exhaust fan to draw in fresh air (unless you are doing an air conditioner cooled sealed CO2 room)
oscillating fan to circulate air within the grow space.
The exhaust fan should be used to help keep VPD in an appropriate range.
Clones / Seedlings 0.6 – 0.9 kPa
Vegetative Stage 0.9 – 1.2 kPa
Flower 1.2 – 1.6 kPa
Grow Boxes
A rigid Grow Box will allow you to securely mount your exhaust fan and carbon filter. The carbon filter can be hung from the top or mounted on top of your veg room if you have one.
Grow Tents
Tents are less sturdy than grow boxes and thus require extra care when installing exhaust fans. An improperly installed fan can pose a fire risk.
Exhaust fans in grows tents should be hung using bungee cords whenever possible to minimize vibrations to the frame. Oscillating fans like this one clip right to your grow tent poles.
Fan Size for Grow Box
I no longer recommend anyone use a 200 CFM 4″ fan unless they are only running a 100Watt LED Board. Anyone running 240W LED Board or stronger should have a 6 Inch 400+ CFM fan. Even if you are going to be reducing this down to 4″ to exhaust out of an existing dryer vent the 6″ fan will provide much faster air movement.
You always want to have excess fan power, you will thank yourself in the summer when temperatures are at their highest. It only takes one heat wave to scorch an entire batch. There are two main ways to control your exhaust fan:
Option 1: Temperature Controller
My preferred option for fan control is a temperature controller. This allows you to set a target temperature at which your fan kicks on. During veg I set my temp to 75F. During flower I lower the temperature to promote resin development and bring out purple colours.
You fan will run less during the night period when the lights are not generating heat. This will help save power. Photosynthesis does not occur during the night phase so constant fresh air intake is not required. I find this helps me maintain more stable humidity since my fan does not run needlessly during the night phase. The plants love it.
Option 2: Fan Dimmer
The second option for controlling your fan is a dimmer. This is option is not as adaptive as the temperature controller but it does let you fine tune the speed. If your grow is within earshot this can be less distracting than a temperature controller kicking the fan on and off.
Light Blocker: The flowering room (left) will be running lights at 12/12 while the vegetative room (right) will have a longer light cycle of 18/6. To prevent light leaks you can make a light blocker using coroplast, a hot glue gun, and a small fan.
Exhaust Junction
Grow boxes produce heat. I built this ‘exhaust junction’ with the idea of conserving heat in the winter months and exhausting it outside in the warm summer months.
Air Intake Filters
There is no shortage of carbon scrubbing exhaust filters for cannabis growers. Even the most basic grow setups typically have a carbon scrubber to eliminate exhaust smell.
For some reason, air intake filters are an afterthought in most grow space designs. I think this is because intake filters are difficult to incorporate into grow tents that do not have the same type of airtight seal a rigid plywood grow-box does. As we enter an era of legalization the air intake filter should become more important than the carbon-scrubbing exhaust filter that was designed to hide smells during the prohibition.
Air intake filters remove pollen, mold spores and dust mites. They will even filter bacteria (if you buy a filter with an MER of 13+). I recommend a filter with a MERV of 8 or more (MPR 600+) to filter out mold spores.
In my initial design, I used a waffle style filter that you find in most living room air purifiers.
$20 Air Purifier Filter
This worked well but these filters are expensive and even in a small 2 plant 240 Watt grow I found it to be too restrictive on airflow. I upgraded to furnace filters which are actually cheaper, I guess due to mass production. On Amazon and Costco you can readily find them for under $10 each. Get a filter with an MER of 8+ in order to properly filter out mold spores.
I decided to make a frame around the furnace filter for easy removal and so that I could add some light-blocking shutters. This is entirely optional as you could just tape the filter on to the side of your box for installation.
Slide-in Frame to allow for quick changing
My grow box has ambient light around it 24/7 so for good measure I installed light blocking shutters. If you are worried about light leaks you could also get a thicker filter. The filter above is 1″ thick but you can get them up to 4″ thick.
Installed and ready for a filter$10 Furnace Filter
Remember this is why we use air intake filters. Buds are sticky and this dust would otherwise have been stuck in your precious nugs!
I have applied this same idea to many types of grow spaces.
