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CO2 for Free

By David Oppenheimer

We’ve all heard that there is no such thing as a ‘free lunch’; well the process described in this article may be as close to one as you get. Most of us already know the benefits of CO2 enrichment for photosynthesis. To maximize indoor growing and greenhouse potential, CO2 is supplemented to maintain an approximate level of 1500 ppm, this can require frequent trips to an industrial gas supplier and/or a lot of propane or natural gas use, and related costs. It is ironic that many indoor farmers are exhausting CO2 to the outdoors from home heaters and hot water heaters while simultaneously releasing or generating CO2 for an indoor grow room or a greenhouse.

Propane and Natural gas burn clean enough that small non-vented gas devices have been approved for indoor use. These gas burning devices all use oxygen (in the air) to burn the gas, resulting in the by-products of CO2, H20 (humidity), and heat (Reusch). Exhaust from gas appliances can provide 3 essential conditions for maximized growth: humidity, temperature, and CO2 level. Most heat from the exhaust is removed by the heat exchanger of the furnace or water heater; resulting in mildly warm exhaust. Photosynthesis for many plants, including marijuana, in a CO2 enriched environment,is most efficient around 85 degrees F.

If a large gas heater’s exhaust is diverted into the grow area, there is great potential for all the oxygen to be burned up or displaced, as well as CO (carbon monoxide accumulation), resulting in toxic air conditions. With proper equipment, the CO2 from your gas furnace and/or water heater’s exhaust can be safely used to supplement the CO2 used in your grow room. This will save time and money, make the plants grow great, reduce fuel use, and dramatically reduce the amount of ‘Greenhouse’ gasses released into the environment. Using this technique, you will help prevent global warming while optimizing growing conditions. The key to doing this safely and effectively is to divert enough exhaust from your gas appliance into your grow area to maintain a CO2 level of 1500 ppm, and have additional exhaust directed outside.

“The Occupational Safety and Health Administration (OSHA) and the American Conference of Governmental Industrial Hygienists (ACGIH) have set workplace safety standards of 5,000 ppm” and very high CO2 levels can cause undetectable asphyxiation when O2 in the blood is replaced by the CO2 (Minnesota Department of Health). Keep a CO (carbon monoxide) detector in the grow room for safety in case equipment malfunctions! Do not attempt this project if you use oil or kerosene heat, which do not burn cleanly! The trick to tapping into this source of unused CO2 from gas furnace and water heaters is power dampers. A power damper is a duct section with a flap that opens and closes the flow through the duct and is powered by electricity. Some dampers close with current applied and others are designed to open. Most dampers are low voltage so the right size transformer must be wired inline to the damper; there are some 110volt dampers. Quality dampers will seal much better than cheap dampers. This simple addition to a CO2 enrichment system will pay for itself many times over (especially with today’s fuel prices) and reduce house or business emissions into the environment, making your project ‘greener’. It is necessary to have a CO2 levels monitor hooked to a controller (sequencer) to tell the dampers (by applying power) when CO2 is needed and when the threshold has been reached. You can still use your controller to run your CO2 generator and or regulator.

Find the exhaust pipe of your gas furnace or hot water heater. These devices should already be properly vented. Turn off your gas appliance while working on this. Disconnect (or cut open) a section of duct where it will be closest, with least bends, to tap in and route a new duct to the grow area. The few items needed can be found at most heating supply stores. If you can not find matching dampers for the size and type of duct that you have, you may have to convert the duct to a size or type that you can find dampers for. Using a “Y” connector and a power damper that closes when power is added, hook up inline with the duct running to the outside. For smooth flow, install “Y” so exhaust comes in the ‘bottom’ of the “Y” duct section. Now take the damper that opens when power is applied, attach it to the other “Y” opening. Run a duct from this ‘power open’ damper into the grow area above the plants, since CO2 is heavier than air; but you probably already know this. Now all you have to do is either wire the dampers together or use a multi-outlet adapter and plug them into your CO2 sequencer along with your CO2 generator or CO2 tank regulator using a 3 outlet adapter.

If the pipes are far from the outside wall or roof there may be inline duct booster fans. If the grow room is not getting flow through the new duct, a inline duct booster fan may be needed, especially if there is one used on the original duct that exhausts to the outside past the new “Y” section from the appliance. If you add a duct booster fan, wire it or plug it in together with the dampers, they will then power on and off together. Many furnaces will have an adequate exhaust blower so an additional duct booster fan will not be necessary. Keep an eye on any booster fans (if any) on the original duct between the “Y” junction and the outside, there is a chance they could overheat if running when the damper to the outside is closed. Once this is setup, when your CO2 sequencer determines it’s time to add CO2 to the room and switches power on, the exhaust damper to the outside will close and the damper to the grow room will open; resulting in the furnace’s or hot water heater’s exhaust being diverted into the grow room. The CO2 generator or release regulator will be working also, this way the room is guaranteed to always have just the right amount of CO2 even if the furnace or water heater is not currently in use. When the proper level is reached and the sequencer powers off, the damper leading into the grow room will close and the damper in the duct leading to the outside will reopen. All exhaust at this point will vent outside until the room needs more CO2. For safety, be sure all circuits and/or outlets are supplying no more than 80% of their rated load in watts, and are properly wired. Also, be sure to secure duct well. The 200°F rated duct tape holds up better than the regular (for connecting duct sections).

If you are diverting the hot water heater exhaust for this CO2 enrichment supplement, you can further take advantage of this setup by timing showers, dishes and laundry, as the lights come on (time that enrichment equipment runs the most) and during the light cycle in general.

Using this system, farmers will find they make less trips to get propane or CO2 tanks filled, and are spending less money, while the levels in the grow room remain the same.

This addition to an enrichment system will also reduce the volume of CO2 released into the environment from the house or business. The CO2 diverted to the room is used by the plants during photosynthesis, further reduces the gas appliance’s CO2 release into the environment. Using this system, the room will reach the desired CO2 level quicker, and fluctuate less, further enhancing growth.

Works Cited List:

Minnesota Department of Health

This page, located on the Minnesota Department of Health's website, is a good source to show the adverse health effects high levels of C02 will cause. As best as I can tell, this site is run by the State government. The information on this page is consistent with other sources also describing the harmful health effects caused by too much CO2 in air. This page, while short, clearly presents figures and dangers, agreed upon by government scientist, that are associated with high CO2 levels. The statement, "At very high levels, 30,000 ppm and above, CO2 can cause asphyxiation as it replaces oxygen in our blood.” clearly demonstrates the potentially fatal condition that elevated CO2 levels can cause.

“Carbon Dioxide (CO2)” Minnesota Department of Health. Mar. 2004.

27 June 2005

Reusch, William. "Reaction of Alkanes" Michigan State University

This page clearly describes the physical process of propane combustion. After reviewing a dozen sources of propane and natural gas combustion, I found this page to have most precise, in-depth yet understandable descriptions of the potential reactions of propane combustion. Although the article cites no references for the information it contains, the information is consistent with common knowledge and other reference materials and is on the Chemistry Department’s web site of the Michigan State University. By showing the structural formulas for the reactions mentioned along with clear reasoning demonstrating why the reactions may differ, this source will allow a reader of my essay to understand what products can be created by propane combustion. This article shows how CO2 and H2O are the direct products obtained by the burning of propane when sufficient O2 present.

Reusch, William. “Reactions of Alkanes” Michigan State University Department of Chemistry 1999. rev. 2004. 28 June 2005

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