Proper Intake and Exhaust for a Grow Tent

(dpontius)

Can I run the timer every 3 hours for 10 minutes, or do I have to run it 24 hours?

Answer There are three things that running an exhaust fan accomplishes when it exchanges the air inside your grow room with fresh air. First, it lowers the temperature. Second, it lowers the humidity. Third, the fresh air coming into the grow box carries with it a fresh supply of CO2, which the plants need constantly to keep growing. By taking a careful look at these three factors, both during your "lights on" cycle and again during your "dark period", I should be able to give you a very good idea of what you need to do to make sure your exhaust plan is meeting all of these needs, both day and night.

First, let's consider this with the lights on. HID lights, and even fluorescent lights, produce a lot of heat....especially when you close them up into small spaces. Temperatures of 80 degrees or above- measured below the lights, at the plant level- are bad. If your grow tent is inside your home, the ambient air temperature is around 70 degrees. As soon as the lights kick on in the grow tent, the temperature begins to rise.

In the hot spot where you should be taking your temperature readings at, it will probably reach 80 degrees in less than ten minutes. Even with the exhaust running constantly at that point, the grow room will never go below 70 degrees, because that is the temperature of the air in the rest of the house (unless you want to pump AC directly into your grow tent, which I personally have had to do before to make this kind of setup work).

In reality, it is very UN-likely that the grow room will go below 75 or 78 degrees, because you have a screaming hot light burning inside the grow tent and it is not only producing convection heat (which you can remove much of with your fan), but it is also producing radiant heat, which is warming up the temperature of the light itself, and the walls of the tent, and and the PVC that the hydroponic system is made of, etc..

Just to give you an idea of what you are up against, another visitor mailed me just last night. She only has a 105 watt compact fluorescent in her broom closet. After a half an hour with the light on, the closet was 115 degrees!

Well, with the fan running constantly during the lights on cycle, at least you won't have to worry about the air inside the tent becoming depleted of CO2....and you won't have to worry about the humidity getting too high either. However, you will most likely have a heat problem to figure out- that's why most people quit for the summer. It is much easier to do this kind of thing when you can just crack open a window and suck some cool air from outside into your grow room.

Now let's look at this with the lights off. Well, you won't have to worry about the temperature. However, plants do a LOT of growing during the night...using all of the starches that they manufactured during the day and stored in their roots. That means, if you want good plant growth, you also need to make sure your plants got some CO2 during the night. You see, plants and animals are also known as Carbon based life forms, because several components of animal cells and plant cells cannot be made without atoms of Carbon.

So the plants will not (can not) make any new plant cells (which is exactly what they NEED to do in order to grow) unless they have access to Carbon. And where does that Carbon come from? You guessed it! The CO2 in the air. Unfortunately, the level of CO2 in our atmosphere is only 400 parts per million. When the CO2 levels in the air drop below 250 ppm, plant growth stops.

When plants are confined to a small space, they can drop the CO2 levels from 400 ppm to 250 ppm very quickly. It really depends on how much plant matter and how small the air volume in the grow room is. If you want to keep your plants supplied with CO2 and growing during the dark period, I recommend running the exhaust fan long enough to completely exchange the air volume of the tent with fresh air twice an hour. How long you must leave the fan on will depend on how much air volume is inside your grow tent and also on the CFM (cubic feet per minute) rating of your fan. Step by step instructions for doing this calculation are on my exhaust fan calculator page.

By the time you are exhausting your grow tent twice and hour (or at the very least once an hour), high humidity should no longer be a problem during the dark period (which it often is). I realize this all seems like a big pain in the butt, but if the plants don't get it the way they like it, they will not do for you what you want them to do.....so really, all of this stuff needs to be the way YOU like it too. I hope this helps you out, and Happy Growing!