Help deciding how to rig my box's exhaust?

I'm pretty much done building a 4' x 2' x 4' grow box in my attic, with usable internal grow area of more like 3' x 1.5' x 3.5'. It will have a semi-passive intake that is a direct tap into an air conditioning duct for my house. I say semi-passive because any time the a/c is running, it will be pushing cold air into the box. Otherwise, the air will need to be pulled from the house through the duct. The box is currently lined with 5 mil black plastic, and I will be adding another layer of a reflective material, possibly reflectix or mylar. The goal is to have it fully airtight, but it's possible I will not achieve that.

This will be an all-CFL grow, with a total of about 200 or 250w of CFLs (ranging from 23 to 42w apiece), so hopefully the heat produced by the lights won't be too big of an issue. However, since it's in an attic, I will want to make sure it's pulling up enough air from the house to fend off the heat of summer. There are trees over my house, so the attic doesn't get ungodly hot, but it's still pretty toasty up there in summer, easily 90F+. There is an attic exhaust fan on the opposite end of the attic that is hooked to a thermostat, which I plan to adjust so that it's pretty much always running.

The box is situated near one of the attic's venting windows, and I would like to run some sort of duct from the box to that window, with some kind of exhaust fan either inline or at one end or the other. Near-absolute silence is of paramount importance, so that there aren't sounds echoing all through my house's a/c ducts.

Is there a really good, extremely quiet fan I could buy for this setup? Hopefully not ridiculously expensive? I'm thinking something on the order of 50-100 CFM will be more than enough. I might add a DIY carbon filter to the exhaust during flowering if odor becomes an issue, so I do want slight overkill on the CFM rating.

Any product suggestions?

My friend has 4 plants in a 10sqft box and a 400cfm blower doing nothing but circulating the air in the box through a can charcoal filter. In the box it is still quite odorous. Outside it is odor free. He tried reducing the cfm output with a controller but anything less than full allowed the odor to invade the room outside the box.

As for venting out the ducting window and low noise, just vent into the attic as far from any opening as possible to keep noise to a minimum. The attic gable fan should vent enough to clear the attic of that heat.

The odor of flowering plants is much more than most think it will be.

CannaMatt said:

As for venting out the ducting window and low noise, just vent into the attic as far from any opening as possible to keep noise to a minimum. The attic gable fan should vent enough to clear the attic of that heat.

The odor of flowering plants is much more than most think it will be.

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Yeah, for my first grow (10 years ago), I vented directly into my bedroom. My whole apartment reeked by a few weeks into flowering.

However, I really don't want my attic full of the odor. I would much rather vent it outside directly, where it can quickly dissipate, rather than allowing it to stink up my house by seeping in from the attic. The attic is basically a 3rd floor window, and the ceilings are quite high on each floor below, so it might be closer to a 4th floor. The lots are wooded and well-spaced in the neighborhood, so it's unlikely anyone will smell it or know where the smell is coming from, once it's blowing through the trees. I'm not worried about the noises from the outside, I'm worried about people inside my house being able to hear the fans or smell the grow.

So, back to my original question: Is there an affordable, extremely quiet fan that can effectively vent a grow box that will be about 20-25 cubic feet? Or at least kinda relatively affordable?

And I forgot to add that I will add a charcoal filter to the exhaust if I start to notice the smell from my driveway, or even worse from the street in front of the house. So I want a fan that is quiet but capable of venting that space through the added resistance of a charcoal filter, if necessary.

I have the same size box for drying buds and i use this fan and filter can't smell a thing. http://www.thehydrosource.com/Fan_Filter_Bundles/4_Inline_Fan_Carbon_Filter_Combo

GreenMeds4All said:

I have the same size box for drying buds and i use this fan and filter can't smell a thing. http://www.thehydrosource.com/Fan_Filter_Bundles/4_Inline_Fan_Carbon_Filter_Combo

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How quiet is it? Would you probably be able to hear it in a room if it were basically attached to the a/c duct that ran to that room through the ceiling? My passive intake will be directly tapping into an a/c duct, so there will be nothing to muffle the sounds between the grow box and the a/c ducts in the house. That's why I want silence above all else. I want the quietest fan I can get that will move enough air to cool the grow box. I am thinking I probably won't even need a filter since I'm venting directly into the outside air, but I might add one later if I'm proven wrong.

Two words, Electrostatic Filters. They have worked like a charm for me. jmi as I said they have worked wonders for me. 3M makes them and for my flower room I built a box and bought a 22"X28" 3M premium electro filter that costs just under 30$. The thing lasts over 90 days at a time and is washable and reusable several times over. Another nice feature or result is NEVER a trace of static. That was a big difference. Anywho,,,,,,, it has worked nicely so far.
Cheers,
Chase

So no one has a suggestion for a super quiet fan? The electrostatic filter sounds interesting for a filter, but right now I'm trying to find a fan for my exhaust. Supremely stealthy quiet.