here's some info i pulled together for a grow room construction tutorial:
13. how to plan your ventilation system? Weed likes temperatures between 65 f and 75 f without CO2 and up to 85 f with CO2. The basic decisions on how you construct your ventilation system are determined by whether or not you need to cool the air and whether or not you’re using enhanced CO2. If the answer to those two questions is yes, then you want to minimize the exchange of air with the outside. If the answer is no to cooling and enhancement, you are going to need a lot of air exchange in your system. In our case we’re experiencing a typical 100 f summer so cooling is absolutely required, and as said we’re adding CO2.
a. How much cooling -
this calculator is excellent for sizing an a/c. but it gives the theoretical cooling that can be impossible to reach unless you carefully plan the design of your system. Can’t go wrong by doubling what the calculator tells you.
b. What kind of cooling - These are your choices, in order of highest to lowest cooling capability:
i. Tie into your home central a/c. by far the best choice if you can do it.
ii. Add a split mini a/c system. These systems are almost as effective as a central a/c but are sized for single rooms. Downside is price and professional installation or charging required. We have a splt mini in our veg room and have never had a temperature problem.
iii. Add a dual hose portable a/c. these can be almost as effective as a central a/c if you take special precautions to handle the heat they generate.
iv. Add a single hose portable a/c. we recommend against single hose systems because of poor efficiency and the fact that they exhaust room air without being treated for smell.
v. Add a swamp cooler. We recommend against this option because of relatively little cooling achieved and added moisture that’s not wanted during flowering.
c. What kind of room exhaust hole is needed - in our case, exhaust of room air is limited to occasional use to remove trapped hot, stale air every couple of hours. A four-inch vent hole to the outdoors is sufficient for this limited use. If you are building a room without cooling, plan on as large as an 8-inch exhaust hole. Make your final choice of hole size based on the fan size you choose later in the process.
d. What kind of air input is needed - The smallest grow spaces can get enough incoming air from leaks around the room. But most grow spaces will benefit from one input hole, even without a fan. Ideally, you want your exhaust fan to create a slight vacuum so that there is no chance of smells leaking out of the room, so plan your input with or without a fan to create less airflow than your exhaust.
e. Treating your air with carbon filter - you will want a carbon filter on the inside of your grow room directly connected to a fan. That fan can be simply recirculating air in the room if you’re using a/c and/or CO2, or ducted out the exhaust if you’re not cooling/enhancing. There is a lot of debate about the placement of the carbon filter and fan, here’s our take: security is the most important factor and you will be assured of treating the most air with this recommended placement. If you move the fan away from the filter, then untreated air can get pulled into the ventilation line through any leaks. If you move the filter outside the grow area and blow air through it then all of the air in your room will still smell. This is not the most efficient use of the fans but we trade security for a little efficiency.
f. Cooling your light - The light itself usually produces the most heat of anything in your room, not including the a/c exhaust. Adding an air-cooled hood and dedicated ventilation line can help this. The other benefit of an air-cooled hood is that you can keep your light closer to the plants, enhancing growth and yield. You need an entry hole and exit hole for the light’s ventilation, either 4-inch, 6-inch or 8-inch depending on the hood. If you are not cooling/CO2 enhancing then many people combine the carbon filter line with the light cooling ventilation line as follows: carbon filter->fan->ducting->light->ducting->duct muffler->exhaust.
g. Sizing your fans - you will not know how much air movement you really need until you’ve built your system and turn it on. A practical way to handle this is to get a relatively large capacity
industrial inline fan and a separate
variable speed controller. a fan that’s running faster than you need is going to be louder than you want and a fan that’s running slower than you need is going to be worthless. The guidelines for fan size are very broad (from one room air exchange every five minutes for fresh air up to 2 or more exchanges of room air per minute to handle high temperatures). As an example, our flowering space is 270 cubic feet so could require a fan of 540 cfm or greater if we were using it to bring cool air into the room. We settled for a 440 cfm fan on our carbon filter line with a 170 cfm fan on our light cooling line, both with variable speed controllers. We probably could use a stronger fan on the light cooling line judging by the high temperature of the air leaving the light.
h. Adding recirculating fans – you want to provide good airflow inside your grow space to even out temperatures, remove waste gasses from your plants and strengthen the plant via air movement. We have one oscillating fan and one still fan in the room in addition to the carbon filter’s fan that recirculates air most of the time and the a/c blower.
Tip – when I thought I had everything dialed in I noticed a very obvious vibration with my hand on the outside wall of the growing room. I searched all my over-sized industrial equipment but it turned out to be the little oscillating fan causing the problem.
i. Ducting – it is very important to use insulated ducting on all lines that are carrying relatively hot air or you may not be able to cool the space at all. This is especially true for the a/c exhaust line. Also, plan all ducting lines as short as possible consistent with few kinks or corners.
