How much fresh air do plants need at day time?

Ventilating a room serves 3 purposes.

1) to reduce temperature
2) to reduce RH
3) to get fresh air

Let's forget about number 1 and number 2.

How much fresh air exchange is optimal for plant growth at day time?

I really want to know this. Most of the answers are related to RH or temperature, but I want to know how much would be enough to not be causing a deficiency of Co2 in air?

Talking about the peak of growth when plants are most hungry for fresh air and about 50% of area is being used for canopy.

For tents, the rule of thumb is to replace the entire volume of air every 5 minutes. A 4x4x7 tent is 112 cu. ft. That's 22.4 cfm of air flow necessary to replace the air every 5 minutes.

Is that what you're looking for?

Plants need fresh air all the time. Not just during peak grow times. Air exchange needs to be done every 5 minutes during lights on. Find out the cubic foot of your room. Height x width x length.
Example: 8 x 8 x 8= 512 cu ft.
Next get a fan that has at least 512 cu ft per min air movement.
Set a timer to turn the fan on for one minute every 5 minutes.

Or.... If temps and humidity are not an issue, use co2. That's what I do. Hope this helps.

Yes, that's what I was looking for.

I was thinking myself maybe 0.2 air exchanges per minute (once every 5 minutes) wouldn't be enough in some cases when plants consume a lot of Co2 from air.

In this case I think more is definitely better than less, but I'm just trying to understand if there's a way to accurately calculate it.

If you're saying it's a rule of thumb, then I must trust you.

So if RH is definitely not going to be an issue, then 1 air exchange per every minute would be overkill/unnecessary?

0.2 air exchanges per every minute would be definitely enough for optimal growth?

CouchlockOR said:

Set a timer to turn the fan on for one minute every 5 minutes..

Click to expand...

I never thought about on/off cycles like that. That's interesting. I use a variac ($60) to reduce the speed of the fan. I think I would prefer the constant low fan sound to what I imagine full on/off would sound like.

Measuring airflow is a little more tricky than simply dividing theoretical flow by 5. I had to use a 33gal trash bag, time how long it took to fill, extrapolate cu. ft. to gallons, etc. Kind of pedantic. Dividing theoretical by 5 is easier. (But, I wouldn't trust that entirely because theoretical depends on restrictions to flow.).

BTW: If the op goes for a speed controller, I advise against those cheap $20 dimmer-like controllers. They chop the AC sine wave creating 60 on/off pulses per minute. It's noisy. The variac is *much* quieter. I've read this kind of clean electricity is easier on the motor.

You can't just forget about temperature and humidity, because they play a critical roll in the "Ventilation Process" of peoples' grow areas. So, unless you have no temperature or humidity issues to deal with, you must include these as legitimate concerns - and plan accordingly.

I don't really think that there is enough consensis of oppinion to say that there is any "Rule of Thumb" - let alone to exchange the air every 5 minutes! I have seen all the following "Exchange Times" quoted on Rollitup many times (make that many-many times) each.

1. 5 minutes.
2. 3-5 minutes.
3. 2-3 minutes.
4. 1 minute.
5. 2-3 times per minute.

If you are deffinatally not using air extracton to cool or reduce humidity (only to provide fresh air) then every 5 minutes is probably ok. Otherwise I would say every 1-2 minutes, maybe 1-3 minutes.

The place where I live, we have an average RH of 82% outside.

If I ventilate fast enough, my room would be filled with humid air and no matter how many lamps are shining, they wouldn't be able to reduce enough RH.

The less I ventilate, the more RH will be reduced by devices that emit heat, but of course there is a sweet spot too, considering that plants evaporate and raise RH themselves.

My upcoming grow project will take place in winter, where outside temperature averages -7C (19F).

I will need to add a lot of extra heat to maintain healthy temperature for growth. Adding heat will itself already reduce RH.

So yes, I'm pretty much in a situation where I need ventilation mainly to provide gas exchange

For example, if plants are at peak in their maturity and take up most of the room, I don't see how could I get RH under 80% by ventilating faster when outside of the grow room is 80% RH at night, do you?

if you only have to vent for gas exchange then maybe you should rig up your own co2 system seen as you could pretty much have a sealed grow then and have less worries about smell maybe just vent/filter your air a few times a day or something ?

Otherwise I would say it seems like you need a co2 meter and it would all depend on how many plants you had and how hard you were working them

Also things like where the grow is would matter as grow on a low level in a building with a lot of people or animals can get a pretty high level of co2 once all them people start breathing lol

Okay, yea , I see what you mean. 82%, wow! You should probably consider a de-humidifier of some kind.

With RH that high, mold and mildew issues must be a real problem. Sorry, I really can't help with that; I live in So. California - we have very low humidity.

So, as long as you don't have heat issues, a 5 minuet exchange should fine for you. Good luck!

CRAZINESS!! Listen friend, all things will be good if you have a 6" or 8" induction system with the appropriate inline fan. Put the exhaust end outside somehow (if in a residential neighborhood a charcoal filter is the only other thing you need) and and the intake inside your grow room pulling fresh air across your lights , you will do fine. All other issues come down to you being with them enough every day to mist them with water and make sure they are happy. Think about being outside in nature, that plant can take ANYTHING you throw at it and bounce back so long as you keep the air moving and watch them closely. You will know in time when they need what they need. All grows are different so nobody elses advice will be exactlty right for you. Good luck

5 min rule has always been my standard