CFM Needed for Grow Room

Hello,

My grow room is 25ft (L) x 10ft (W) x 9ft (H) = 2250 cubic feet. My room will have CO2 supplementation and only needs to exhaust heat as needed. Otherwise, no exhaust will be happening. I will have 6x1000watt HPS lights which will create almost 21000 BTUs of heat per hour. My goal with the extra CO2 is to keep temps between 85 and 90 degrees for flowering.

I know the typical calculation for an open loop grow room aka intake and exhaust using normal air is "cubic feet of room / how average you want you air completely replaced" aka if my room is 2250 and I want the air exchanged every three minutes it would be 750CFM needed.

Since I am not replacing the air and my room but only exhausting heat when needed and beyond that using CO2 to keep it "fresh" I am not sure what CFM I need.

Any help would be appreciated

xIPhobiaIx said:

Hello,

My grow room is 25ft (L) x 10ft (W) x 9ft (H) = 2250 cubic feet. My room will have CO2 supplementation and only needs to exhaust heat as needed. Otherwise, no exhaust will be happening. I will have 6x1000watt HPS lights which will create almost 21000 BTUs of heat per hour. My goal with the extra CO2 is to keep temps between 85 and 90 degrees for flowering.

I know the typical calculation for an open loop grow room aka intake and exhaust using normal air is "cubic feet of room / how average you want you air completely replaced" aka if my room is 2250 and I want the air exchanged every three minutes it would be 750CFM needed.

Since I am not replacing the air and my room but only exhausting heat when needed and beyond that using CO2 to keep it "fresh" I am not sure what CFM I need.

Any help would be appreciated

Click to expand...

It doesnt matter, the fan will run until it reaches set point. Bigger fan gets it done quicker. You will be wasting alot of CO2 doing this. If you cant seal the room and run AC, I wouldnt bother running CO2.

Get an AC.

redi jedi said:

It doesnt matter, the fan will run until it reaches set point. Bigger fan gets it done quicker. You will be wasting alot of CO2 doing this. If you cant seal the room and run AC, I wouldnt bother running CO2.

Click to expand...

So you think I would have to exhaust often enough aka not like "15min every couple of hours" that it would waste the CO2? I mean that is a concern of mine.

If that is the case what is a good AC you all recommend that will not break the bank and get get the job done in the room I have?

xIPhobiaIx said:

So you think I would have to exhaust often enough aka not like "15min every couple of hours" that it would waste the CO2? I mean that is a concern of mine.

If that is the case what is a good AC you all recommend that will not break the bank and get get the job done in the room I have?

Click to expand...

You want to break the bank with the ac, get the wrong one and your gonna be out of luck when the time comes. I’d find a lot of BTUs something that uses more than you need, get a good portable ac unit if you can’t build an actual unit outside.
you’ll be losing a lot more money losing co2 than you will be anything else if you exhaust.

User1534 said:

You want to break the bank with the ac, get the wrong one and your gonna be out of luck when the time comes. I’d find a lot of BTUs something that uses more than you need, get a good portable ac unit if you can’t build an actual unit outside.
you’ll be losing a lot more money losing co2 than you will be anything else if you exhaust.

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Price does not always determine quality. Usually, something can be found that is good at getting the job done that it needs to while also being more affordable. I dont' want $ but prob don't need $$$ so $$ I am ok with.

xIPhobiaIx said:

Price does not always determine quality. Usually, something can be found that is good at getting the job done that it needs to while also being more affordable. I dont' want $ but prob don't need $$$ so $$ I am ok with.

Click to expand...

This is true when it comes to other things. But some things you are going to want to spend more money on because in the end it is cheaper. With ACs, yes most price is quality.

You could always buy an inkbird temp/RH controller:

Or a microcontroller (like an arduino or raspberry pi) and temp/RH sensor and log the data required to answer your question. I don't think there is a simple numerical answer like your hoping for, there are many variables (including the plants themselves) and a microcontroller will allow you to plot in realtime the VPD, temp, RH, etc. to ascertain the time to keep your fan on (and if you buy a CO2 sensor the quantity of CO2 you're losing in each fan cycle).

guitarguy10 said:

You could always buy an inkbird temp/RH controller:

https://www.amazon.com/Inkbird-ITC-308-Temperature-Controller-Humidity/dp/B01N56KEU6/ref=sr_1_8?crid=1XHYIS14UA3HR&dchild=1&keywords=inkbird+temperature+controller&qid=1613505829&sprefix=INKBI%2Caps%2C211&sr=8-8

Or a microcontroller (like an arduino or raspberry pi) and temp/RH sensor and log the data required to answer your question. I don't think there is a simple numerical answer like your hoping for, there are many variables (including the plants themselves) and a microcontroller will allow you to plot in realtime the VPD, temp, RH, etc. to ascertain the time to keep your fan on (and if you buy a CO2 sensor the quantity of CO2 you're losing in each fan cycle).

Click to expand...

AKA another wards collect the necessary data and adjust from there. Any way to do this you know without exposing the plants to a potential fast changing environment. Maybe hook everything up and pull in the data without the plants, etc.

xIPhobiaIx said:

AKA another wards collect the necessary data and adjust from there. Any way to do this you know without exposing the plants to a potential fast changing environment. Maybe hook everything up and pull in the data without the plants, etc.

Click to expand...

Well the plants contribute massively to the temp, rh etc. When my girls are in late flower it can be a struggle for me to keep the RH down because they are so large, active and transpiring quickly, whereas when they are in veg or early flower I need to put a humidifier in there because the air here is very dry (<30%).

That's why I said I don't think there is a simple numerical answer and that the only way to get the best number is to log them yourself with your plants as they grow.

The inkbird controllers would make this very easy, which is their selling point, but you would miss the data on CO2. I champion the idea of a microcontroller because they're not that hard to use (there are ppl here including myself who can try and help you along) and will give you real time data that you can use (and even plot in excel realtime with PLX-DAQ or using a web interface like one that's already been written called Mycodo). You might have to make the decision between regulating their VPD, temp, RH at the loss of CO2 or the loss of some of those variables if CO2 is a limiting factor. I don't know what you mean by keeping it fresh, CO2 is CO2, but a CO2 sensor on the microcontroller would tell you how the CO2 concentration correlates with your ventillation system.

xIPhobiaIx said:

So you think I would have to exhaust often enough aka not like "15min every couple of hours" that it would waste the CO2? I mean that is a concern of mine.

If that is the case what is a good AC you all recommend that will not break the bank and get get the job done in the room I have?

Click to expand...

Yes, any exhausting during lights on is gonna waste CO2. And I doubt an exhaust alone will keep temps in check.

As to a good AC, you get what you pay for. With only 6kw of light, you dont need alot of horsepower so you could go with window shakers or portables. I would suggest two units at opposite ends of the room.