How often should you exchange all the air in your grow room?

I'm not asking for personal opinions so much, I'm more asking if there's like a clear scientific consensus. I keep hearing people say "Every 5 minutes" or "Every 3 minutes" or even "Every 1 minute"

But in reality, there has to be a "correct" answer, right? Maybe some literature someone can direct me to? Or perhaps one of the weed Youtubers has done an experiment of some sort?

Any help is appreciated, thanks!

No correct answer as everyone has different spaces with different amount of plants etc..
Basic rule about forced air exchange is 3 min yes.
Anyway, if you want some help you can check out this

In my own grows I go for the lowest air exchange the heat management lets me get away with.

My thinking is to exchange air 100% of the time. I have a small vent fan (computer fan) that runs 24-7.
There is a carbon filter in the tent and a carbon filter setup outside the tent, which allows me to smoke in room.
So it is not scientific but it covers the reaction exchange.....

You need to exchange it 100% of the time indeed just the rate is at question,
If you work in tents, and heat is not a problem, the lowest vent / carbon filter will work perfectly.
When you need to combine heat management with you vents it gets somewhat tricky and it's better to overbuy a stronger than what u need vent so you can compansate the heat

most scientific research you are searching for all have control environment there for they're supplementing co2 not air exchange...

that part like @DanKiller said also

So your x times a minute is to replenish co2 .

But the main thing is to exchange enough air to maintain temperature.
That's why there's no one single figure..

There is some equation you can do which is based on
starting temp (ambient air) X wattage / temp difference.

But without going back several years through my own posts , I've got no way of finding it. As it was on my old laptop

DanKiller said:

No correct answer as everyone has different spaces with different amount of plants etc..
Basic rule about forced air exchange is 3 min yes.
Anyway, if you want some help you can check out this

Filter Calculator | Can-Filters

Calculating your requirements early is important because in an exhaust setting the air only passes through the filter one time so it is imperative that we remove as close to 100% of the contaminate as…

canfilters.com

In my own grows I go for the lowest air exchange the heat management lets me get away with.

Click to expand...

That's a good point about the different amount of plants. That's a variable that's hard to control.

formularacer said:

My thinking is to exchange air 100% of the time. I have a small vent fan (computer fan) that runs 24-7.
There is a carbon filter in the tent and a carbon filter setup outside the tent, which allows me to smoke in room.
So it is not scientific but it covers the reaction exchange.....

Click to expand...

Maybe my phrasing was off, of course, you should be exchanging air 100% of the time, but there's usually some discussion about whether you should be replacing all the air in your growspace every 1, 3, 5 minutes, etc. As Dankiller says, it's the rate at which you do it I'm curious about ^^ Non-native speaker, so I sometimes struggle getting the correct meaning out!

DanKiller said:

You need to exchange it 100% of the time indeed just the rate is at question,
If you work in tents, and heat is not a problem, the lowest vent / carbon filter will work perfectly.
When you need to combine heat management with you vents it gets somewhat tricky and it's better to overbuy a stronger than what u need vent so you can compansate the heat

Click to expand...

Gotcha, yes the rate is what I was referring to. What do you mean by the lowest vent / carbon filter will work perfectly? As in, at the lowest setting?

Yes, the lowest you can go on the vent output and matching carbon filter (if needed, if not you can even go lower like PC fan etc)
Because in tents even the 8x4 ones, small vents will exchange air in less than 3 min usually.
Maximizing air exchange without relation to added co2 will not improve anything but heat reduction or rh control if you have too much.

Calculating the airflow required to maintain a stable temperature, and so determine the size of fan needed, is surprisingly simple.

It boils down to CFM = 3.16 x Watts / DT(°F)

Watts is the lighting power (and any other 'hot' things in the grow space) in Watts.
DT is the allowable temperature rise within the enclosure (i.e. desired temperature minus ambient temperature) in °F.

So, to work out the size of fan required (in CFM) simply plug your lighting Wattage into the equation along with the DT value.

An example is:
Ambient temerature = 20 °C
Target temperature of the enclosure = 25 °C
DT in °C = 25 - 20 = 5
DT in °F = 5 x (9 / 5) = 9
Lighting Wattage = 250 Watts

Plugging these values into the equation gives:

CFM = 3.16 x 250 / 9 = 87.77

That's the equation for calculating the cfm needed to maintain the desired temperature.

Which for most people is greater than the requirement for maintaining co2 levels.

It's not my math, just something i came across years ago

DanKiller said:

Yes, the lowest you can go on the vent output and matching carbon filter (if needed, if not you can even go lower like PC fan etc)
Because in tents even the 8x4 ones, small vents will exchange air in less than 3 min usually.
Maximizing air exchange without relation to added co2 will not improve anything but heat reduction or rh control if you have too much.

Click to expand...

Ah perfect. That helps clear some things up. Because I was wondering like, what benefits the plants themselves are seeing at 1 minute, 3 minute, 5 minute, but if it's more about heat and humidity then it's simpler.

coreywebster said:

That's the equation for calculating the cfm needed to maintain the desired temperature.

Which for most people is greater than the requirement for maintaining co2 levels.

It's not my math, just something i came across years ago

Click to expand...

Thanks! Useful if you're not using a controller. (Which I'm not, yet!)

Even with a controller you need to buy a fan big enough in the first place.

Obviously if you live in the desert and van only allow a 1 degree rise your going to be buying a much bigger extractor than the guy who lives in a colder climate. All these fans have a range.

HandyGringo said:

Ah perfect. That helps clear some things up. Because I was wondering like, what benefits the plants themselves are seeing at 1 minute, 3 minute, 5 minute, but if it's more about heat and humidity then it's simpler.





Thanks! Useful if you're not using a controller. (Which I'm not, yet!)

Click to expand...

I use a 3" pc fan for extraction in my 4ft x 4ft tent.

I've used pc fans, what ever works.

I don't care about exhange rate, my fan has temp/humidity control and there are times it dosen't run much at all. In my experiance, the exchange needed to control humidity always prevents co2 depletion.

My PC fan exhaust to the outside. My lights on temp is 80 -81 lights off temp is 75. I am planning to run two warm location strains. So the plan is to put a ceramic heater in the air intake and jump the temp up a few degrees along with the humidity. My second carbon filter setup picks up near the tent output and then feeds it back into the intake. I have caps on the system which I can remove for temp change I will automate it but it is still under development. Aiming towards a temp change of ten degrees when lights go off.
I have birds in the room before tent and they generate CO2 and the air flow heads to the tent. Fuzzy baby quail CO2 generators.

HandyGringo said:

I'm not asking for personal opinions so much, I'm more asking if there's like a clear scientific consensus. I keep hearing people say "Every 5 minutes" or "Every 3 minutes" or even "Every 1 minute"

But in reality, there has to be a "correct" answer, right? Maybe some literature someone can direct me to? Or perhaps one of the weed Youtubers has done an experiment of some sort?

Any help is appreciated, thanks!

Click to expand...

A few weeks ago, I was being pestered by someone (on another site) about air flow in my 2' x 4' tent. In response, I did a Google search, read through about a dozen links, and captured the information in an Excel document.

The attached document was written from the perspective of determining the speed setting on my AC Infinity 4" inline fan but the data, links, and comments at the top of the document should provide some insight.

Delps8 said:

A few weeks ago, I was being pestered by someone (on another site) about air flow in my 2' x 4' tent. In response, I did a Google search, read through about a dozen links, and captured the information in an Excel document.

The attached document was written from the perspective of determining the speed setting on my AC Infinity 4" inline fan but the data, links, and comments at the top of the document should provide some insight.

Click to expand...

Thanks for this! Laid out like this it becomes even clearer that there's not a "perfect" number, it depends on your local conditions.

1 minute is fine as long as it doesn't have a negative impact on your temperature or your relative humidity, the same for 5 minutes, it seems. It's about striking a balance. If you can get the air exchanged every 1 minute while staying within your ideal humidity, then that should be better than 5, right?

Feel free to correct me if you reached a different conclusion!

HandyGringo said:

Thanks for this! Laid out like this it becomes even clearer that there's not a "perfect" number, it depends on your local conditions.

Click to expand...

The majority of the values in the ∆ column run between 1 change per minute and 1 change per three minutes (0.33 changes per minute) so I suspect that somewhere in the middle is good. Good air flow helps ensure that the grow gets an adequate supply of CO2 and it reduces the chances of bud rot. The latter was the big issue for me since I lost a beautiful plant to bud rot a year ago.



"local conditions" - bud rot thrives at temps <76° and RH >70%. If wind speed it high, that will make it harder for the spores to settle but it's still best to steer clear of cool, moist conditions. I used an AC Infinity heater to keep the tent at 76° during lights out.

HandyGringo said:

1 minute is fine as long as it doesn't have a negative impact on your temperature or your relative humidity, the same for 5 minutes, it seems. It's about striking a balance. If you can get the air exchanged every 1 minute while staying within your ideal humidity, then that should be better than 5, right?

Feel free to correct me if you reached a different conclusion!

Click to expand...

I would agree - 1 change per minute better than 1 change every 5 minutes. The only caveat would be to ensure that you're not aiming a fan that's blowing a lot of air directly at your plant. I bought an anemometer from Uni, the company that make the lux meter, because I wanted to see what 2 meters/second looked like. That value is one that I've seen kicked around re. wind speed (I don't recall the source) and it's pretty high, higher than having leaves "fluttering".

My grow is in a 2' x 4' tent and one fan was an AC Infinity oscillating fan blowing on 3/10 which was pushing just over 2m/s at that setting. Given that it was an intermittent, there were no issues with "excess wind".

HTH.

Hi all. I’m new myself in the sense I haven’t even bought anything yet. This is a very interesting conversation because I’m on a shoe string budget. My thoughts are start small, think big. So with that in mind, some of the equipment I’ve chosen could be carried over to a bigger future grow.

My space is around 40sqft but I have a 4” inline exhaust with carbon filter, Inkbird controller for the ppm and a solenoid for the c02. My thoughts are if I can make 4-6 plants work, then expand gradually while only buying certain components once (outside of failure). Am I wrong for doing so? I can always slow the air down with smaller ducts, filters etc.