Quick Answer: For most small greenhouses, the ExHale CO2 Bag (roughly $35-$60 depending on size) is the simplest fix — a passive, no-power, no-combustion mushroom-culture bag rated to raise nearby CO2 by roughly 500 ppm for about six months. If you want precise, logged control over a CO2 tank and regulator, AC Infinity’s CO2 Controller (about $129) handles the timing and monitoring. For a large walk-in that needs real volume, a 4-burner propane CO2 generator (roughly $390-$480) puts out the most CO2 per hour, at the cost of needing power, propane, and active ventilation planning.
Every growing guide on this site eventually mentions CO2 — our greenhouse ventilation guide covers how airflow can deplete it, and our strawberry growing guide cites a real yield boost from supplementing it — but the equipment that actually adds CO2 back into a sealed structure has never gotten its own comparison here. That’s the gap this roundup fills.
By the numbers: Oklahoma State University Extension research found that a tightly sealed, actively growing greenhouse can see daytime CO2 fall to as low as 200 ppm on a sunny day, down from roughly 420 ppm in outside air — a drop that cuts photosynthesis by about as much as raising CO2 all the way to 1,300 ppm would boost it. Separately, Ohio State University’s Controlled Environment Agriculture Center found that supplementing strawberries up to a 1,000-1,500 ppm range delivered a 15-20% yield boost over unsupplemented plants.
CO2 bag vs. controller vs. propane burner: what’s the actual difference?
All three categories raise CO2 levels, but they solve the problem in very different ways — and that difference matters more than any single spec:
| ExHale CO2 Bag | AC Infinity CO2 Controller | Propane CO2 Generator | |
|---|---|---|---|
| Power needed | None | Standard outlet | Standard outlet + propane tank |
| CO2 source | Mycelium culture (included) | Separate CO2 tank + regulator (not included) | Propane combustion (ongoing cost) |
| Output | ~500 ppm rise nearby, passive | Precise, set to a target ppm | Highest — 10.6+ cu ft/hr |
| Best space size | Up to ~128 cu ft (4x4 ft) | Scales with tank/regulator size | Large walk-ins, 600+ sq ft |
| Byproducts | None | None (tank-fed) | Heat + water vapor from combustion |
| Typical price | ~$35-$60 | ~$129 (controller only) | ~$390-$480 |
If you’re growing in a small enclosed space and don’t want to deal with tanks, propane, or timers, the bag is the whole solution in one box. If you want to dial in an exact target ppm and already have (or are willing to source) a CO2 tank and regulator, the controller is the brains of that setup. If you’re running a large walk-in that needs serious volume, only a burner generator puts out enough CO2 per hour to matter.
Best greenhouse CO2 generators at a glance
| CO2 Generator | Best for | Output | Price | Rating |
|---|---|---|---|---|
| ExHale CO2 Bag (Original/XL) | Best for small/no-power setups | ~500 ppm rise, 6-month life | ~$35-$60 | ★★★★☆ |
| AC Infinity CO2 Controller | Best for precise, logged control | Set-point accuracy (tank-fed) | ~$129 | ★★★★½ |
| 4-Burner Propane CO2 Generator | Best for large walk-ins | 10.6-11 cu ft/hr | ~$390-$480 | ★★★★☆ |
1. ExHale CO2 Bag — Best for Small/No-Power Setups
ExHale CO2 Bag (Original / XL)
- A sealed bag containing a live, non-fruiting mycelium culture that inhales oxygen and exhales CO2 through a microporous breather patch — no electricity, no combustion, no refills.
- Rated by the manufacturer to raise CO2 roughly 500 ppm in the air immediately around it, continuously, for about six months.
- Original size fits spaces up to about 4x4 ft (128 cu ft); the XL version covers more room for a higher price.
- Zero maintenance beyond hanging it near plant height and not puncturing the breather patch — replace the whole bag once production tapers off.
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The ExHale is the easiest entry point into CO2 supplementation precisely because it isn’t a piece of equipment at all — there’s no power draw, no propane to refill, no controller to program, and nothing that can short out or run with a bad flame. That makes it the right pick for a small greenhouse, a grow tent, or anyone who just wants a “set it and forget it” boost rather than a system to manage. The tradeoff is you can’t dial in an exact ppm target or scale output up on demand — it does what it does, passively, until the six-month cycle ends.
2. AC Infinity CO2 Controller — Best for Precise, Logged Control
AC Infinity CO2 Controller (2-Outlet)
- Two independently programmable outlets (up to 1,100W combined) — run a CO2 regulator on one and an exhaust fan on the other, synced to the same target ppm.
- Built-in CO2 sensor with a 12-ft probe cable, plus historical data logging so you can see actual ppm trends over a day, not just a snapshot reading.
- Does not include a CO2 tank or regulator — you supply those separately (a standard 20 lb CO2 tank, refillable at most welding-supply or beverage-gas shops, plus a basic regulator).
- Pairs with AC Infinity's own inline fans and controllers if you're already running their ventilation hardware elsewhere in the greenhouse.
The honest caveat here: the $129 price is for the controller alone, not a complete CO2-delivery system. You’re buying the “brains” — the part that reads actual ppm and decides when to open the regulator or kick on a fan — and sourcing a tank and regulator separately, which is a real added cost and errand most bag or burner buyers skip entirely. What you get in exchange is the only option in this roundup that logs real ppm data over time instead of just promising a number, which matters if you’re trying to actually verify you’re hitting that 1,200-1,600 ppm target rather than guessing.
3. 4-Burner Propane CO2 Generator — Best for Large Walk-Ins
4-Burner Propane CO2 Generator
- Burns liquid propane (natural-gas versions also exist) to produce CO2 directly, rated around 10.6-11 cu ft/hr on 4-burner models — enough for roughly 600-800 sq ft of growing space.
- Runs on a standard 120V outlet to power the ignition and solenoids; needs its own propane supply, not included.
- By far the highest output of the three categories here, and the only one that scales meaningfully for a large commercial-style walk-in.
- Availability note: this is a thin, specialty-retailer product category — several well-known models (including Titan Controls' Ares line) show up as discontinued or out of stock across multiple hydroponics retailers. Confirm current availability before building a plan around a specific model.
A burner is the only option here that adds real heat and water vapor as a byproduct of combustion — useful in a cold-climate winter setup where you want both, a problem in a greenhouse that’s already running warm or humid. Before relying on one, check the flame color: a correctly tuned burner runs blue, and a red or yellow flame signals incomplete combustion and potential carbon monoxide release, not just a cosmetic issue. Pair it with a timer or controller (like the AC Infinity unit above) so it only fires during daylight hours — running CO2 enrichment overnight, when plants are respiring instead of photosynthesizing, wastes propane and can stress the crop rather than help it.
How to choose a greenhouse CO2 generator
- Match the category to your space, not just your budget. A bag covers a tent or small greenhouse; a controller-plus-tank setup scales to whatever regulator and tank size you buy; a burner is really only worth the propane and maintenance for a large walk-in.
- Don’t skip a monitor if you’re running a burner. NIOSH’s 5,000 ppm occupational exposure limit is well above the 1,200-1,600 ppm greenhouses target, so the gas itself isn’t the risk — but a badly tuned flame producing carbon monoxide is. A basic CO2/CO monitor is cheap insurance on top of any combustion-based system.
- Only enrich during daylight. CO2 supplementation helps photosynthesis, which only happens in light — running any of these three options at night doesn’t help plants and, with a burner, actively works against them.
- Check your structure’s air-tightness first. If your greenhouse already vents or leaks air freely, see our greenhouse ventilation guide before buying a generator — a leaky structure won’t hold enrichment long enough to matter.
- Pair CO2 with humidity tracking, not just temperature. Enrichment works best alongside the kind of VPD-aware monitoring covered in our best greenhouse humidity controller picks — CO2, light, and humidity all interact on the same photosynthesis curve.
The bottom line
For most hobby growers, the ExHale CO2 Bag is the best starting point — zero equipment to manage, a real passive CO2 boost, and no propane or electricity required. Want to hit and verify an exact ppm target, go the AC Infinity CO2 Controller route with a separately sourced tank and regulator. Running a large walk-in that needs serious volume, a 4-burner propane generator puts out the most CO2 per hour, as long as you pair it with ventilation awareness and a flame-color check. Whichever you pick, remember CO2 enrichment only pays off alongside good airflow and light — our greenhouse ventilation and humidity controller guides cover the other two legs of that stool.