CO2 Supplementation and Its Hidden Energy Cost

Why growers add CO2 at all

Plants take carbon dioxide in through their leaves and combine it with water and light to make sugar. Outdoor air carries roughly 300 to 500 parts per million of CO2 depending on season, time of day, and what is nearby [ngma-co2-enrichment]. In a tight growing space the crop can pull the concentration well below ambient during the day, which slows photosynthesis more than enrichment above ambient speeds it up, so the first job of any CO2 strategy is simply to stop the room from running short [ngma-co2-enrichment].

Enrichment above ambient does increase photosynthesis, and the right target depends on the crop, light intensity, temperature, and growth stage [ngma-co2-enrichment]. Greenhouse guidance notes that many crops begin to show undesirable responses above roughly 1,200 to 2,000 ppm [ngma-co2-enrichment], which is why most indoor cannabis rooms run somewhere between ambient and that ceiling. Enrichment only matters during the light period, because that is when the plant is consuming CO2 [ngma-co2-enrichment].

None of that is the expensive part. The expensive part is the side effects of how you make the gas.

Three ways to enrich, and what each does to the room

Compressed or liquid CO2. Cylinders or a bulk tank feed a regulator, a solenoid, and a controller that opens the valve when a sensor reads below setpoint. Liquid CO2 has to be fully vaporized before it enters the room, and handling has to follow the supplier's instructions and local codes [ngma-co2-enrichment]. What lands in the grow space is cold, dry CO2 and nothing else. The electric load is a controller and a solenoid. The cost is the gas itself, delivered by the pound, plus tank rental or cylinder swaps.

Fuel-burning CO2 generators. These appliances burn propane or natural gas and release the exhaust into the room. Gas-fired appliances generate CO2 and water vapor as primary byproducts of combustion, and incomplete combustion can add nitrogen oxides, carbon monoxide, and ethylene, which is why greenhouse guidance insists on third-party certified appliances and on adequate ventilation air for complete combustion [ngma-co2-enrichment]. The fuel is cheap per pound of CO2. Everything else that comes out of the burner is a load on your HVAC.

Boiler flue-gas recovery. Some greenhouses run a hot-water boiler with a flue gas condenser designed for CO2 enrichment, capturing CO2 from heating they were doing anyway [ngma-co2-enrichment]. It is rare in indoor cannabis because most grow rooms are cooling, not heating, but it is the model for the general principle: get the CO2 without putting the combustion heat where the plants are.

The choice interacts with room design. Greenhouse guidance is direct that CO2 enrichment is uneconomical when ventilation rates are high, because the gas leaves with the exhaust air [ngma-co2-enrichment]. That pushes indoor operators toward sealed rooms with recirculating HVAC, and a sealed room is exactly where a burner's heat and moisture have nowhere to go.

What one gallon of propane actually puts into the room

Two things are worth pinning down with numbers: the heat and the water.

Heat. EIA's conversion factor for propane is 3.841 million Btu per barrel, based on NIST enthalpy-of-combustion data [eia-mer-appendix-a]. A barrel is 42 gallons, so that is about 91,450 Btu per gallon. That figure is a gross (higher) heating value, meaning it counts the heat released when the water vapor from combustion condenses back to liquid. In a sealed grow room the dehumidifiers do condense that water, so the room's cooling system ends up handling essentially the full gross figure: the sensible heat directly, and the latent portion through the dehumidifier's coil.

Water and CO2. Combustion chemistry for propane is one molecule of C3H8 plus five of oxygen giving three of CO2 and four of water. By molecular weight (about 44 for propane, 44 for CO2, 18 for water) that is 3.0 pounds of CO2 and 1.6 pounds of water per pound of propane burned. Natural gas (methane) gives 2.7 pounds of CO2 and 2.2 pounds of water per pound of fuel by the same arithmetic. These are stoichiometric figures, not measurements, and a real burner is slightly less than complete, but they are the right order of magnitude for planning.

Put the two together and the trade is clear. To get 3 pounds of CO2 from propane you accept 1.6 pounds of water and roughly 21,000 Btu of heat in the room, using an assumed density of about 4.2 pounds per gallon, a number your propane supplier can confirm for your tank. Tank CO2 gives you the 3 pounds and nothing else.

Where the cost lands on the electric bill

Grow rooms are cooling-dominated. In the Northwest Power and Conservation Council's survey of licensed producers, cooling was 15 percent of total electricity use, ventilation 12 percent, and dehumidification 4 percent, against 66 percent for lighting [nwpcc-cannabis]. A burner adds to the first and third of those.

There are three distinct bill effects, and they are worth keeping separate because they respond to different fixes.

1. More cooling kWh. Every Btu the burner releases has to be removed by the air conditioning, at whatever efficiency your system achieves. Sensible heat goes straight to the cooling coil.
2. More dehumidification kWh. The water vapor has to be condensed out, which is the dehumidification load problem in miniature: latent load that the HVAC pays for at roughly the same efficiency as sensible cooling, sometimes worse if the dehumidifiers reheat the air.
3. Higher peak demand. Burners run during the light period, which starts at lights-on, which is the interval that most often sets the month's billed demand. See demand charges explained. Because the burner's heat is added on top of lighting and HVAC that are already peaking, the extra cooling kW lands in exactly the wrong 15 minutes.

There is a fourth effect for unvented burners specifically. Codes and manufacturers require minimum outside-air changes per volume of fuel burned, to supply oxygen for complete combustion and to keep water vapor and contaminants from building up [ngma-co2-enrichment]. That makeup air is untreated outdoor air that your HVAC has to condition, which in a humid summer is a real load, and it also carries some of your enrichment back outside. Building envelope and weatherization covers why uncontrolled air exchange is expensive in a grow.

A worked example

Every number below is an input we chose so you can follow it. Replace them with your own.

Assume one sealed flower room of 5,000 sq ft, enriched during a 12-hour light period by a single propane burner rated at 20,000 Btu per hour, running continuously during lights-on (real burners cycle, so this is the upper bound). Assume the room's cooling and dehumidification together remove 3 units of heat for every unit of electricity they consume (a coefficient of performance of 3, a round planning figure; your equipment's rating will differ). Assume an all-in electric price of 14.5 cents per kWh, close to Illinois' June 2026 commercial average of 14.53 cents [eia-epm-5-6-a], and a delivery demand charge of 12 dollars per kW.

So one small burner in one room costs on the order of 125 dollars a month in HVAC electricity, on top of the 2.6 gallons a day of propane. Scale it: a facility with eight flower rooms on the same photoperiod, each with a burner, is adding about 16 kW to its coincident peak and roughly 1,000 dollars a month in HVAC electricity to make CO2. Whether that beats tank CO2 depends on what your supplier charges per pound for the roughly 260 pounds a day those eight rooms would otherwise consume, and on whether your dehumidifiers had headroom for another 18 pounds of water per room per day. Sometimes the burner still wins. It just does not win by as much as the fuel invoice suggests.

You can put your own room count, fuel, and rates into the facility energy cost calculator.

How to keep the hidden cost small

• Control to a setpoint, not a timer. A CO2 sensor tied to the controller only fires the burner when concentration drops, so most rooms burn far less than the continuous upper bound above. Greenhouse guidance notes a good metering system pays for itself in gas savings alone [ngma-co2-enrichment].
• Stagger burner operation across rooms. Rooms on opposite photoperiods already split the lighting peak; do the same with burners so their cooling load does not stack in one interval.
• Do not burn into a room that is already overwhelmed. If a room's dehumidifiers run flat out at lights-on, a burner's 18 pounds of daily water is the wrong addition. Fix the HVAC sizing or switch that room to tank CO2.
• Use certified appliances and keep them tuned. Incomplete combustion wastes fuel and produces ethylene and carbon monoxide [ngma-co2-enrichment]; a poorly tuned burner costs you twice.
• Compare against your benchmark. If your facility's kWh per square foot is already above the range in grow facility benchmarks, the burner's HVAC penalty is a candidate for the difference.