Open Vs. Closed Loop Grow Rooms

By: The Guru

2009 turned out to be a good year for the growing community of San Diego. Finally the government is listening to what the people of California voted for and is easing off the medical growing community. But what to do with all these new people coming into the growing game, and in some cases flooding the market? It’s simple. Be educated and prepared; be a good gardener and produce the best product you can by having the proper grow room environment.

During winter many growers are having their best harvests. The main factor responsible for this is the ENVIRONMENT! The environment is the most important factor in a grow room. If the environment is good then most anything will grow well. Factors such as humidity, temperature, air quality, air circulation, ventilation or air exchange, and CO2 levels determine an environment. All of these factors must be perfectly in line. There is a complete line of environmental controllers and products available to control all of these factors, for the times you need it most, such as when spring and summer approach us again.

There are two different ways to set up a grow room. Open loop or closed loop, also known as a sealed room. An open loop system has open ventilation, or a fan pushing air into the room and another fan exhausting air out of the room. This is known as ventilation or air exchange. It is exactly what it says; exchanging the old air in the room with fresh, cool CO2 enriched air. In a normal grow room you need to exchange the air at least once every one to two minutes at minimum. Personally, I like to exchange the air about once a minute up to every 30 seconds. Some say that’s not necessary but the more air exchange the better in an open loop room. The air exchange is controlling three different elements in the grow room. It is controlling the air temperature, the CO2 levels in the room, and the air quality. As the fresh air is delivered to the room it brings cooler temperatures, flushing out the hot air in the room and replacing it with fresh CO2 enriched air. The fresh air exchange for air quality and CO2 replenishment is one of the most critical factors in a successful grow room.

Air circulation is just that, oscillating fans or blowers moving and stirring up the air constantly and keeps the plants moving gently. Bugs and molds don’t like wind and fresh air. Somewhat like exercise, a plant works the stocks and stems making them stronger, healthier and larger by being gently blown around. By stirring up the air and moving it around the air never has a chance to get stale or stagnant. All parts of the garden should have air gently blowing through it. There should be no dead spots. Dead spots where air is not moving is the most likely place that molds, fungi, diseases and bugs will first appear and develop. Oscillating fans should be spaced all around your garden on the walls and at different levels ensuring that the garden and plants have plenty of fresh air flowing all around them.

As for the temperature, plants are most comfortable in temperatures between 76 and 80 degrees. If temperatures get too high, even in the mid 80s, the plant is not performing properly. Using an infrared temperature gun on the top leaves of the plant, any hotter than 82 degrees and the plant is not at optimal performance. A plants fruit or flowers can develop airy and leafy as a result of the plant trying to cool itself off. A plant will stretch out and develop loose rather than tight fruits as a method of keeping air flowing through it. If temperatures get too hot the plant is not processing photosynthesis at 100%, and molds, fungi, diseases and especially bugs are likely to establish their colonies as they often prefer warmer environments. If the room gets too cold, below 70 degrees or so, then the plant is not performing or growing to its full potential. Approximately, a ten degree difference in temperature between daytime and nighttime is helpful in keeping the plant healthy and the fruit or flower clusters tight and firm.

If your room is completely sealed, and the vents bringing in and exhausting air have filters on them, the chances of bugs and diseases getting into the room are greatly lessened. As long as you don’t bring in any dirty or infected plants into the room, and don’t transport something in from outside on your clothing as you enter the room then you will have a bug and disease free grow room. You should always wrap and line your walls with black and white panda film with the white side facing in to the grow room. Molds and fungus can’t absorb into the plastic, as they can into wood, drywall or any other porous surface. It’s easy to put up using a contractors staple gun. Also make a tight seal and cover your floors, and walls up to the ceiling. It’s cheap and easy to put up and replace when sterilizing or cleaning a grow room.

Plants breathe or consume CO2 gas and convert the CO2 into oxygen. Without plants in this world the CO2 in the air would kill us instantly. Plants are basically the oxygen filters of the world. This is why it is so important that our few rain forests left like the Amazon do not get destroyed for profit any more. Day by day, as they strip the forests of the trees, the giant lungs and filters of the world, they are stripping away our natural filters. The rain forest is the perfect example of a sustainable organic garden.

A plant must have the proper CO2 levels in the air to be a healthy plant, with a healthy immune system, which will fight off molds, diseases and bugs, and produce healthy bountiful fruit. Outside, the air we breathe has about 400-450 ppm (parts per million) of CO2 all the time. For plants, this is a happy level of CO2. However at levels around 200 ppm, a plant can actually start choking or suffocating. At levels this low the plant will not be healthy and may even die. Plants can use levels of CO2 up to 2000 ppm 4-5 times that of normal ambient levels, and achieve optimal growth and production. Performance can be enhanced by adding CO2 to the room, increasing the levels significantly. This can only be done efficiently in a sealed or closed room. This is why air exchange in the room must be happening constantly, ensuring the CO2 does not drop below ambient.

In an open loop system, supplementing with CO2 is a bit wasteful since the fans are constantly exchanging air in the room, bringing it back to ambient CO2 levels in just minutes. In most cases especially here in San Diego, a grower can not turn off his exhaust and intake fans for more than a few minutes without the temperatures climbing too high. Come summertime, don’t even think about running CO2 in an open loop system. The temperatures are too hot in the summer and ventilation systems are running 24/7, constantly exchanging the air in the grow room. There’s really only one way to beat the heat and other factors that affect the grow rooms environment; build and run a sealed room with no exhaust!

Humidity is also a very critical element that must be maintained at proper levels. The humidity controls the transpiration rate of the plant and how the nutrients are received by the plant. Just as with humans, if the humidity gets too low, our skin can become dry and flaky. We transpire by sweating more fluids out in lower humidity levels. The humidity level is like a pressure cap on the plant, keeping the moisture in the plant, allowing it to have proper transpiration rates of the fluids. Ideal humidity levels in a grow room range between 50% to 70% in vegetative growth, and 50% to 60% for flowering plants.

When humidity levels drop too low, the plants transpire at a rate much quicker than that of nutrient uptake. The nutrients or minerals do not transpire thru the plant, only the water does. So this leaves behind a concentrated level of nutrients in the plant that will actually cause a nutrient burn. Most people don’t realize in situations like these that the humidity could be responsible; usually thinking that it is too many nutrients in the reservoir. Just as a lack of CO2 can cause a plant to go dormant, low humidity can cause a plant to have nutrient problems, resulting from the transpiration rate being much too high in low humidity level environments.

Conversely, when humidity levels get too high, moisture is building up on the plants and walls, forming whole colonies of molds, fungi, and mildews. Even if you had none of these developing before the high levels, the moisture would create the perfect environment for all of these to start developing, in a very short amount of time. These pathogens will destroy your garden if not taken care of immediately. This is one more reason why a controlled, closed loop, sealed room is the best way to build and run a grow room, as you can control every element of the environment with the right equipment.

Compared to an open loop room, a sealed or closed loop system has no fresh air intake and no main exhaust. The only air running into the room is for cooling the lights, removing the heat from the room. Since the fan is connected by ducting to an outside fresh air source such as a window or adjacent room, the air never comes in contact with the room air, as long as you use proper air cooling lights with a sealed glass lens. The air is simply brought in through the ducting (always pushed, never pulled through the lights) and pushes the hot air out of the lights through further ducting, normally up into the attic as heat rises.

With no fresh air exchange to control CO2 levels, air quality, and humidity, how do you control the environment? Simple, by creating and building a sealed closed loop grow room! By using equipment such as air conditioners, humidifiers and dehumidifiers, and supplemental CO2, either as a CO2 tank with a regulator or a CO2 burner. You must also have a CO2 monitor on both of these, and possibly an environmental controller to synchronize all the components together to create the optimal grow laboratory. Check out to view a complete line of environmental controllers, CO2 generators and burners. The Fuzzy Logic CO2 controller and monitor is one of my favorites (all come with a 3 year no hassle warranty, and a winner of the High Times Stash Awards). A simple charcoal filter, with an inline fan for scrubbing and recalculating the air, is also needed to accomplish this properly. The filter or scrubber in this case is running through the room, constantly cleaning the air free of molds, fungi and diseases.

The air conditioner is definitely the main component to this sealed closed loop room. A true AC is recirculating the same air in the room. The air never leaves the room! These types of AC units are usually referred to as split unit ACs. It has vents in every room putting cold air into the room, and also has a return duct of equal CFM (cubic feet per minute) creating equal pressure, cooling the same air over and over again. This allows CO2 to be run efficiently and not be wasted as the air is moving and recirculating. The only loss of CO2 is that which the plants have consumed. The AC also acts as a natural dehumidifier, so you need to watch your humidity levels in case the AC is dropping the levels in the room below ideal. The closed loop and sealed room only work with the right AC unit, one that has a return and recirculates the air.

The amount of BTUs produced by the AC unit you will need to cool the room is based on the amount of heat that is being produced by the lights. With quality AC units, you will need 4000 BTUs for each 1000 watt light, even with air cooled lights. I like to rate them at about 4500 – 5000 BTU’s per 1000 watt light to ensure that it covers my CO2 burners heat as well, and that the AC motor is not running at max load which is harder on the AC motor and costs more in electricity. Any motor running at lower than max load will run more efficiently for the most part. So when purchasing an AC, it’s not a bad idea to get one larger with more BTUs than you actually need. One of the most impressive AC units I have seen is the Kwikool. They are portable and true recirculating commercial, they do not allow any of the air from the room to contaminate the outflow exhaust duct which could allow unwanted odors out of the room.

Finally you have the CO2, the life of the plants and the grow room. Since the temperatures, humidity, and air qualities are controlled at this point, you just need to supplement CO2 with a burner or tanks. But instead of being at roughly 450 ppm of CO2 in the air as in an open loop room, we can increase the levels up to 1500-2000 ppm with the right equipment. By doing this we can enhance the plants metabolism rate and its growth and flowering will be dramatically enhanced. Plants don’t use CO2 when they’re sleeping so it would not be on during the night cycles. With high levels of CO2 and all other factors in the ideal range, you have the perfect grow room.

bringing you that fire! stay tune for more posts.

Related Articles

1 Comment

Avarage Rating:
  • 0 / 10
  • Kade , July 9, 2020 @ 12:50 pm

    Thank you so much for this article. It is very hard to find information regarding this. The weakest kwikool is still a bit powerful for my small grow, but I now know exactly what I am looking for, thanks!

Leave a Reply

Your email address will not be published.