CO2 Generator vs. Tank System

CO2 enrichment in a greenhouse requires choosing between two delivery methods: a combustion-based generator (burning propane or natural gas to produce CO2 as a byproduct) or a compressed CO2 tank with a regulator and solenoid. Each has clear advantages and disadvantages that depend on your greenhouse size, climate, and growing goals.

Generators: Cheap CO2, Extra Heat

CO2 generators burn propane or natural gas, producing CO2, water vapor, and heat. The CO2 cost per pound is very low — a $20 propane tank produces CO2 for weeks in a hobby greenhouse. The byproduct heat is welcome in winter (free supplemental heating) but problematic in warm weather (adding heat to an already warm greenhouse). Generators also increase humidity, which may require additional ventilation or dehumidification.

Tanks: Pure CO2, No Byproducts

Compressed CO2 tanks deliver pure CO2 with no heat, no moisture, and no combustion byproducts. A regulator and solenoid valve release CO2 at a controlled rate, and a CO2 controller maintains the target concentration precisely. Tank refills cost $15 to $30 for a standard 20-pound cylinder — more expensive per pound of CO2 than generator fuel, but with zero unwanted byproducts.

Which System Fits Your Greenhouse?

Choose a generator if: your greenhouse is 500+ square feet (lower cost per CO2 at scale), you need supplemental heat (winter use), and you can manage the moisture output. Choose a tank system if: your greenhouse is under 500 square feet (more practical at small scale), you do not want additional heat or moisture, or you need precise concentration control for sensitive crops. Both systems require a CO2 controller for safe and effective operation.

CO2 Enrichment Basics

Ambient outdoor CO2 concentration is approximately 420 ppm. Most greenhouse crops benefit from enrichment to 800 to 1,200 ppm during sealed morning hours when vents are closed and photosynthesis is active. Above 1,500 ppm, most crops show no additional benefit and some (lettuce, herbs) may develop physiological stress. Human safety limits apply at 5,000 ppm — always use a controller to prevent over-enrichment.

CO2 enrichment only benefits plants when other growth factors (light, temperature, water, nutrients) are not limiting. Adding CO2 to a poorly lit winter greenhouse provides little benefit because photosynthesis is already constrained by low light. The greatest returns from CO2 enrichment occur during spring and fall shoulder seasons when light levels are moderate to high but the greenhouse is sealed enough to retain the enriched atmosphere.

Controller Selection

Both generators and tanks require a CO2 controller for safe and cost-effective operation. Controllers measure ambient CO2 concentration with an infrared sensor and activate the CO2 source when levels drop below the target. Quality controllers cost $200 to $400 and include high-level alarms, data logging, and integration with ventilation controllers (which shut off CO2 delivery when vents open, preventing waste).

For hobby growers who want CO2 enrichment without the controller cost, a timer-based approach provides approximate enrichment: set the CO2 source to activate for short intervals during sealed morning hours. This approach is less precise than controller-based management but provides some benefit at a fraction of the equipment cost. Monitor plants for signs of over-enrichment (leaf curling, necrotic spots on thin-leaved crops) and reduce timer intervals if symptoms appear.

Neither system is maintenance-free. Generators require periodic burner cleaning and combustion safety checks. Tank systems require regulator inspection, solenoid testing, and periodic tank refills — which means transporting a heavy cylinder to and from a gas supplier. Choose the system whose maintenance requirements best fit your routine. For growers who already use propane for greenhouse heating, a generator shares the fuel supply and feels natural. For growers who prefer clean, simple equipment, a tank system is less intrusive and requires no combustion in the growing space.

Safety considerations favor tank systems in small, tightly sealed greenhouses where combustion byproducts (carbon monoxide, nitrogen oxides, water vapor) can accumulate to problematic levels. Generators must be properly vented and maintained to ensure complete combustion — incomplete combustion produces carbon monoxide, which is dangerous to both plants and the grower. A CO detector is mandatory in any greenhouse using a combustion-based CO2 generator. Tank systems produce zero combustion byproducts, making them inherently safer in enclosed spaces where the grower spends significant time working.