Running exhaust fan and carbon filter in reverse? Help?

[dark horse]

This might should be in micro growing as my cab is very small, only 3 feet tall and 1.5x1.5 feet wide. So the point is, I have extremely limited space and can't put a fan or filter inside of my box without taking up toooo much space.


I have made a crude drawing trying to explain what I'm trying to ask. I have a closet that's level with the attic. Can I drill a hole in the wall and simply attach the duct to the top of my box with maybe only 1 or 2 inches inside the box? and put the fan and filter at the EXHAUST end on the other side of the wall, as I've laid out in my drawing?

Not only would this not take up any space in my box, this would also be the only acceptable place to put the fan due to the noise. So please help, is this a sound and tested way of doing things? Also, don't forget to add anything I may have forgot to ask but need to know. I'm a bit on the noob side.

 

[trichster]

Yes you can push through carbon filters. Take the pre-filter off the outside of carbon filter and stick it inside of it.

 

[Friend]

It'll work, it won't be ideal, but it will function.

More ducting usually means more turns and bends so your airflow is going to be slightly reduced. Plus you're going to have to worry about dust accumulating on the inside of your filter instead of the outside.

 

[dark horse]

Ok so I read on another forum that you can put panty hose over the intake if you reverse it to prevent dust from entering...sound good?

I also see some carbon filters advertised as "non-reversible"....I assume that means you can't do what I'm talking about. The filter I have my eye on doesn't say non-reversible, but I'm scared to assume it is without getting more info.

Pre-filter questions, what's the purpose of a pre-filter? And How hard is it to take the pre-filter off the outside and stick it on the inside? Is that mandatory?

I can NOT have ANY smell...very important,,,when you say "not ideal" does this mean you think the effectiveness of smell elimination will be reduced?

I could probably use a minimum of 10 feet of duct in a crunch, but would prefer to run it a little farther up into the attic, maybe 20 feet if possible.

 

[NotaProfessor]

You're doing it right. Few people know that this is the way to put a load(filter) on a centrifugal fan, i.e. load on the output side.

A centrifugal fan can easily apply pressure on the exhaust side but is inefficient with a load/restriction on intake side. Apply a little restriction on the intake side and the fan will just spin faster not having anything to push out the other side (exhaust). This is also the case for centrifugal water pumps, where the recommendation is always to limit flow via the output side.

Only in a few rare instances is the load placed ahead of a centrifugal fan, namely in the HEPA fans used for asbestos abatement. This is done only to prevent the fan from getting contaminated by asbestos.

 

[trichster]

Prefilter is to keep dust out of carbon. Its easy to remove and put inside. Yes mandatory if running as exhaust. What CFM is fan? Running filter at end is just as effective as long as no leaks and set up right

 

[joe guy]

It will cost a lil bit more but if your Gona run it so the air is blowing thru the filter then you will want add a dust shroom or any other pre filter on the end that will be in your cab
To prolong the life of the carbon, it's not Gona work if the carbons pores are clogged with dirt.
Another thing is try and keep it straight as possible to reduce the drag the fan will feel.
Also keep your eyes peeled for any holes in the ducting that may form.
I have a buddy that uses abs plastic tubing instead of the flex duct to make sure it never happens.. ( the filter won't scrub air it dosnt touch)..
Hope I made sence just wakin up so I maybe a lil scatter brained and babbling

 

[dark horse]

I didn't think about leaks springing in the duct, that sounds like a frightening situation. I assume you could just tape it up though w/ foil tape if it happened, right?

Here's the filter I was eyeballing: http://www.hydrofarm.com/product.php?itemid=6831

and the fan would just be whatever 4" one looks good, and falls within the filters "optimal CFM range", I guess. (hopefully optimal CFM is the same whether in reverse or not).

How's it look?

 

[NotaProfessor]

I didn't think about leaks springing in the duct, that sounds like a frightening situation. I assume you could just tape it up though w/ foil tape if it happened, right?

Here's the filter I was eyeballing: http://www.hydrofarm.com/product.php?itemid=6831

and the fan would just be whatever 4" one looks good, and falls within the filters "optimal CFM range", I guess. (hopefully optimal CFM is the same whether in reverse or not).

How's it look?

In your configuration, a cut in the duct would not leak any air out but would suck ambient air into the duct since it is under negative pressure.

That filter is fine and will work either way (suck or blow).

 

[STUNKY]

UR GOOD MAN . Just hook it like the picture.. been running them like that for years. with the same fan, with homemade filters...try to position the bulb to not loose light up into the duct if possible.. use all your lumens..

 

[joe guy]

Yeha tape should work I would not use the paper foil tape I'd use actual HVAC approved tape. And if you go with flex ducting make sure it's pretty rigid and it won't suck in and collapse in on itself with 200 cfms running thru it..ie. don't use the foil dryer vent from Walmart use actual flex ducting mine is some kinda plastic material that's pretty streachy before it rips.. And has a pretty thick gauge metal spinning around the inside

 

[dark horse]

Thanks guys my confidence level has been tremendously boosted so far.

Should any 4" ducting that I buy from a hydro shop be sufficient? Anything in particular to look for there?

Also, 1 more thing. I was thinking it might be safer to do a longer run once I get into my attic so as to place the fan/filter at the rear of my house instead of immediately above the front porch which is where the closet is above.

But that would involve running the line probably 20 feet or more. Would that be cool? You can see now what I cannot have ANY smell AT ALL...because it would be exhausting literally feet from my front porch. I hope these carbon filters are really truly 99.5% effective, and that 0.5% won't creep.... like I said, my grow will be very tiny (micro probably 70 or 100 watts) but still...

 

[Friend]

Buy the ducting at Lowe's or somewhere, the 4" stuff they sell at the hydro shops is exactly the same as dryer ducting but they overcharge you for it.

 

[STUNKY]

I have always used dryer duct ,but if your running a big fan u might need extra support... As far as positioning the filter.. If its in your attic SMELL and EXHAUST will exit where it wants alotta factors determine this like roof vents gable vents soffit heat cool air etc.. Your filter is gonna stop the enough smell so just make ur run as short as possible while taking into consideration the noise of the fan.. Which if needed can be muffled...Sometimes you just have to do it and see how it works.. You have a good plan, so roll with it..

 

[joe guy]

Yeha anything from the shop will work fine, and if u get a fan speed controller and dim the fan down a lil it will slow the air down and keep it in contact with the carbon longer In turn scrubbing the stink slightly more.. I don't know if its sicentificly true just works in my head lol
I'm not 100 percent positive how much juice a 4 in inline has for 20 plus foot run you may need a booster some where along that run or use a bigger fan .. Just get a reducer like 6in to 4 in and use it upside down so the 6 inch fan will hook up to your 4 in filter..and I really like the noise reducing clamps the ones with the neioprine insert inside the clamp for a nice air tight seal..

 

[dark horse]

1 more question. Say I run a 4" duct through a 4.5" hole (in the cab or through the wall), that leaves some space. What's the preferred method for sealing it all off? Am I looking for a "flange" to put on both sides of the passage? like this.. http://www.hydrofarm.com/product.php?itemid=8305

and then just tape it up really good with HVAC foil tape?
http://www.hydrofarm.com/product.php?itemid=8305

 

[STUNKY]

Whatever floats your boat!!!!

 

[dark horse]

I'm just asking if that's how you're supposed to do it?

 

[festerous]

Those flanges work well except for the fact that they are slightly too small to use with the clamps. You will need to use tape to get a good seal.

 

[dark horse]

i have another question if anyone knows... does the ideal CFM zone for a filter change if you're running it in reverse? actually what i really mean is does the CFM change when a fan blows instead of sucks?

i see certain filters advertised as "non reversible" and even fewer as "reversible" whereas most have no mention of either one. any comments on this or am i over thinking it?