Carbon Dioxide Enrichment

Carbon Dioxide Enrichment will dramatically increase the growth of green plants. Green plants use carbon dioxide ( CO2 ) and water in the presence of light to synthesize organic compounds. The plant then converts these organic compounds into elements that it can use (food). This process is called photosynthesis. If any of these ingredients ( CO2, water or light ) are at levels below what the plant can use for maximum efficiency, it will only be able to perform at that level and no greater. Adding carbon dioxide to a growing environment that is not receiving proper nutrients or is low on light will not produce the desired results. Similarly, plants that are receiving plenty of sunshine and nutrients will only perform as well as the ambient level of CO2 will allow. The ambient level of CO2 in the earth's atmosphere is generally between 300 and 600 ppm. Most plants receive far more water and energy in the form of sunshine than they can use. Increasing the CO2 in the growing area will let the plant use the excess water and energy that is stored in the leaves. The result is a substantial increase in the growth rate of any plant that uses chlorophyll in the process of photosynthesis. Enrichment should commence at sunrise or when photoperiod begins and refrain during darkness hours. The average CO2 level that is recommended is 1000 to 2000 parts per million (PPM). The object is to maintain a constant level in the atmosphere. You can run a small unit continuously during photoperiods or you can use a larger unit and time it to function periodically. Where exhaust requirements are frequent, the latter method is preferable. After the exhaust function the generator can be cycled to replenish CO2 quickly to maintain rapid plant photosynthesis. Use the operation time charts to determine the size of generator and the number of minutes of operation required for the level of CO2 desired. Frequency of recharge depends on the rate of consumption. For the greatest fuel economy and accuracy of recharge it is recommended that the Green Air Products CO2 Monitor Controller (CDMC) be used with all generator and emitter systems.



Carbon dioxide generators operate by burning carbonaceous fuels such as propane or natural gas. The burners used in Green Air Products CO2 generators are specially designed to maximize the production of CO2 and minimize heat as a by-product. Green Air Products generators provide CO2 far more economically than any other means of enrichment. A constant burning pilot is used to ignite the gas when the generator is turned on. A thermocouple is used to monitor the pilot flame. If the pilot flame is lost, a safety valve will close so that unburned fuel will not be released into the enclosure. CO2 Generator operations may be automatically functioned according to actual ambient values when installed in conjunction with the CDMC. This control device continually samples the level of carbon dioxide in the growing area and functions the generator whenever CO2 levels do not meet your predetermined values. Generators are easily installed, dependable and efficient. Transformer included. Operates on 110 VAC.


Choosing the right size Generator

CD-3 through CD-36

Carbon dioxide enrichment equipment and methods have improved as experience and the understanding of atmospheric control and plant physiology has advanced. Plant metabolism and growth rate are dependent on the available atmospheric CO2. Plants are keenly sensitive to CO2 variations and optimum values must be constant and precisely controlled. The method, timing, and size of equipment must be properly determined to provide maximum benefits and economy. Carefully follow the operation charts to choose the proper size generator for the cubic foot area. Keep the recharge times to a minimum for best results. Do not undersize the generator or optimum results may never be achieved.

Size and Timing Charts


CO2 Charge Time


CO2 (ppm)

Total minutes =

(minutes generator must be on to
reach the CO2 level you chose)

This chart will give you minutes of operation required for areas and models shown. Top row of the chart represents the cu. ft. of area (Length x Width x Height) in the enclosure. Left column is model size. Model numbers represent maximum CO2 output per hour in cubic feet (CD-3 = 3 cu ft/hr). Example: A CD-6 will take 10 minutes to produce a CO2 level of 1000 ppm in a 1000 cu. ft. enclosure.

Large greenhouses logically require longer charge times and multiple units may be necessary. The CD-36 takes 1.66 min. per 1000 cu. ft. area. Therefore a 20,000 cu. ft. area would be 20 X 1.66 = 33.2 min. to charge to 1000 ppm. For this application this may be an acceptable charge time. The area should be charged as quickly as possible for maximum advantage. Charging the atmosphere quickly insures that the rapid photosynthesis process will not be interrupted and the plants growth rate reduced Five to ten minutes is a good charge time. Try not to exceed twenty minutes when possible although this is not always practical in very large spaces. Your microclimate will need to be recharged every one to four hours depending on how much atmosphere is lost due to natural convection or plant usage.

CO2 Generator 1000 PPM Operation Time Chart
CU / FT 100 200 400 600 800 1000 1200 1400 1600 1800 2000
CD-3 2.00 4.00 8.00 12.00 16.00 20.00 24.00 28.00 32.00 36.00 40.00
CD-6 1.00 2.00 4.00 6.00 8.00 10.00 12.00 14.00 16.00 18.00 20.00
CD-12 .50 1.00 2.00 3.00 4.00 5.00 6.00 7.00 8.00 9.00 10.00
CD-18 .33 .66 1.33 2.00 2.66 3.30 4.00 4.66 5.33 6.00 6.60
CD-36 .17 0.33 0.66 1.00 1.33 1.66 2.00 2.33 2.66 3.00 3.30
Based on 1000 ppm CO2 enrichment

Click here for printable chart

Fuel Consumption

This chart is used to decide how long a tank of propane will last. This chart is fairly self explanatory first you will look in the left column to pick a model,  then follow it across to the propane tank size you have.  The number that corresponds with the two selections will be the number of hours that you can operate the CO2 Generator.


Generator Installation


Bracket wall mount screw holes must be 10 inches center to center. Mount brackets to wall and housing will slip over screw heads and fit snug into slotted holes in housing back. This allows for easy removal of housing from wall brackets. Make certain bracket screws are secure in wall and use good judgment when choosing mounting location.


Use chain, eyebolts and ceiling hooks found in hardware pack. Hang unit from sturdy location at least 18 inches from ceiling. The unit must operate in level upright position. "CAUTION" Should unit fall during operation fire may result.


Remove four screws holding bottom plate on housing. Locate holes on bracket to match and replace screws .





Test.gif (2552 bytes) 1. Plug the transformer into a grounded 110 V timer and power source.

2. Depress the RED button located on the inside of the generator for 90 seconds to clear the air from the hose.

3. Once gas is present at the pilot let up button and wait 60 seconds for excess gas to clear from the housing. Now depress the RED button again and light the pilot burner. Maintain holding the button down for an additional 30 seconds to allow the thermocouple to heat to operating temperature.

4. Push the front on-off switch to the "ON position. If your timer is also in the on position fuel will pass through the solenoid valve and the main burner will be ignited by the pilot flame. As your timer cycles on and off so will the main burner flame.

5. For the CD-18 and CD-36 , the brass needle valve may be used to adjust the flame size. This will increase the time that the unit needs to charge the space with CO2.



Propane model


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2. Carefully thread the regulator flange nut in the tank valve counterclockwise with your fingers until you feel the flange seat. Tighten firmly with adjustable end wrench. DO NOT USE PLIERS! Fasten hose between regulator and generator gas inlet in same fashion.

3. Turn the propane tank valve to wide open "ON" position.

4. Check for gas leaks. A solution of 25% hand dish soap and 75% water in a spray bottle will work well for detecting gas leaks. Apply solution to all previously connected fittings. Bubbles will occur around loose connections. Always use two wrenches when tightening multiple fittings.

1. For propane applications use a propane tank that has been filled to only 80% of it’s capacity. This is very important for all propane burning mechanisms. Failure to observe this common rule will make your generator hard to ignite and will not stay lit (refer to troubleshooting).

Replacement LPG regulator and hose: GAP propane CO2 generators use a standard small appliance type propane regulator such as might be used with a small heater or barbecue. It can be obtained at most hardware or home improvement stores. Original equipment comes with a Marshall Brass model 140 LPG tank mount regulator. The outlet uses a 1/4 inch male pipe thread to 1/4 male hose fitting to facilitate a flexible hose connection between the regulator and the generator. The regulator inlet uses a standard LP tank POL fitting to connect to the LP tank. The fuel hose is a typical flammable gas type welding hose such as might be used with an acetylene welding torch. These parts are available at a welding supply store. 

Natural gas model


1. Turn gas supply off before you begin work. Now connect a gas shut off valve (not included) onto your incoming gas supply line. If needed reduce to 1/2 male nipple to accept enclosed regulator. Note gas flow direction indicator arrow. Use gas compatible pipe compound on all pipe thread fittings and tighten securely.

2. Connect regulator to shut off valve nipple with compound and tighten.

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3. Check for gas leaks. A solution of 25% hand dish soap and 75% water in a spray bottle will work well detecting gas leaks. Apply solution to all previously connected fittings. Bubbles will occur around loose connections. Always use two wrenches when tightening gas fittings.

Replacement  NG regulator and hose: GAP natural gas CO2 generators use a standard small appliance type NG regulator such as might be used with a residential stove or water heater. It can be obtained at most hardware or home improvement stores. Original equipment comes with a Harper-Wyman second stage small appliance regulator. It is 1/2  P.S.I.G. (with inlet pressure standard relief spring setting is 7.0" W.C). The outlet uses a 1/4 inch male pipe thread to 1/4 male hose fitting to facilitate a flexible hose connection between the regulator and the generator. The regulator inlet uses a standard 1/2 pipe fitting to connect to the NG supply line. The fuel hose is a typical flammable gas type welding hose such as might be used with an acetylene welding torch. These parts are available at a welding supply store. 

Helpful Information

Instruction Manual..................................

Click here for instructions PDF   acrobat-icon.gif (336 bytes)

Q and A .................................................



Gas Conversion Instructions.....................

Click here for instructions PDF   acrobat-icon.gif (336 bytes)

Converting a CO2 generator from propane to natural gas or vice-versa involves changing the pilot and burner orifices and the gas regulator. This involves a conversion kit and partial disassembly of the unit. We strongly recommend that you have this service performed by the manufacturer or one of its authorized repair stations. Should you decide to undertake this modification yourself be advised that you are tampering with flammable gas connections and the manufacturer or its representatives will not be liable and warranty obligations by the manufacturer may be void. Contact your dealer or the manufacturer at 1-800-669-2113 to arrange for conversion services. 

CO2 Generator

Control Diagram

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This example shows the simplest method of CO2 generator control. Set timer for short intervals during light hours only.

This diagram demonstrates an exhaust synchronized system where the CO2 equipment is disabled during exhaust functions and immediately replenished when exhaust is completed. The CT-DH-3P temperature and humidity controller activates the exhaust fan and shuts off the CO2 during the exhaust cycle. It has a built in photo sensor to limit CO2 enrichment to photoperiods only. Enrichment  will only occur when exhaust fan is off and light is present.The CO2 "On" cycles are timed by the Cyclestat repeat cycle timer  which cycles the CO2 equipment according to your preset periods. An example might be that the timer is set for 5 minutes every hour. The CO2 generator would be operated according to that hour schedule. In the event that there is an exhaust function at any time the CO2 will come on for 5 minutes immediately after the exhaust cycle is completed. It will repeat again one hour after that point. Replenishment of CO2 will always follow an exhaust cycle to maintain a constant and continuous enrichment level.

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CO2 Monitor Controller


This system is the ultimate in precise automated CO2 control. The CDM-7001 sensor continuously determines atmospheric CO2 values. The CDDS-2 controller interprets these values and provides flexible set point adjustments to sequence CO2 equipment functions. The CDDS-2 has a built in photo sensor for night time defeat and is disabled during exhaust functions when plugged into CT-DH-3 Atmospheric controller.

The CO2 generator (or emitter system) and the monitor are plugged into the CDDS-2 controller. The CDDS-2 controller and the fan are plugged into the CT-DH-3 as shown.


CO2 Output CD-3 CD-6 CD-12 CD-18 CD-36
Cu Ft CO2 / Hr 3.2 3.4 6.2 6.4 12.4 12.8 18.6 19.2 37.2 38.4
Pounds CO2 / Hr .376 .388 .753 .776 1.51 1.56 2.27 2.33 4.54 4.66
Max BTU / Hr 2183 2172 4366 4343 8733 8686 13098 13029 26196 26058
Fuel Consumption
Cu Ft / Hr 1.03 2.01 2.05 4.02 4.1 8.04 6.15 10.06 12.3 20.12
high-wide-deep (inches) 12-10-5 12-10-5 15-12-7 15-12-7 15-12-7 15-12-7 15-12-7 15-12-7 15-12-12 15-12-12
Weight  (lbs) 9 9 12 12 14 14 16 16 27 27
Weight  (lbs)
11 11 14 14 16 16 18 18 30 30
Transformer 110 / 12vdc 500 mA  center positive





The LPCV-1 liquid propane crossover valve will automatically switch from the empty tank to the full one. This will enable you to fill the empty tank at your convenience and suffer no downtime.


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