Reddit mentions of VIVOSUN 8 Inch 740 CFM Inline Duct Ventilation Fan with Variable Speed Controller for Grow Tent

Also a fan of enginerding cheaper solutions:
I live in the south so I punched through the wall and have an inline moving air out of a vent coupled with a space heater does the trick even when cold... like the whopping 50 degree winters here...

But for heat retention I would emulate the tree growers solutions (duct fan hooked to a charcoal filter ). Keep in mind even the "quiet" duct fans still make a loud hum if exposed in your garage. Model I bought.

And potentially expect a knock on the door...but hey, relocate your grow house somewhere else first and it wont be a problem.jk

Seriously though, I'd weigh the costs of making something + the filter media vs buying a commercial air cleaner as those suckers really work. I know if I had to retain air and had 2k to drop I'd buy one.

Fellow Maine-ah here, just setup an 8 inch fan and it's almost overkill. https://www.amazon.com/gp/aw/d/B01CTM0LLO/

That's an ongoing investigation on my end.

So far what I've discovered is that adsorbent technologies, carbon and otherwise, do work but with a number of caveats.

Here's some devices I've tested - with a high grade industrial PID meter directly on the output of the exhaust:

1. Austin Air HP+ - Immediate lowering of VOCs anywhere from 20 - 35%. Also includes 15lbs of carbon and HEPA filter. I do have concerns regards filter life. Kinda of expensive at >~ $700.00
   
2. Terra Bloom carbon cylinder and in line fan. This device marketed to the marijuana market but delivers excellent performance and a very low price <~$250. I've seen immediate reductions of 50 - 70 %. It's loud and not pretty but the bang for buck is outstanding.
   See:
   https://www.amazon.com/gp/product/B06X3VFK2T/ref=ppx_yo_dt_b_asin_title_o03_s00?ie=UTF8&psc=1
   https://www.amazon.com/gp/product/B01CTM0LLO/ref=ppx_yo_dt_b_asin_title_o09_s00?ie=UTF8&psc=1
   
3. Vortex, a Canadian company is currently my favorite. Quality construction with patented innovations to increase the life of their carbon filter. They make 4 filters with the smallest having 30lbs of carbon - WoW- and that's the smallest. They allow the filter to be turned over. they claim this results in extend filter life. I've seen reductions up to 85 - 100%.
   See:
   https://www.hvacquick.com/products/residential/Indoor-Air-Quality/Carbon-Air-Filters/Vortex-PROFilter-Reversible-Activated-Carbon-Filters
   https://www.hvacquick.com/products/residential/Fans/Hydroponics-Fans/Vortex-Powerfans-VTX-Series-Inline-Fan
   
4. I also tested a non carbon adsorptive device. These devices are attractive as they over come some fundamental deficiencies of carbon. Unfortunately the device I tested was not near as efficient as any of the devices I mentioned above, but it was only using a 1 lb filter. If heavier filters become available at a reasonably price I will reconsider.
   
   Limitations of carbon:
   
5. When carbon is saturated trapped VOCs can be released - Not Good
   
6. When carbon is warm - upper 70s - trapped VOCs can be released - Not Good
   
7. In humid conditions carbon trapped VOCs trapped can be released - No Good.
   
8. Carbon is biased. Some VOCs are trapped more efficiently than others.
   For instance carbon is not very effective (not totally ineffective) for formaldehyde.
   
   Often times carbon is mixed with additives to breakdown VOCs that carbon has a hard time catching. That's true with the Austin Air, but these additives can break down faster than carbon.
   
   
   The quickest most effective method to bring VOC levels down is to ventilate. Getting temps up and opening Windows and blowing air out of rooms/homes is the best method of all. Remember VOCs are liquids at ambient temperatures. What gets into the air are molecules that off gas from the liquids on surfaces. Just like taking a how shower creates more humidity.
   
   All said, I would really, really like PCO/PECO technology to work. The concept is very attractive. Instead of trapping (carbon), or exhausting (ventilation) lets just destroy the VOCs by turning them into water and CO2. I was really disappointed when measurement with the Molecule were nil. I was expecting it to working judging by the 3rd party test that MoleKule funded. Problem is the environment the MoleKule was used in does not resemble what we find in our homes. That difference seems to be significant.
   
   I haven't given up yet concerning the MoleKule or other PCO or plasma systems yet.
   
   Finally keep in mind that the MoleKule might be very effective in killing biologicals. Not something I tested for.

Here is what I used. 2 wire shelves. 1 shelf holds 4x 8 card rigs (last rig sits in the front 'exhaust' area on top of a normal PC box). 1 shelf holds the fans that blow on the rigs. Air is blown over the rigs and to the front of the tent, and exhaust fans placed in that area for exhaust.
Tent: https://www.amazon.com/gp/product/B00MB68BEI/ref=oh_aui_detailpage_o08_s00?ie=UTF8&psc=1

Wire shelves:
https://www.amazon.com/gp/product/B00B7E8Y9M/ref=oh_aui_detailpage_o06_s00?ie=UTF8&psc=1

Blowers for exhaust (I use 2 of these, exhausted out the window):
https://www.amazon.com/gp/product/B01CTM0LLO/ref=oh_aui_detailpage_o02_s00?ie=UTF8&psc=1

Fans for airflow, sitting inside the tent on one of the wire shelves (I have 3 of these):
https://www.amazon.com/gp/product/B004IS6JBY/ref=oh_aui_detailpage_o02_s00?ie=UTF8&psc=1

Hope that helps.

I've had good luck with this one

Should be plenty of CMF for a grow tent that size. Plus it has a built in controller so you can turn it down if needed. It pulls around 130w at full blast and 100 at min setting.