Ac infinity duct fan

My friend gave me his old 4 in ac infinity with the temp screen and I wanna know if it will do good in my 4x4 because I have a 8in duct fan and it works to good and creates a lot of negative pressure

AC Infinity makes great fans. They are efficient and very reliable.

The 4" will certainly improve airflow, but it’s underpowered for a 4x4 if it would be your only fan. Not big enough to draw a vacuum in a 4x4 on its own.

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Can you open a port near the bottom to reduce the draw.
I’ll ask the obvious…is it on lowest speed?

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I would try to leave the 8 inch fan in the exhaust, and put the 4 inch in the intake. If this seems to work, then I would increase the the duct on the intake side of the fan to 8 inch, and clamp a screen over the opening.
Also AC Infinity makes some very nice intake filter boxes, at a good price.

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In a tent that big it could be used as a forced air intake to reduce neg. pressure and bring more fresh air into the tent in general.

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Yes I did open a port at the bottom but it still gets sucked in and makes me have less space and it is on the lowest speed

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Could also hook it to a top and bottom port on the outside to circulate air from the top to the bottom of the tent and saves interior room.

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Keep going. Provided you have an appropriate/efficient exhaust setup, it sounds like you simply don’t have enough intake area. But that’s mainly what you need to do if you want to approach static pressure & give your exhaust fan its max flow.

Putting an ‘intake fan’ over your existing intake will just make your efficiency worse than it already is. Your intake area is already too small. Putting a fan on it makes it smaller. The hub & to some degree the blades take up intake area. Your 8" fan will now be forced to drag air through an intake that was already too small & is now smaller than before, since the ‘intake fan’ is taking up area. The intake fan also won’t work very well as a fan since the exhaust fan will be pulling most of the air off of the blades of the intake fan. The intake fan won’t have much bite on the air. It will mostly just be in the way & causing turbulence. Turn up the intake fan? Turn down the exhaust fan? One way or another, fans in series will always be fighting each-other & hurting efficiency.

Max out the flow of the exhaust fan by giving your exhaust fan enough intake area to reach its max flow & static pressure when the fan is on max speed. Then take a little intake area away to get a little negative pressure back to keep in the smell & put some velocity on the intake air. My exhaust fan needs around 5x exhaust port area in unobstructed intake area before it hits static pressure. My intake filter area needs around double that to maintain that level of flow, since the intake filter screen is an obstruction.

@PhotoFinisH, I disagree a 8 inch duct booster fan will match CFM fairly closely to a 4 inch in-line fan. Anyway what will it hurt to give the 4 inch fan a try. It maybe worth trying the 4 inch on the exhaust, and the 8 inch on the intake. May need the aid of some balancing dampers. I would definitely try to use what is on hand first.

@Tyler2, Here is a couple examples of some very simple air balancing techniques that I have used.
First is a cardboard box that can block off some of the exhaust from a 4 inch fan. This on a 2x3 tent and the fan need CFM needed to reduced, (was really sucking the tent). The closer that I slide the box too the fan the more I reduce the air flow. I have since added an intake fan with both on speed controls, and this works much better.


This the little booster exhaust fan out of my clone cabinet. The hydrometer setting in the outlet reduces air flow, this helps out the the humidifier efficiently. This also gives me a better idea of the tent’s actual temperature and humidity.

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Answering the question of fan usage and single tent, with a possible suggestion.
Use the 8" AC-Infinity for exhausting and the 4" for intaking. Using the Cloudline 67 controller (or 69) and the devices will control both fans, independently (69) , or simultaneously (67). Is the fan T6,A6 or potientiometor (dial) controlled? The Dual Outlet Controllers (75, or 79) could control both fans, together or seperately. Good Growing to you. Exhasut fan is oversized for intake fan, resulting in negative air pressure.Should extesive exhausting tubing become utilized, the 8-incher will handle distance or charcole filter, quite well. I have one, too.

I also believe ac infinity makes great fans. Unfortunately the 4" is small for a 4x4, really it’s probably too small for any tent if you plan on using carbon filter. It just doesn’t deal with static pressure very well. The 6" duct fans are much better application for exhaust with filter.

Again it depends on what type of fans @Tyler2 is using? If he is using a 8 inch duct booster fan, with a 4 inch inline fan it may match up well.
Now that I think about it a little more, since the booster fan doesn’t handle static pressure as well. It maybe best to use the 4 inch for exhaust and the 8 inch booster as the intake, and this may allow the intake to self balance a bit to match the exhaust.
If he is using both a 8 inch and a 4 inch inline fan then yes this will be a mismatch of sizes.
The previous post is correct the existing fans are not big enough to pull through any filter finer than the intake screens in the tent.
@DEEPDIVERDAVE, is very correct two 6 inch fans controlled by one of those AC Infinity controllers would be the best way to go. But on the other hand I understand trying to use what you have on hand already. I do not judge either approach, since I see benefits in both.

I can balance my tent or create tent movement (Negative or positive presureing the tents).
I have the 4 inch being used as a temperature trim balance for the tents. Teansfering heat or cool depending upon tent cycles or lung room temps. The AC-Infinity 69 and 67 controllers run the intakes or exhausts thru the filters. Yes, 4 inch is small for one 4x4 tent. 6"s are better and the 8" is awesome for ehausting lung room into attic, causing tent and lung room negative (fresh cool air exchange). Todays needs and future growth, long term investment, as it took 40 years to get here., again.
I got confused on the fan designations, Not all “inline” are the same.
Size/CFM important, not just integer number.
6 inch fans, S6 not the same a T6, and A6 not the same, either.
DC pulse modulated, vs TURBINE NOISE generator (Alternating current motor).
Inventory spares of useless VSun equipment grows.

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Is that what he means by duct fan? If he’s really using a booster fan, their flow rate drops off very quickly as soon as the negative pressure starts to build. A strong 4" true centrifugal fan might even out-flow the 8" duct-helper fan, & then the helper fan would be the bigger obstruction. Either way, one way or the other, fans will be fighting each-other & hurting airflow efficiency if you put them in series.

More air could flow by putting them both in parallel & giving them enough intake area, vs. running them in series.

But if he’s just dumping an 8" helper fan right out the side of the tent & it’s working for him other than the excess negative pressure, then maybe it can stay & he just needs more intake area, because his complaint is too much negative pressure in the tent from the 8".

Also response to negative pressure (obstructions) is important.
Axial fans like duct helper fans don’t like it at all. Personally I wouldn’t want an axial fan in the air exchange system.
Hybrid axial-centrifugal fans don’t like it much either, but they can take a little more than axial fans before crashing.
‘True centrifugal’ fans handle it the best.
Actual setup would help to choose there. Lots of restrictive ducts & filters? Then a true centrifugal might do better. Is everything free-flowing & not obstructing? Then a hybrid might be better since they usually can outflow a true centrifugal of the same size, as long as the negative pressure stays low enough. Once it builds high enough the hybrids fall off a cliff & the trues continue to taper off linearly. Vortex publishes negative-pressure based flow charts of their fans on their website, & you can compare how their true centrifugal and hybrid centrifugal fans respond to negative pressure. Not sure about any other manufacturers but I haven’t seen any pressure-based flow charts from Vivosun or AC Infinity yet, but maybe they give them out if asked.

Curious to know what OPs 8" fan & duct filter setup is specifically.


Requires proficient abacus operations.

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Good info @DEEPDIVERDAVE, here is a copy of some a tech tip from HTG, hopefully it clearer and less boring than I would be.

Indoor Garden Air Circulation Setup

There are several tools which make it a breeze to monitor and control your humidity and temperature, but the gadgets are only as helpful as the actual sense behind the setup. A fan doesn’t do any good if it’s not pointed at something, right? That’s a fairly silly example, but it illustrates a point: your setup has to be, well, set up. You’ll get decent results if you do a decent job, but to get excellent results you have to think it out. For example, let’s say you need to ventilate the excess humid air from your tent. Opening your vents is a solution, but it’s not the best one. What you really need in this situation is an exhaust system, which is much less expensive or intimidating than it sounds. A simple duct fan is all you really need for smaller spaces like a tent – just put it right in your tent’s vent port (preferably one that’s higher up) and you’re set, it’ll suck unwanted air out of your tent and as long as you leave an open vent on the other side, fresh air will replace it. If you do it this way, just be aware that the open vent should be covered with a filter of some sort to prevent pests and mold from coming in with the fresh air.

If you’re feeling inspired, you can employ a slightly more advanced setup using a second duct fan. In this case, you’ll place the first fan in a higher-up position, either on top or high on the wall. Since hot air rises, this will efficiently pull the hottest air out of the tent. Then you’ll place your second fan in a vent near the floor of the tent, pulling air IN. Most tents can get by with just a 4″ duct fan for this purpose. With this system, fresh air will be sucked into the tent at the bottom and exit at the top, and it’ll collect moisture from the plant and spread CO2 the whole way up. We recommend having a slightly bigger and/or stronger fan blowing OUT than sucking IN, though, just to be safe. This helps to create a negative pressure inside of the space, which helps to maintain proper airflow.

Will from Commerce City says: “In a 9’ x 13’ room, I would have about 10 fans in total. This includes an intake, exhaust, floor, and circulating/wall-mount fans. I made a diagram to show you what I mean – don’t judge my drawing skills!” (We totally judged his drawing skills and remade the picture, sorry Will!)

How to Calculate CFM Requirements

There’s a pretty simple formula to figure out how strong your fan needs to be to optimally circulate your air. We’ll take the width x length x height. (Example: a tent or room that is 12f x 10f x 8f = 960 cubic feet.) Now you need to decide how often you want to exchange the air the room. For this example, we’ll use 4-minute intervals. Dividing the cubic feet (960) by the number of minutes (4) will provide the Cubic Feet Per Minute, or CFM – in this case, 240. So we’re looking for an outtake fan (or a combination of outtake fans) that’s around 240, or even very slightly higher. But wait, hang on, there’s one last factor: our lighting. LED grow lights don’t really contribute to the heat of the tent/room in any substantial way, but if we’re using lights that heat up (like CMH or HPS lights), we’ll need to account for the extra heat they cause. In that situation, we just add 10% to our total for every 1000w of lighting. So if we have four 1000w lights, we add 40%, which brings our total up to 336. Don’t worry, all our duct fans are specifically labeled with their CFM information and if you’re ever not sure about anything, just ask us! Also, it’s important to know that the strength of the fans increases exponentially with their size; the 4-inch GrowBright HV fan is rated at 170 CFM, but the only slightly larger 6-inch unit is more than twice as powerful at 400 CFM (and it’s only about $25 more). If you can’t find a model that offers a CFM directly in line with your calculations, you can always grab a larger fan and use it with a fan speed controller to dial in the optimal airflow. So if you’re doing the math and worrying about how many fans it’ll take to handle the big numbers you’re coming up with, worry no more – chances are, it’s MUCH cheaper and simpler than you think.

Even in a sealed space, we recommend a full air exchange at least twice a day or up to every four hours if a CO2 generating burner is being used, because combustion leads to ethylene buildup. Ethylene buildup can become an issue even before you can visually see the problem because it stunts plant growth, but advanced symptoms will manifest in wilted-looking leaves that have retained their turgidity (water pressure inside the plant). You should swap out air more often in smaller spaces, of course, but people sometimes assume that a large enough room negates the need for ventilation. If you don’t properly supply your plants with the air they need, they won’t grow to their potential.

FACTORING FILTERS INTO YOUR CFM CALCULATIONS

For those of you with more advanced needs, another consideration to account for will be air filtration. We touched on it briefly earlier, but if you have a big/complicated setup and you want to be sure that your plants are protected, HEPA intake filters are the way to go. GrowBright HEPA filters catch 99.97% of airborne particles 3 micrometers or smaller in size, and all you have to do is slip one over your intake fan/port. For further protection (and for odor control purposes), an activated carbon filter for your outtake fan will do wonders. Same principle, just connect it directly to the duct fan that’s blowing air out of your tent and you’re set! Using these filter types is a best practice for indoor growing, but they will lower the airflow rate of their partner fans in the ventilation system. Be sure to factor this into your calculations if applicable to ensure proper air movement is maintained.


We hope we’ve given you something to think about – your air circulation and quality should be just as carefully planned as your lighting and nutrition to ensure a bountiful harvest! Be sure to see below for this weeks coupon code and sale information! As always, drop by your local HTG Supply store if you have any questions, and don’t forget to follow us on social media to be notified whenever we post more articles just like this one (and whenever we have our awesome sales). From all of us here, good luck, stay safe, and Happy Growing!

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I haven’t found info containing the actual flow-rates of their fans as negative pressure builds.