Increase air volume by adding more fans

ductsfanhvac

I have a straight 25 foot rectangular duct that is 0.9 sqft in cross-sectional area (11" by 12"). There are openings on either end, and nowhere in the middle. I wish to move about 1000 CFM through the duct.

My plan is to use "duct booster fans" to do all the air movement. I can get a 10" diameter 650 CFM fan, and a 12" diameter 800 CFM fan (which is slightly larger than the duct). Here are some options I'm considering:

buy two 650 CFM fans, and install them in series: one where the air goes in, one where the air goes out. So the fans will be about 25 feet apart. Will their throughput add to about 1300 CFM? I suspect the answer is no, because nothing good ever adds linearly. But would I get close to 1000CFM?
buy two 650 CFM fans, and install them in parallel at one end of the duct. This would require either widening the duct or somehow splitting it into two ducts, as two 650 CFM fans side-by-side will not fit in the duct. My intuition is that this would be worse than putting the fans in series, but maybe not?
buy one 650 CFM fan and one 800 CFM fan, and install the 650 blowing into the duct and the 800 sucking out of the duct (or maybe the other way around?). Because the 800 CFM fan won't fit in the duct, I would have to build a larger frame for it, then connect it with some sort of funnel to the duct.
same as above, only with two 800 CFM fans.

Any advice would be greatly appreciated.

Best Answer

Its been a long time since I did any fluids, and even then, it was not much. But I'll take a shot just from simple logic.

Suppose we have a single fan at the front end of the duct. It is rated for 650 CFM, and without any duct there, you will get essentially that. But suppose you put this fan in a duct. Moving air will see friction against the walls of the duct, slowing down the air flow. The back pressure reduces the flow, so you really don't get a full 650 CFM out the end of the duct. Of course, bends in the duct hurt even more, but you said it was a straight duct.

Now, suppose you add a second fan at the tail end of the duct. This is sucking air out at a nominal 650 CFM rate too. But in the end, all it does is give you something closer to the full 650 rating for the two fans. You don't get 1300 CFM, you get roughly 650 CFM.

With two fans in parallel, both feeding into the duck (or both pulling air out) if they are not limited by air flow through a duct that is big enough to handle one fan, then you might get more air through in theory. But the problem is, now you have higher velocity air. The link that gregmac provided tells us that pressure losses are proportional to the square of velocity. If you could double the velocity (and in order to double the air flow, you must double the velocity of the air as it moves through the duct) then you would quadruple those frictional losses.

It gets worse. As is pointed out in gregmac's link, with two fans right next to each other, the air flow into those fans will interfere with each other. So you really wont achieve the full rating for those fans if they are set side by side.

How about the 800 CFM fans? If they are too big for the size duct you have, then you again will be limited by the back pressure. You may end up with only a net of 650 CFM anyway.

It seems to make most sense to have a second duct. Run two ducts in parallel, both adequately sized for the fans they are fed by. Yes, I know this may not be an option.

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I don't see a problem with that, other than the existing duct doesn't look wide enough to fit both. You'll have to get/build a reducer that goes from something slightly wider (that can support both circular ducts coming out of it) to the existing square duct size. Most metal shops can build this sort of thing for you - I had a couple 45-degree elbows built to re-route some ducts in my basement, it cost me around $50 I think.

 | |   | |
 | |   | |    <--  existing two round ducts (one moved)
 | |   | |
+---------+
|        /
|       /    <-- new reducer
+------+
|      |
|      |     <-- existing square ducts
|      |

The main thing is to keep the number of elbows to a minimum. In fact, in your case it may work better as it looks like you'll reduce the number of elbows by one.

Another option would be to extend the main duct, and split the two round ducts off at the end. Not sure how long those are now, but the box stores carry a few regular square duct sizes you can buy and assemble/install yourself.

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I have a couple of long runs too and I used a fan until I upgraded the HVAC unit. Before you get a booster you need to measure the air flow out of the two vents and compare them to other vents in the house.

How to measure air flow? Well pick up an Air Flow Meter. However I have a suggestion before you spend $$$ on this. Get a large trash bag, configure it to open about the size of your duct opening (take off cover), hold it tight on the duct opening, measure how long it takes to fill... compare several vents. Note the air should be running when starting this - since how long it takes air to come out factors very little into the cooling equation. If you are confident with the bag results and they are obvious then skip the pricy tool. ##Side Note## (A friend of mine uses one of those garden wind spinning things and measures the rotations over a given time to compare air flow. You have to have the angles/distance the same from the vent and he has to have his iphone slow wdown the frame rate to count. But damn that mcgyver-bastard, pretty sure its more accurate than my empty trash bag! But it does involve slow-motion so my solution is more practical)

I would also take temperature readings inside each duct while on after 5-10 minutes. How cold is the air you are getting?

If you are getting comparable air flow and temp you can add another duct or bigger. Really up to you.

If you aren't and I am guessing you aren't then you need to first get rid of the flex lines. They squash air flow, they are affected by outside temperatures more, and they leak. Put rigid metal ducts in. If the ducts are in the attic then you need to insulate the crap out of them.

Then if you still have issues you need to measure air flow and temp again. If air flow is poor you are looking at a booster. If it is relatively the same, add another duct.

Then you are talking about adding a return to the room and over-insulating the attic above the room.

Best Answer

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