m having trouble understanding how adding a booster fan would help.
I’ve assumed a 200-CFM booster in my figure because, if it was 100 CFM, for example, the exhaust fan would be limited to 100 CFM.
In the “proposed configuration,” the booster is trying to pump 200 CFM into the tent, but the exhaust fan allows only 137 CFM to exit. I believe that imbalance would produce positive pressure inside the tent, but not much. The booster could only push 137 CFM in and, if its RPM was set to produce its 200 CFM maximum, it would be working against the restriction caused by the exhaust fan. That would increase the load on the booster without producing any increase in system CFM.
@OGIncognito: Is something missing from my figure, or wrong with my reasoning?
AC Infinity told me that a carbon filter reduces an inline fan’s CFM by 20-30%, typically. My 137-CFM figure assumes 30%.
So would I! Can you point me to a source that explains how to do those measurements?
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I thought you were going to do some pressure readings with gauges.
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I’m interested, but have no gauges or knowledge of how to use them. Can you point me to a source that explains?
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Maybe I can try to walk you through a DIY swing-gauge. You should be able to leave your fan installed as-is (it looks horizontal in your pic), but you would need to pull the carbon filter off during the first few steps until we would check it for flow losses by putting it back where it is now.
I should say that I think you might need a more powerful fan for a 2x4 even if you optimize the flow of what you have now, especially if ambient temps are problematic, but if you want to check the flow of what you have now, I can try to walk you through.
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I tilted mine up and aimed it under my plant. Gives them a gentle sway 
Some even come with a dial. Pretty helpful.
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Nice, I have a 4” and 6” similar to that one
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Those ‘booster’ fans start to lose flow very rapidly as soon as restrictions start to increase.
You’re basically making a bad design (running fans in series) worse by using a fan with a poor performance curve. What’s the real problem? Fix that instead.
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Is this the type of gauge you mean: Dwyer gauge Would the 25-400 FPM model be suitable?
Do you use it inside the tent, as a circulation fan, or is it pushing air into the tent through a port? If the latter, please look at post #24 above and see if you can explain the error in my reasoning.
Taking another look at your carbon filter & wondering if it’s their 12" or 16" model. If it’s the 16" then maybe it is only stealing 25% of the flow. If it’s the 12" then I’d think it would steal maybe around twice that, & I was thinking it was the 12" model. But either way it would show up on a swing gauge.
Also wondering about the house hvac & the room that the tent is in. Is the hvac circulating the air in the room that the tent is in? And the ambient temps where the tent is are 84 - 86f? Is that what you keep the house at with the hvac? Or is there a cooler room where you can move the tent to or at least duct in cooler air from? (No booster fan needed if you dial in the setup, but you still might need a bigger fan in the first place no matter what) Or is 84 -86f & maybe dropping it a couple degrees the coolest ambient air you have to work with anywhere in the house?
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Can’t help with that sorry
Mine reduced my tent temp because it was pulling cold air in from the ground better. The flap was open prior and I had one of those dollar store desktop fans pointed at it to try and help.
Might have reduced the what’s it, but number go down so 
.
Perhaps the exchange of whatever it cost me was in my favor.
Regardless, it’s too hot here so my flowers don’t smell too much and I just leave the tent open now, day and night, to keep the vpd high with 3 fans in and around it. So it has little impact now.
Doubt that helped.
Also, does the fan in line block additional air being pulled by internal tent suck if it’s indeed pushing lower than is being pulled?
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Sorry, I hadn’t seen your question about the gauge before I posted my questions above. Maybe that would work, but off the top of my head not sure if it’s sensitive enough or not. Maybe? But really you usually don’t need to buy anything to do what I’m talking about. I made mine out of card stock, a piece of foam, some tape, & a marker. And some Scott Steiner math.
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Air in is air out. Air out is air in. No more, no less, & always at the same rate. However you get there is however you get there. Whatever power that isn’t turned into flow becomes pressure or vaccum within the space.
If you started seeing more flow through the intake & exhaust after installing a fan on your intake, then you missed something(s) elsewhere.
A 6" fan in a 6" intake hole takes up more intake area of what’s probably an undersized intake hole to begin with, based on what something like a 4" hybrid centrifugal/axial blower such as OPs exhaust fan would need. Lets say the exhaust side is flowing ideally through the 8x12" port at full power, but then he switches instead to a 6" port & booster fan on the intake. If that intake booster fan is not powerful enough to run with the cenrifugal/hybrid exhaust fan, which even a 6" booster fan might not be (blade design, housing design, etc), then it is just burning power to spin & beat up the air (turbulence), while not moving much if any air, while also taking up area of what has become an undersized intake hole.
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I really have no clue what half of that means, sorry.
I’m sure it’s correct.
But I like my free under canopy fan vs ANOTHER inside the tent fan that also makes me feel like I’m adding more air, even if it isn’t, but still lowers the temperature.
Also my exhaust is not on full blast 24/7, as I embraced the heat and just use VPD and myco. So pressure and CFM and etc. is all a ? for me.
I see why @OGIncognito bowed out though xD
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Number 2 is fairly significant. Attic or out of the house will help some.
Not having a closed lung room is a challenge I haven’t faced.
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It’s their 12" refillable model for 4" fans.
The room (15’ x 16’) is open to the house – archway, no door. It has two HVAC vents. I’ve got a ceiling fan running 24/7 in there now and my digital controller reports 74°, 50% humidity, and 1.4 kPa inside the tent and 74°, 52%, and 1.3 kPa outside.
This source has a table that shows the late flowering stage requires the lowest tent temps: 75° max daytime (I suppose “daytime” means “lights on”), 65° max nighttime. I can meet those criteria in the winter, but there’s no practical way to get the room down to 65° in the summer. I think a Terraform 7 is in my future.
Piping more cold air into the room isn’t feasible, but there’s a furnace room with a door in the basement that’s large enough for the tent and would make fan noise irrelevant. I’m not keen to have HVAC repair people eyeballing my rig, though, and I like the convenience of the current location: 2nd floor, just off the master bedroom.
I believe that whichever fan has lower CFM will prevent the other from pulling or pushing its rated CFM; hence, system CFM will be whichever number is lower. The tent can’t change its volume significantly as its pressurization changes. The ideal would be to have matching fan CFMs, so they aren’t working against each other, but I’m unsure what advantage that yields over an inline fan alone. If you get lower temperatures, maybe it’s because your booster is drawing air from a different location than the tent’s air vent, and that air is cooler.
I’ve seen another post of yours that discusses this method. The Dwyer gauge works the same way. I don’t know how the sensitivity compares (or who Scott Steiner is ).
Sorry, but I can’t tell which post you’re responding to.
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Yeah it’s not intuitive…
…like you might think two of the same fans inline would have some benefit over one alone, or running them in parallel. Go back to thinking about tuning the intake area again, & how much one fan needs to reach max flow. If one fan needs 4x of its own intake port area in passive intake area to reach max flow, then putting two of the same in series (one intake fan one exhaust fan), knocks the intake area back down to a 4" circle, & on the other side the intake fan is also trying to push through the 4" circle, of the exhaust fan. So neither one is going to be able to reach their max flow. You might reach the max flow of one like that, but you bought a second fan & are paying to run it to do the same thing as you could have on one fan & tuning the intake area. Beyond that if you run them in parallel (two exhaust side by side) with enough intake area, you would be able to add both max flow numbers together. (Minus whatever you might be losing to carbon filter, ducting, etc.,)
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74f ambient in the room where the tent is sounds nice. That might be OK if you have enough flow through your tent, unless the exhaust or for whatever reason it gets too hot in the room. But if it gets too hot in the room, can you cover the arch with a cloth barrier, & does the room have a window that you could run a window a/c in? That’s kind of what I do for my lung room. One door is covered with a sheet that goes to a room with no a/c, & the other door is open but it leads to another room with another window a/c, since I don’t have central air. But the sheet over the door blocking off the unconditioned room makes a big difference in keeping my lung room cooler.
You might also be able to fiddle with the hvac registers to maybe get more cold air into your lung room, although sometimes you end up causing issues elsewhere in the house doing that.
You might want to probably test-flow that filter to see how much you’re actually losing to it. Also, even if it flows well enough, it might not be big enough in terms of odor control if you have a 4x2 full of dank. But if it flows well during a test, may as well try it. If it flows like crap during a test, you might want to get a filter that can flow more yet still be effective at odor control. If it flows like crap, I’d probably jump to testing flow through the tent with the fan & no filter, just to get an idea if you might need a more powerful fan to begin with or not, & then worry about a different filter if you think you need one.
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