A Guide to CO2 in the Grow Room

Tara | December 26, 2021

Big Yields Do Come in Bottles

Plants use CO2 in photosynthesis. Without CO2 photosynthesis effectively ceases. By increasing the atmospheric levels of CO2 it is possible to increase the rate of photosynthesis. This means cell division (growth) occurs more rapidly; which means plant growth occurs more rapidly. CO2 can increase yields by 10 – 30% and reduce growing times by 10 – 30%.

This means that if you are growing a crop with an 8-10 week growing cycle you could feasibly finish in as little as 6 weeks. In addition to this, yields could increase by 10 – 30%.

Creating a CO2 Enriched Environment

Atmospheric CO2 levels vary between approximately 320 – 500ppm. These levels differ depending on location and other environmental factors. By increasing and maintaining CO2 levels at between 1500 – 5000ppm you are able to enhance plant growth in most crops.

CO2 enriched environments in greenhouses (tomato crops etc) typically maintain CO2 levels at the 1500ppm mark. While maintaining higher CO2 levels may further enhance plant growth, the 1500ppm mark tends to be the most cost effective level when taking into account leakage rates of CO2 gas – hence costs of gas) and yield increases (cost/benefit ratio).

Leakage refers to gas escaping from the growing environment. This could mean leakage through partially open doors or cracks in the greenhouse etc. Here lies part of the problem when dealing with the complexities of creating a CO2 enriched environment. That is, for CO2 to remain at optimum levels within the environment, airflow out of the environment must be significantly reduced. This means that the artificially enriched (1500ppm etc) CO2 environment must be contained, as leakage will result in decreasing levels of CO2.

Let’s now consider the average indoor grow room. Firstly, we have HID lighting that creates large amounts of heat. Secondly, we have exhaust fans that deal with this heat (by extracting hot air out of the environment and replacing it with cooler outside air).

For instance, if you had an inlet fan and an outlet fan that both moved, for arguments sake, 250 litres of air per second, then you would be moving a cubic metre of air (1000 litres) every four seconds. Now, let’s say that your growing environment was 3mtrs long, 2mtrs wide, and 2.5mtrs in height; this would give you a total of 15 cubic mtrs (i.e. 3 x 2 x 2.5 = 15). Therefore, complete air replacement would occur in one minute. You can probably see that, given this scenario, it would be impossible to constantly enrich the atmosphere with CO2 to 1500ppm. So, the question is, given this, how do you maintain 1500ppm of CO2 for any period of time?

Maintaining CO2 Levels

In order to maintain constant CO2 levels it is necessary to contain the CO2 enriched air within the environment. Therefore, exhaust fans need to be turned off for an extended period of time. There is very little point in extracting the expensive CO2 gas straight out of the growing environment and into the outside atmosphere. In addition to this, any possible leakage points need to be sealed. This includes fan holes etc (fan holes are often sealed by baffles which close when the fans turn off).

This ensures that the CO2 enriched air is able to remain stable in the environment without leakage or extraction. A standard CO2 enriched cycle would ideally mean no air replacement (fans off) for about 30 minutes. After this the fans would come on for five minutes or so (to allow for air cooling and reduction in humidity), and then re-gassing would take place. This cycle would occur throughout the entire daylight (lights on) period.

Gas and Fan Cycles

Enriching the environment with CO2 will require gassing and exhaust cycles. The gassing cycles will only need to occur during the lights on period, as the plants can only benefit from increased levels of CO2 during this period.

So, when the lights come on…

Step one: When the lights first come on, the exhaust fans come on for five minutes to exhaust the stale air out of the growing environment.

Step two: Fans turn off and baffles close.

Step three: CO2 is released for five minutes at required flow rate to ensure 1500ppm of CO2.

Step four: No exhaust for thirty minutes.

Step five: Exhaust fans come on for five minutes

Step six: Go back to step two. This cycle continues throughout the duration of the lightson period.

Providing the growing environment isn’t becoming overheated or too humid this cycle tends to work well. If the growing environment is becoming too humid, or overheated you will need to either adjust the gas/fan cycles (e.g. gas for five minutes, fans off for 15-20 minutes, fans on etc) or improve your controls over the environment to allow for longer fans-off periods. This may mean improving lamp cooling and/or introducing a dehumidifying unit. Increasing the regularity of the gas/fan cycles would mean using more gas, which over a period of time could prove more costly than improving environmental controls.

Some Simple Rules to CO2

  • Plants take in CO2 and release oxygen during daylight hours. At night they take in oxygen and release CO2. Therefore, CO2 enrichment is only of value during the grow rooms daylight hours.
  • Plants can handle higher temperatures than they normally can when the levels of CO2 are significantly higher than standard atmospheric CO2 levels. However, extremely high temperatures are not desirable (e.g. 350C – 95F). While the plants are able to cope with higher temperatures, optimum temperatures between 28-300C (82.4-86F) will result in optimum yields.
  • When air movement ceases (fan/s off cycle), relative humidity (RH) levels within the environment will begin to increase. Transpiration rates of the plant/s and the volume of plant mass within a given space will affect the rate at which this will occur. When RH levels go past approx 65% the plant’s ability to release moisture (transpire) is adversely affected. At 80% RH plant growth is greatly restricted. Enriching the atmosphere with CO2 increases photosynthetic activity (hence transpiration). Because of this RH levels can become unreasonably high within a short period of time. It is important to ensure RH levels aren’t excessive during the gas/fans off cycle, as any benefits gained from a CO2 enriched environment will be negated by the high RH levels. RH levels may affect how you set your gas and exhaust cycle times. Dehumidifying units are extremely effective at reducing RH levels.
  • CO2 is heavier than air. The best method of delivering CO2 to plants is by dropping it directly over them. After this, adequate air movement within the growing environment (through internal air movement) is essential.

Equipment Required – Optimising CO2 in the Grow Room

CO2 Gas

The most efficient means of supplying CO2 to the environment is through the use of compressed CO2 gas (gas bottles).

Different grades of CO2 are available. Always ensure that you are using pure CO2. Food grade CO2 is the ideal. Refrigeration and beer grade CO2 generally contains nitrogen (N) and should be avoided because of this.

Flow Control Regulator Unit with Solenoid

A flow control regulator unit is required for releasing gas from the bottle at the required dosage rate.

Timing Gear/Environmental Monitoring Gear

A unit that controls the gas and fan cycles is required. These vary in sophistication and automation.

Timer Equipment Choices

Many growers have suffered crop losses due to equipment failure. When it comes to purchasing timing equipment think practicality and safety first!!

Choosing the appropriate CO2 timer equipment should be done with careful consideration. It is important to remember that the timer gear that runs the CO2 system (gas release and exhaust fans) can cause major problems if it fails to perform accurately.

It is probably worth pointing out that a single timer unit that controls the whole room (lights, fans, CO2 etc) is not a good idea. Based on my experiences I have found that no one timer unit should control all of the electrical elements in the grow room. This is because, should one part of the timer fail (e.g. light timer or pump timer etc) due to a technical fault you are left without timing equipment for the entire room while repairs are made on the unit. For this reason, it is better to split your timers up into separate, stand-alone units (I.e. a timer that runs the lights, a timer that runs the pump, and a timer that runs the C02 etc).

Low Tech CO2 Timers

In many ways low tech is often best. Low tech, with regards to CO2 timers, refers to a simple programmable unit that turns on the exhaust fans and the gas. A feature that you might want to look for in this type of unit is a decent liquid crystal display that constantly tells you where you are in the gas and fan cycle.

Some of these units will also run the air conditioner. One point to make here is that most modern air conditioners have built in timers and thermostats that will turn on the cooling when necessary (based on the programmed time and temperature). For this reason, the air con point on some timer units is largely unnecessary.

Older air cons (non-thermostatically controlled) may require this feature. Take into account how much draw (amperage) the air con will require, and whether the CO2 timer can handle the draw load. For instance, your CO2 timer may be 10 amps max draw load while your air con requires 12 amps. If this were the case you could easily overload the timer and damage it. You will need the appropriate equipment for the job.

High Tech CO2 Timers – Computer Controlled Auto Dosing Units (Sniffer Units)

More sophisticated units will automatically dose the room, as CO2 becomes depleted, during the fans-off cycle. These units will maintain the atmospheric levels of CO2 within a desired set of parameters (that you set). That is, you programme the unit to auto dose at 1500 – 1800ppm and as the CO2 levels fall below this, the unit automatically adds more CO2 to the growing environment. This means that the optimum levels of CO2 are constantly maintained within the environment, regardless of plant usage and/or leakage rates.

This type of unit incorporates an atmospheric CO2 monitor (“sniffer”) that relays information (CO2 ppm levels) to a computer that regulates further CO2 gas release.

Temperature Control

Lamp Cooling Devices

In order to control the grow room temperature it is necessary to remove lamp heat from the environment without removing the CO2 enriched air. A couple of options are available.

Air-Cooled Shades

Air-cooled shades fully enclose the lamp in a sealed shade. The shade has a glass base, which permits light to penetrate largely unimpeded. Air is drawn through the shade via a fan and drawn out of the environment via ducting. The amount of heat drawn off the lamp relies heavily on the volume of air passing through the shade/s. High volumes of air should result in approximately 95% efficiency (i.e. 95% of lamp heat being drawn out of the grow room environment). Cool tubes or Air Cooled Shades are best suited to the task.

Air Conditioner/Cooler

Air conditioners that recycle the environment’s air (while cooling) are highly desirable where CO2 is in use. This element enables us to control heat build up that can occur due to outside air temperatures and heat from lamps (that is emitted, to some degree, even when cooling devices such as air cooled shades are in use). Ideally, the air conditioner is a refrigerated unit as water based units increase the RH levels within the growing environment (which is less than desirable).


A dehumidifying unit is a great investment for the CO2 grow room. These units draw moisture from air and convert it to water which is captured in a reservoir (and dumped). The use of a dehumidifier enables you to keep RH levels within optimum parameters, reducing the need for fan cycles (and, hence, needlessly venting CO2 from the environment). Because these units reduce the need for fan cycles (and removal of CO2) they can quickly pay for themselves.

Baffle Units for Fans

Baffle units are placed inline on the inlet and outlet fan ducting. When the fans turn on the baffle unit (through an electric solenoid) opens and when the fans turn off the baffle unit closes. Through this method, air is not able to escape from the environment through ducting and, thus, leakage of CO2 from the environment is significantly reduced.

Other CO2 Gas Creating Options

Propane Burner Units : In cooler climates (e.g. Northern Hemisphere) these are an ideal way of creating heat and CO2 in the grow room and greenhouse. Propane burner units are also ideal for winter conditions in places such as New Zealand and Australia (where additional heating is required). However, something that needs to be considered when using propane burner units is that they also add humidity to the atmosphere. Approximately one pound of water is added to the atmosphere for every pound of propane gas that is used. For this reason it is advisable to use a dehumidifier in conjunction with a propane burner unit.

CO2 Breathe Bags: Breathe bags create Carbon Dioxide (CO2) 24 hours a day with no need to refill bottles or use expensive CO2 production units. The power of these breathe bags lie in the mycelial mass inside the vented cultivator. This mycelial mass cultivates CO2 and the microporous breather patch releases CO2 continually for up to 6 months. Rated to support 2-4 plants for up to 6 months

Brewing Beer releases only minute amounts of CO2. This means that it would take forever for the CO2 levels to reach 1500ppm. Remember that your fans will be coming on every half hour in order to cool the environment and to reduce humidity that has been building up during the gas cycle.

Dry Ice is an expensive product given the amounts that are needed to create a CO2 enriched environment. In addition to this, dry ice is nearly impossible to store – it even breaks down in the freezer. This means that you would be visiting your dry ice supplier on a daily basis.

Acids and Sodium Bicarbonate (CO2 Crystals) : This method of creating CO2 gas, in the long run, tends to be more expensive than either using CO2 gas (from the bottle) or propane burners. That is, the volumes of CO2 crystals required to constantly enrich the atmosphere over the lights-on cycle tend to make their cost prohibitive. The key to creating a CO2 enriched atmosphere is to, firstly, release CO2 gas and then contain it at optimum atmospheric levels within the growing environment. Keep in mind that we are also ducting every half hour and therefore, CO2 crystals would be required every half hour to ensure that atmospheric CO2 levels are maintained at 1500ppm. This means that you would be using large amounts of CO2 crystals and would likely be required to be constantly (manually) dosing. For what it is worth, if you are going to create a CO2 enriched atmosphere, invest in gas bottles, regulators and timing gear. This type of equipment has become the standard for a very good reason…. It allows automation and control to a degree where CO2 levels can be constantly maintained within optimum parameters.

Carbon Scrubber and/or Carbon Filter Extraction

Because air remains static for some time in the CO2 grow room, it is advisable to use a carbon scrubber to remove volatile organic compounds (odour) from the air before venting it outdoors.

Other than this it is also advisable to place a carbon filter inline with your extraction fan.

Fan Silencers and Centrifugal Fans

When using carbon filters it is advisable to use either mix flow fans (e.g. Can Max Fan) or centrifugal fans.

Carbon filters will restrict airflow. Therefore, equipment choices pertaining to filter and fan type are important. On this note, centrifugal fans are the ideal fan type for a situation where a carbon filter is in use. Centrifugal fans (due to their design) force air through lines far more effectively than axial or mixed flow fans. However, they also produce more noise than both mixed flow and axial fans due to creating more air pressure. Unfortunately the more air pressure (the more effective the fan) the more noise. For this reason, if grow room noise is an issue, it is advisable to use fan silencers (e.g. Can Silencer)

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