[Mrkline006]

Originally Posted by 2011rex87

You did list the height actually sorry about that so yeah do the math on 12x9x8 is 864 cubic feet. Depending on your setup, you do not need to replace the air inside your room every minute. If you put a scrubber on your 740CFM fan, you may only get 600CFM out of the exhaust. That would replace the air in your space every 90 seconds or so. If you need to exhaust HEAT from your grow space, you may want to replace the air every minute or even twice or more times per minute. You'll have to see what your needs will be. On the other hand, if you supplement your grow space with CO2 then you will not want to exhaust the CO2 out before it has had an effect on your plants. If you use CO2, you may want a controller to maintain ppm (800-1500ppm CO2) for example and then set to exhaust the air when humidity builds up or on a timer to refresh the grow space air on a fixed schedule.

You should default to more airflow when making a decision on HVAC. Always overbuild. You can put the fans on a variable controller to dial the power back if you have too much air exchange. I would go ahead and use 2 separate 740CFM exhaust fans or upgrade to 1 larger CFM fan. Now that you can calculate your cubic feet in your grow space, you can calculate your air exchange. Question is how quickly do you want to exchange the air? As said above, at least once a minute if not twice or 4 times a minute should be enough as long as you are not struggling with heat issues or humidity issues.

Thanks.. I'll be running 4x600 watt HPS lights. along with a dehumidifier and AC unit in the room so should help keep temps and humidity decent. all 4 lights are aircooled so i may have to look into an extra fan to help with exhaust. Thanks.

[Mrkline006]

Originally Posted by 2011rex87

You did list the height actually sorry about that so yeah do the math on 12x9x8 is 864 cubic feet. Depending on your setup, you do not need to replace the air inside your room every minute. If you put a scrubber on your 740CFM fan, you may only get 600CFM out of the exhaust. That would replace the air in your space every 90 seconds or so. If you need to exhaust HEAT from your grow space, you may want to replace the air every minute or even twice or more times per minute. You'll have to see what your needs will be. On the other hand, if you supplement your grow space with CO2 then you will not want to exhaust the CO2 out before it has had an effect on your plants. If you use CO2, you may want a controller to maintain ppm (800-1500ppm CO2) for example and then set to exhaust the air when humidity builds up or on a timer to refresh the grow space air on a fixed schedule.

You should default to more airflow when making a decision on HVAC. Always overbuild. You can put the fans on a variable controller to dial the power back if you have too much air exchange. I would go ahead and use 2 separate 740CFM exhaust fans or upgrade to 1 larger CFM fan. Now that you can calculate your cubic feet in your grow space, you can calculate your air exchange. Question is how quickly do you want to exchange the air? As said above, at least once a minute if not twice or 4 times a minute should be enough as long as you are not struggling with heat issues or humidity issues.

to figure out the exchange rate. Is it just the total area (864) divided by what, the minutes? or something else?

[Koondense]

Total volume should get changed in one minute = cfm value.
I don't get what you don't get.

[Mrkline006]

Originally Posted by Koondense

Total volume should get changed in one minute = cfm value.
I don't get what you don't get.

I get you say that... I've read 3 mins, ive read 5... he said 90 seconds or so (1.5 mins) so im trying to figure out whats the best figure to use.

[2011rex87]

So many factors. Humidity and heat are the big 2. Honestly you can get away with much less than 1 exchange per minute but is that ideal? Nobody can answer that for you until you put it all together and get some information on environmentals for us. That is why we suggest you overbuild your intake and exhaust to meet any future needs and that you put everything on variable power supplies so you can dial things back when needed.

[Mrkline006]

Originally Posted by 2011rex87

So many factors. Humidity and heat are the big 2. Honestly you can get away with much less than 1 exchange per minute but is that ideal? Nobody can answer that for you until you put it all together and get some information on environmentals for us. That is why we suggest you overbuild your intake and exhaust to meet any future needs and that you put everything on variable power supplies so you can dial things back when needed.

yeah, im aiming for 1min exchange. like you said, rather OVER build, than under.