hello i would like to start by saying a common problem with rooms running constant exhaust is a lower concentration of co2, not as low as a room with no exhaust but not as good as a sealed room with co2. the problem some people have with sealed rooms with co2 is excessive temperatures. i believe plants can utilize up to 5000 fc at 90-85 farenheit co2 with a ppm less than 1200 at a relative humidity below 55% but above 50%. with a sealed grow room all of these factors are much harder to control, while maintaining a consistent level of co2 i think i have a solution and it is a sealed grow room with recirculating exhaust. 1st make sure to seal up your grow room completely. a good way is with duct tape. i envision this in a 3x3x5 ft grow box with three walls and the front being a sheet of pandafilm/mylar/reflectix etc. on one side will be the intake fan or pusher fan, not the most efficient design as far as air movement but necessary to recirculate exhausted co2 rich air. this fan will be on the lowest part of the pusher fan side as this will provide colder air by the means of the exhaust duct acting as a heat sink. cut a hole in the pusher fan side and install the fan and seal around with duct tapemaking sure there are no leaks. next we install the puller fan side. install puller fan on the side that is opposite the pusher fan and about 2 inches above where you feel the top of the canopy will be . secure the puller fan on wall opposite the pusher fan and make sure it wont fall duct tape is good for sealing but may need other support such as hooks etc. now we install the recirculation duct. install ducting the exact diameter of both fans so that the puller fan connects to the pusher fan so you have one fan pushing air into the room and another pulling that same air out of the room in a constant loop.i find its best to run the exhaust duct from the puller fan, behind the box and around to the pusher fan. also to reduce humidity , install an intermediate cylinder or box( coffee can works) and fill with silica beads and screens to keep them from moving. attach the pusher duct and the puller duct so the air goes through the cylinder pulling moisture from the duct. to reduce heat and vent problematic ethylene from the box we install a small fan on the ceiling of the box. this will be installed closest to the pusher fan side so that it sucks heat as well as ethylene out of the box while leaving room temp air and co2 in the recirc system. now this part is very important, we need to cut a small hole in the box so that the ethylene/ heat vent pulls fresh air from the hole and not from any leaks around the bottom of the box. if we neglected to cut this hole the negative pressure would force a leak anywhere on the box and it could pull air that is not enriched with co2 , defeating the purpose. the hole needs to be as close to the fan as possible. it should be on the same wall as the pusher fan above the canopy by at least 5 inches and within 1-3 inches below the ethylene/ heat exhaust fan. we basically want the fresh air to make it easier for the fan to pull heat ethylene out of the box, while keeping that same fresh air from entering the recirculating duct system. density of co2= 1.98 kg/m cubed density of ethylene= 1.18 kg/ m cubed density of air= 1.225 kg/m cubed as you can see ethylene is very light even lighter than air, naturally it will rise to the top , hot air as we know also rises to eliminate heat issues. the co2 being the densest of the 3 gases will stay in the bottom or pulled through the recirc system. it is important for the heat/ ethylene extraction fan to have a lower cfm rating than the recirc sytem fans. this will ensure they are exhausting or recirculating their respective gases. now you are done. it is important to be able to control at least one of the fans speeds for most control how ever if you can only use one fan controller make it for the pusher fan as the puller fan will always be limited by the rpms of the pusher. it is even possible to use one fan but make sure it is the puller fan up to to increase upward air movement maximizing co2 uptake now you have much more control as opposed to running a fan timer in your grow room plus these added benefits: 1 less wasted co2 2 more consistent level of co2 3 ability to control humidity more easily 4 easier temp control 5 less risk of outside pests entering 6 increased air movement in the grow room, co2 is very dense and tends to pool on the bottom so upward air movement and negative pressure are helpful 7 reduced odor i hope someone found this useful i will be implementing it soon a side note this setup is intended for co2 only, without co2 enrichment the plants would use up available co2 as well as recirculate hotter air since it is a sealed system to create this you should need to spend =166$ (updated price) 1. 50 cfm ventilation fan =7$ for ethylene heat extraction ceiling of box (updated price) yes iv found at least 5 that cost 7$ 2. 2 250 cfm 6 inch duct fans= 40$ total (updated price) 3. 6 ft 6 inch ducting = 12$ 4. duct tape =3$ 5. co2 monitor= 104$ (is also a temp/ humidity monitor) or you could buy a 166$ temp humidity co2 monitor timer and controller. i figured id give the cheapest monitor first for people who dotn want to use a burner or tanks 6. 500 grams silica beads-5$ (updated price) with this method you can use the more ghetto methods of co2 production ie baking soda and vinegar, seeing as it is sealed save for the small exhaust fan up top, or yeast fermentation or yeast reproduction method(preferred) with a co2 monitor you will easily able to see if this method has any validity and I hypothesize that it will save you money compared to a timed vent with other sealed grow rooms. I know 166 sounds expensive, but most co2 basic kits for a tank will cost you 60-120 and for a co2 sensor that releases co2 at a given ppm will cost 225- well over 1000$,not to mention the 200$ 20 lb tank you need as well. this setup will tell you every thing you need to know and wont waste co2 while making the baking soda vinegar method more effective and eliminating ethylene from the area now with picture!!!! aw gee that's swell!