Coal & Char

Propane Heater in a Greenhouse

You need to hold a greenhouse above freezing on propane without cooking the plants or filling it with combustion products. Here is the fix, then the heaters that suit it.

By Stephen V.Last updated How we pick

How this is funded:we earn a commission if you buy through our links, at no extra cost to you. It never changes which pit or heater we recommend, and we’ll tell you when we’d skip one. Full disclosure.

The problem

A greenhouse is a thin-walled box with a huge glazed area, so it loses heat almost as fast as you put it in — and it is also nearly airtight when you shut the vents. Those two facts pull in opposite directions. The heat loss makes you want a big heater. The airtightness makes a big unvented heater genuinely dangerous, because everything burning propane produces carbon monoxide along with the heat, and a sealed greenhouse has nowhere to put it.

The second problem is the plants. Burning propane produces water vapor, and a lot of it. In a cold greenhouse that vapor condenses on the glass and sits on the leaves, which is exactly the condition mold and botrytis want. Growers who lose plants to a propane heater usually lose them to humidity and still air, not to heat.

The fix, in five steps

  1. Decide what you are actually heating for.Frost protection — holding above 32°F — is a completely different job from keeping a greenhouse at 60°F all winter. Almost every hobby grower needs the first. Size for that and the heater gets small, cheap and easy to run.
  2. Ventilate at two heights, permanently. Air in low, air out high. A single cracked vent is not enough, because you need a flow path rather than a gap. This is the step people skip and it is the one that makes the rest safe: it carries combustion products out and lets the water vapor go with them.
  3. Put a carbon monoxide alarm inside, at head height. Not optional, not expensive, and the only thing in this list that will tell you something is wrong before you feel it. Replace it on the schedule printed on the back.
  4. Insulate before you upsize. Bubble-wrap glazing film on the north-facing panes, a sealed door, and a thermal blanket over the most tender bench will each do more per dollar than another 10,000 BTU. Heat you do not lose is free.
  5. Add a small circulation fan. A heater warms the air near it; a fan makes the greenhouse one temperature instead of four. It also keeps the leaf surfaces drier, which is the single best thing you can do for plant health in a heated greenhouse.

Do those five and a modest propane heater will hold a hobby greenhouse through a frost comfortably. Skip step two and no heater on this page is safe to use in there.

What to look for on the heater itself

  • An oxygen depletion sensor. An ODS shuts the burner off when the oxygen level in the space drops. In a confined, poorly ventilated volume this is the feature that matters most, and it is why the Portable Buddy is our pick rather than a bigger, cheaper radiant heater.
  • A tip-over switch. Greenhouses are full of trip hazards, hose runs and pots. A heater that cuts out when it falls over belongs in one.
  • Modest output, ideally adjustable. 9,000 BTU for a small greenhouse, 18,000 for a bigger one. The adjustable 50,000 to 80,000 BTU convection heaters are for polytunnels and commercial spans, not an 8 by 10 glasshouse.
  • Radiant or convection, chosen deliberately. A radiant heater warms what it points at, which suits protecting one bench. A convection heater warms the whole volume, which suits holding an entire tunnel above freezing. Our portable propane heater picks explain the difference in more detail.
  • A 20 lb tank connection. Running a greenhouse heater on 1 lb cylinders is expensive and means changing bottles in the cold. A hose to a 20 lb tank outside the greenhouse is cheaper per hour and keeps the fuel out of the structure.

When propane is the wrong answer

Three cases, stated plainly, because a product page that recommends a purchase in all circumstances is not much use:

  • You have power at the greenhouse. A thermostatic electric heater adds no moisture and no combustion products, and can be left unattended with far less thought. For overnight frost protection this is simply the better tool. Our propane versus electric comparison covers the running-cost side.
  • You cannot ventilate it. If the structure has no openable vent at high level and you are unwilling to add one, do not run an unvented propane heater in there at all.
  • You need it overnight, unattended, every night.That is a vented or externally mounted heater’s job, or an electric one. The vent-free Blue Flame unit is the closest thing here to a permanent install, and even that wants a fresh-air provision and a CO alarm.

If none of those apply, propane is a perfectly sensible way to keep a greenhouse alive through a cold snap — and it is by far the cheapest way to get a lot of heat to a structure with no electricity. Just buy the small one, open the vents, and fit the alarm. For the broader safety argument that sits behind all of this, read is a propane heater safe indoors.

The propane heaters that suit a greenhouse

Sized for the problem above, not for maximum output. A greenhouse is a small, badly insulated volume with living things in it, and the mistake almost everyone makes is buying far too much heater.

Quick picks

Ranked on published specs, beginner-friendliness and value. Select a row to jump to the full write-up. We haven’t cooked on every one of these — here is exactly what we do instead.

#ProductBest forPrice
1
Mr. Heater 9,000 BTU Portable Buddy Radiant Propane Heater

Mr. Heater 9,000 BTU Portable Buddy Radiant Propane Heater

The Portable Buddy is the one radiant propane heater almost everyone in cold-weather outdoor hobbies already owns, and it runs on a 1 lb cylinder rather than a 20 lb tank — which is what makes it portable in the real sense. Nine thousand BTU is small next to a patio heater but enormous inside a tent, an ice shanty or a blind. Mr. Heater builds in a tip-over switch and an oxygen depletion sensor. Use a carbon monoxide alarm with it anyway, every time.

Best for a small greenhouse
$89.99 · View on Amazon

Price as of October 27, 2026. #ad How we’re funded

2
Mr. Heater 18,000 BTU Big Buddy Heater

Mr. Heater 18,000 BTU Big Buddy Heater

Double the Portable Buddy's output and the ability to run two 1 lb cylinders at once, which turns a two-hour heater into an all-evening one. It is the right size for a garage, a workshop, a large tent or a covered porch — still not an open patio. The same caveats apply and apply harder at 18,000 BTU: combustion heater, enclosed space, carbon monoxide alarm, every single time.

Best for a larger greenhouse
3
Mr. Heater 30,000 BTU Vent-Free Blue Flame Dual Fuel Heater

Mr. Heater 30,000 BTU Vent-Free Blue Flame Dual Fuel Heater

Blue flame means it heats the air rather than radiating at objects, which is what you want when the job is holding a whole garage at a temperature instead of warming one bench. It is vent-free, so the ventilation rules apply exactly as they do to any unvented propane heater, and it mounts to a wall rather than following you around.

Best permanent install
4
Dura Heat LPC80 50,000-80,000 BTU Propane Convection Heater

Dura Heat LPC80 50,000-80,000 BTU Propane Convection Heater

A convection heater rather than a radiant one, which is the distinction that decides whether this is the right buy. Radiant heaters warm what they point at; this one warms the air in a 360-degree column and lets it circulate, so it is the better answer for an open garage, a barn or a large tent where you want the whole volume warmer rather than one bench. The listing states an adjustable 50,000 to 80,000 BTU range. It is a lot of combustion in one place — ventilation and a CO alarm are not optional.

Best for a polytunnel
$136.37 · View on Amazon

Price as of October 27, 2026. #ad How we’re funded

#1Best for a small greenhouse

Mr. Heater 9,000 BTU Portable Buddy Radiant Propane Heater

The Portable Buddy is the one radiant propane heater almost everyone in cold-weather outdoor hobbies already owns, and it runs on a 1 lb cylinder rather than a 20 lb tank — which is what makes it portable in the real sense. Nine thousand BTU is small next to a patio heater but enormous inside a tent, an ice shanty or a blind. Mr. Heater builds in a tip-over switch and an oxygen depletion sensor. Use a carbon monoxide alarm with it anyway, every time.

What’s good

  • Runs on a 1 lb cylinder — genuinely carryable
  • Tip-over shutoff and oxygen depletion sensor built in
  • 9,000 BTU is a lot of heat in a small enclosed space
  • The long-standing default in cold-weather outdoor hobbies

Trade-offs

  • Far too small to warm an open patio
  • 1 lb cylinders are expensive per hour of heat
  • Still a combustion heater: use a CO alarm, always
  • Not rated for use in every enclosed space — read the manual
TypePortable radiant propane heater
Heat output9,000 BTU (per the listing)
Fuel1 lb propane cylinder
SafetyTip-over switch and oxygen depletion sensor, per the maker
SeriesMidnight Series

Specs read from the product listing, on October 7, 2026. “Not published” means the maker doesn’t state that figure there.

#2Best for a larger greenhouse

Mr. Heater 18,000 BTU Big Buddy Heater

Double the Portable Buddy's output and the ability to run two 1 lb cylinders at once, which turns a two-hour heater into an all-evening one. It is the right size for a garage, a workshop, a large tent or a covered porch — still not an open patio. The same caveats apply and apply harder at 18,000 BTU: combustion heater, enclosed space, carbon monoxide alarm, every single time.

What’s good

  • 18,000 BTU covers a garage or a large tent properly
  • Runs two 1 lb cylinders, or a 20 lb tank with the right hose
  • Variable output rather than one fixed setting
  • Same proven Mr. Heater safety features

Trade-offs

  • Still a combustion heater — a CO alarm is mandatory, not optional
  • Hose and filter for a 20 lb tank are extra
  • Heavy and bulky next to the Portable Buddy
  • Not a substitute for a patio heater in open air
TypePortable radiant propane heater
Heat output18,000 BTU (per the listing)
FuelTwo 1 lb cylinders, or 20 lb tank with a hose (hose not included)
ModelBig Buddy

Specs read from the product listing, on October 7, 2026. “Not published” means the maker doesn’t state that figure there.

#3Best permanent install

Mr. Heater 30,000 BTU Vent-Free Blue Flame Dual Fuel Heater

Blue flame means it heats the air rather than radiating at objects, which is what you want when the job is holding a whole garage at a temperature instead of warming one bench. It is vent-free, so the ventilation rules apply exactly as they do to any unvented propane heater, and it mounts to a wall rather than following you around.

What’s good

  • 30,000 BTU published on the listing
  • Blue flame convection warms the air, not just objects
  • Runs on propane or natural gas
  • Wall mounted, out of the floor space

Trade-offs

  • Vent-free: ventilation is mandatory, not optional
  • Ships in two to three days rather than immediately
  • Heated area not published as a figure
  • Fixed installation, not portable
TypeVent-free blue flame gas heater
Output30,000 BTU (per the listing)
FuelPropane or natural gas (per the listing)
MountingWall (per the listing)
Heated areaNot published

Specs read from the product listing, on October 13, 2026. “Not published” means the maker doesn’t state that figure there.

#4Best for a polytunnel

Dura Heat LPC80 50,000-80,000 BTU Propane Convection Heater

A convection heater rather than a radiant one, which is the distinction that decides whether this is the right buy. Radiant heaters warm what they point at; this one warms the air in a 360-degree column and lets it circulate, so it is the better answer for an open garage, a barn or a large tent where you want the whole volume warmer rather than one bench. The listing states an adjustable 50,000 to 80,000 BTU range. It is a lot of combustion in one place — ventilation and a CO alarm are not optional.

What’s good

  • Adjustable 50,000-80,000 BTU per the listing — real range, not one setting
  • Convection heats the air all round, not just what it faces
  • Runs off a standard 20 lb tank
  • Cheaper per BTU than any cabinet patio heater

Trade-offs

  • Needs serious ventilation and a CO alarm
  • Tall, open-flame tower — not a heater to put near anything
  • No thermostat control
  • Overkill, and wasteful, for a patio or a single person
TypePortable propane convection heater
Output50,000-80,000 BTU, adjustable (per the listing)
Fuel20 lb propane tank
Heat patternConvection (360 degrees)
Indoor useNot published

Specs read from the product listing, on October 13, 2026. “Not published” means the maker doesn’t state that figure there.

A note on fire safety. Clearances and extinguishing steps on this page follow Smokey Bear’s campfire-safety guidance. Your local burn rules, HOA and the manual that came with your unit all override anything here. We’re enthusiasts sharing what we’ve researched, not fire officers — when in doubt, follow your local fire authority.

Frequently asked questions

Can you use a propane heater in a greenhouse?

Yes, and growers do it routinely — but only with real ventilation. Unvented propane puts water vapor and combustion products into the air along with the heat, and a sealed greenhouse concentrates all three. Leave a vent open at both a low and a high point, keep a carbon monoxide alarm inside, and never run an unvented heater in a greenhouse you are working in with the vents shut.

What size propane heater do I need for a greenhouse?

Far less than people expect. A hobby greenhouse of roughly 8 by 10 feet is usually served by something in the 9,000 BTU class if the goal is holding above freezing, not growing tropicals. Doubling the output does not double the result — it mostly means short, hot cycles, big temperature swings and more moisture. Size for frost protection and add a thermostat or a timer instead.

Will a propane heater hurt my plants?

Clean, well-adjusted propane combustion produces carbon dioxide and water vapor, which plants tolerate. Incomplete combustion is the problem: a dirty, starved or badly maintained burner can produce by-products that damage foliage, and the extra humidity raises the risk of mold and botrytis on dense planting. Keep the burner clean, give it air, and keep air moving inside.

Is it safe to leave a propane heater on overnight in a greenhouse?

Not an unvented portable one, unattended, on a frosty night with the vents shut — that is the scenario that produces carbon monoxide incidents. If you need overnight frost protection, the safer arrangements are a vented or externally mounted heater, or an electric heater with a thermostat. Where propane must run overnight, keep permanent ventilation open, fit a CO alarm, and check it before you sleep.

Propane or electric for a greenhouse?

Electric if you have power out there: a thermostatic electric heater puts no combustion products and no moisture into the air, and it can be left unattended with far less thought. Propane wins when there is no supply, when you need a lot of heat quickly, or when a cable run would cost more than the heater. Many growers use propane for cold snaps and electric for the baseline.

Does a propane heater add humidity to a greenhouse?

An unvented one does — water vapor is a direct product of burning propane, and in a sealed greenhouse it shows up on the glass and in the soil. That is the strongest argument for ventilation beyond the safety case: moving air, not just warm air. A small circulation fan does more for plant health in a heated greenhouse than another few thousand BTU.

Sources

  • CPSC — Carbon Monoxide Information Center — The US Consumer Product Safety Commission on carbon monoxide: “More than 200 people in the United States die every year from accidental non-fire related CO poisoning associated with consumer products,” and its standing recommendation to “Install battery-operated CO alarms or CO alarms with battery backup on every level of the home and outside sleeping areas” (accessed October 7, 2026)
  • 16 CFR § 233.1 — Former price comparisons (FTC Guides Against Deceptive Pricing) — The federal standard for a legitimate “was” price: it must be “the actual, bona fide price at which the article was offered to the public on a regular basis for a reasonably substantial period of time.” A price is fictitious “where an artificial, inflated price was established for the purpose of enabling the subsequent offer of a large reduction” (accessed October 7, 2026)

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