What size fan for an ac box?

[Guest 18340]

This is actually a 2 part question.
I'm building a "box"for my 5k btu window unit and need to know what type of fan is prefered to suck the hot air out the back of the box (can-fan, dayton type, etc), and whats the minimum cfm rating needed.
I'm gonna be ducting the back of the ac "box" into my garage attic.
Thanks in advance

 

[OgreSeeker]

Use a 6" Vortex (preferred) or an 8" Canfan -minimum-. Both of these fans put out around 450cfm.
This will ensure adequate airflow/cooling.
If your fan is to small (low cfm) then your ac will cycle continuously and may cause damage...at best it will not be efficient.

 

[hoosierdaddy]

In theory, if you do not hamper the size of the intake or exhaust orifices, you will need no fan at all for the unit to function as designed.
The intake is determined by the total area of slots in the side and top vents. The exhaust is determined by the size of the housing that encompasses the fan internally.

 

[OgreSeeker]

Maybe in theory but in real life you need a fan. That hot air has to go somewhere. Even if you used oversize duct, say 12", and didn't hamper the intake or exhaust orifices whatsoever you'll still need to move the hot air out the duct. The ac blower isn't gonna move it adequately.
In theory I should be able to lift my own body weight but I know that shit ain't happening.

 

[hoosierdaddy]

I'm sorry, OgreSeeker, but you are wrong on this one.

The physics behind this are clear and absolute.
And if the air flow going in and coming out are not restricted from what they were from design, then the unit WILL function as designed.

You state the ac blower isn't going to move the air adequately...well, just how do you explain that? The fan moves it adequately hanging in a window. And if you do not choke off the exhaust pathway from what it is now, then you have the same deal. It knows not if it is in a window or in a box, as long as the airflow is not restricted.

Now, I agree that in most every instance, people will have a smaller duct out than design. Like I mentioned before, the housing surrounding the exhaust fan internally is the determining air flow factor. If that is a 10" opening the fan blades are in, then it would take a 10" or better opening from there on out. If 6" duct is used instead of 10", then that means the air flow has been somewhat restricted, and the use of an additional means of getting air past the fins is warranted.
If the grower were to use the same size duct out as the housing enclosure, and he was not restricting the intake at all, then he would need no fan at all. This is a fact.

*edit:
Actually, the exhaust orifice would not have to really be the same size as the exhaust housing diameter...since the fins are restricting the airflow. So, whatever the air flow past the fins is would be what you need. Now, that is very hard to determine, since measuring and math are going to be hard to do with such a configuration. But it is a safe bet that if you are as big as the internal enclosure dia. then you are golden.

On my AC unit, I am not restricting the intake at all, so I am good there.
However, my internal fan housing is approx 10" in dia, which means the air the fan is pulling on goes through a 78.5 sq. in opening.
My exhaust duct only has an air flow path of 51 sq. inches. There is a reason that I cannot increase the size of my exhaust ducting, so instead of ducting it out of the room to the attic, I duct it to my negative pressure exhaust chamber of my cab, where I have an additional fan pulling on the hot air from my exhaust ducting.
Now, like I said before, I probably do not need a duct that is a full 78.5 sq in to be good, and I am probably close to what the actual air flow is with the 51 sq in.

 

[Guest 18340]

I see your point hoosierdaddy but my grow space will be built in the corner of my garage and i cant have the heat off the back of the ac blowing into my garage, it'll make my garage even hotter than it is now. So, thats why i gotta build a box around the very back of the ac unit and attach a fan that is powerful enough to suck the hot air out and into some ducting that will go up in the attic.

 

[OgreSeeker]

If duct, even large duct, is connected to the intake without a fan then how will it draw cooler air? The reason you'll get adequate air flow if it's hung in a window is because there is nothing to restrict the cool air from entering and nothing restricting hot air from being exhausted. The minute you hook up 10 to 20 feet or duct to the inlet and exhaust of your ac box your going to need a larger fan then what the ac can provide because of pressure, friction etc.
In your example, the hot air may eventually be exhausted out of the duct but not before building up/backing up and overheating the unit. It's just not enough air flow to keep things cool.
I'm telling you right now...if you have duct connected to your inlet and exhaust without a fan to move the air through & out the exhaust duct then your ac is not going to work properly. Ask ANYONE who has built an ac box without a fan. Most people have problems using a 4" fan so how are you going to say that an ac box with NO FAN will work?
I think I'm right on this one.

 

[Guest 18340]

Just to be clear, the box i'm building will only be covering the very back of the unit, not the intake vents all around the sides. Actually, it'll look exactly like your "box", hoosierdaddy. But, i need to vent that hot air up into my attic so ill be running maybe a foot of ducting up into the attic thru a hole and then having a fan sucking the air off the back of the unit.
So my question was/is what type of fan should i use and what cfm rating is needed. Its a small 5000 btu unit.
I know that bigger (fan) is better but those "vortex" type fans cost $$$ so im trying to narrow down my choices. I'm sure a 1000cfm fan would work wonders but i aint got the cashh for that.

 

[hoosierdaddy]

I don't have to ask anyone, I already have a firm grasp on the dynamics of this.

Look, the fins on the intake are ducting of sorts. You have to understand that much.
If your lead in to the vents is not smaller than the total area of the vents, then you are golden. Same with the exhaust, if you have large enough exhaust, it will not change the air flow one iota from what it would be in a window.

See, when you start talking about "most people" that is only general terms that means nothing really. We need to see what the set-up is for any particular application that is having problems, and then we could see what the real culprit is. Most liekly they have restricted the airflow at some point. Because if they haven't they will not have problems. Again, this is fact.

 

[OgreSeeker]

I'm sorry, OgreSeeker, but you are wrong on this one.

The physics behind this are clear and absolute.
And if the air flow going in and coming out are not restricted from what they were from design, then the unit WILL function as designed.

That's exactly it. The second you build a box around the inlet and exhaust of an ac then your restricting it's air flow. What your saying would work if you built the box around the ac and cut vent holes directly in the ac box but when you add duct, the flow from the ac is extremely restricted and the ac fan alone WILL NOT adequately cool the unit.

 

[hoosierdaddy]

What you are going to have will probably be fine, evlme2. The size of the ducting to the attic is the determining factor. The smaller it is, the more additional CFM you will need to get the unit back to the original air flow or better.

IF the duct you use from the exhaust was the same size as the back of your unit...say for instance your box was the duct and ran straight to the attic, you would need no fan at all, and the fan provided by the unit would do the job.

 

[hoosierdaddy]

OgreSeeker, I love debates and discussions...but you need to go google air flow and study up before we get much deeper. I don't care how many twists and turns there are in the ducting, if it has adequate open surface area, then it is good to go. There is math available to figure the head of ducting, and you may come across those formulas during your investigation.

Man, the whole world is the duct for a window unit both in and out. The world and it's atmosphere are the ducts and are quite large enough to handle the air flow. And you can make the duct smaller...as long as you don't restrict what air flow the unit has from design.

We can back and forth this all night, but both of us need to understand air flow dynamics.

 

[OgreSeeker]

evlme2,
Go ahead and make your ac box without a fan and report back to us. Then we'll have our concrete answer.
No offense HD but when his compressor craps out or his unit stops working due to over cycling are you going to pay for a new one? No, all you'll be able to do is apologize for offering opinion as fact.


**edit**

your right...I'll stop the bickering here

 

[hoosierdaddy]

If the man follows what I am saying and does not restrict his air flow, he will not have problems. I respectfully disagree on who is providing opinion, and who is basing their side of the debate on physics and air flow dynamics.

OK...just for shits and giggle...imagine the duct that I show on the back of my AC unit is running for lets say 10 feet straight out the back, and then through the wall to the outside. I have ductwork leading from the exhaust to the outside...now, do I need an additional fan to get the air out ten feet downstream? No.
Yes, there is a slight bit of restriction from just the head of the ducting, but it is minuscule, and will not come close to effecting the functionality of the unit.
Now, how small can I go before the ducting DOES start to hamper the air flow? Well, if you have a 10" housing internally, then as I stated before, the ducting needs to be about 78.5 sq in in area to be sufficient.
A 6" duct has an opening of 28.26 inches. If 6" ducting is used and ran straight off the back of the unit ten feet to the outside, there is a restriction and a fan is needed to overcome it.

And although more efficient on the exhaust side, it can also be pushing on the intake side of things and will do the same thing..which is make up for air flow that was restricted.

 

[OgreSeeker]

I think that I understand what happened here...

I may have been confused about the "type" of ac box. Maybe this will clear things up.

I agree that if you make an ac box where the intake is sucking from the actual room and exhausting a very short distance out the back then this may work. I also agree that if your only ducting a VERY SHORT distance and don't restrict air flow then this may work. ...BUT

If your building a true ac box, one that pulls it's intake air 10' or 20' from another room in the house and exhausts it 10' or 20' away to another area it is going to need a fan. I assumed evlme was building a completely enclosed ac box which is what I consider to be a true ac box. I should have asked.

If this case then I think we are both right. I hope I'm making sense here.

One thing I don't understand is why people build ac boxes and use the air conditioned air (grow room air) to be used as the cool air intake...wouldn't this be extremely inefficient? Seems to me that the ac would have to work extra hard to cool the room AND the ac. Also, wouldn't the cooler room air be sucked into the intake and exhausted right out the back...again very inefficient.

 

[Guest 18340]

I wont be restricting the "intake" vents that are punched into the sides and top of the ac unit, only a box around the very back of the unit, where the radiator type fins are.
The only part of the ac unit that'll be in my grow room will be the front, the intake vents punched around it will be on the outside of the growroom, as if the unit were hanging out a window. BUT, i cant have the ac blowing hot air into my garage so thats why i gotta build a box around just the very back and duct that into my attic. Since i'll be mounting the unit high up in the grow room, ill only need to run like a foot of ducting. So, since ill be restrcting the flow somewhat by the ducting i need a fan to suck the air off the back of the unit.

 

[hoosierdaddy]

If your building a true ac box, one that pulls it's intake air 10' or 20' from another room in the house and exhausts it 10' or 20' away to another area it is going to need a fan.

Yes, in most cases it would need a fan, because it has the air flow restricted by the size of the ductwork used. If you used rectangle ducting, like for central air, and it was the right size, the need for a fan is not there. Only if the duct run was long enough to build up enough head to start restricting the flow. (again, there are formulas avail to figure head pressures)

Isolating an AC unit is a three part deal.
1) the intake, which must get adequate fresh air from somewhere.
2) exhaust, which must have adequate air flow to evacuate the hot air properly.
3) the cool side of things that recirculates it's own air.
These three separations must be made, and with no air flow restrictions, to get by without a fan.

 

[hoosierdaddy]

Yes, evlme2 you will need a fan. I use the $40 Stanley blower from walmart with a 6" collar fixed to it.

 

[OgreSeeker]

I wont be restricting the "intake" vents that are punched into the sides and top of the ac unit, only a box around the very back of the unit, where the radiator type fins are.
The only part of the ac unit that'll be in my grow room will be the front, the intake vents punched around it will be on the outside of the growroom, as if the unit were hanging out a window. BUT, i cant have the ac blowing hot air into my garage so thats why i gotta build a box around just the very back and duct that into my attic. Since i'll be mounting the unit high up in the grow room, ill only need to run like a foot of ducting. So, since ill be restrcting the flow somewhat by the ducting i need a fan to suck the air off the back of the unit.

That makes sense.

Time for one of these...

 

[hoosierdaddy]

I don't want to beat the horse to a pulp...but...

You know I say yes you need a fan, but in fact you do not IF you have the right size duct going into the attic.
Take my sketch for an example...

What I have is a piece of duct that has an inside area equal to the total area of the back of the unit. My duct is running straight out the back, and would need no fan at all to allow it to work properly. IF you were to turn this rectangle ducting up 90 deg. and into the attic, and especially since you are only going a foot or so, you would need no fan at all for it to function as designed.
So to bypass buying a new fan, all you would have to worry about is a ~1'x2' hole cut in the ceiling board, instead of a 6" or 8" hole.

This is the whole point I have been trying to make here. And you have a perfect example of a situation that would work just like I am saying it will.

*Imagine taking the top off of my ~1foot ductwork and placing it on the back so my flow is now to the top. What I would have is a slight bit of restriction from what it is now by adding the 90 deg turn. And then you would have the restriction caused by the friction of the duct side walls for the additional 1-2 ft it takes to get to the ceiling. The turn and the side wall friction is what creates the "head" pressure of the duct line.

I can even choke the size of the rectangle duct down a bit if I needed to, because as I stated earlier, the real restriction on the exhaust side is the housing enclosing the fan blades inside the unit, which on my unit is a ~10" (78.5 sq in) opening. What I have now is the back of my unit being 17" x 11",
which gives me a total duct area of 187 inches!
I need a min of 78.5 inches to know I am not restricting the air flow, and I have 187 inches. I am golden for a few turns and some footage at the size of that duct.

I don't know how to explain this any better.