6 Best Greenhouse Heaters

You've given your plants shelter from the winter by building a greenhouse. Now you need a heater to take away the chill. Here are some options.

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Electric oil heater in greenhouse with seedlings of plants, plantingMAYNAGASHEV/GETTY IMAGES

Buying a Greenhouse Heater

A greenhouse protects seeds and plants from the elements and keeps them warm. But when the temperature drops, even a well-insulated greenhouse needs a little help.

A greenhouse heater provides the extra warmth plants need, but it’s important to choose wisely. Oversize the heater and your plants will bake. Undersize it, and you’ll be paying for gas or electricity without the benefits.

Unfortunately, solar greenhouse heaters aren’t available yet. If you want to go solar, the best you can do is install a solar array, then purchase a separate battery pack and use that to run a conventional electric heater. Otherwise, your energy choices are propane, natural gas, paraffin and electricity.

Unless you’re growing tropical plants, your greenhouse heater doesn’t need to maintain the same temperature as your home. But it’s important to set a target temperature based on what you’re growing. That will help determine the type of heater and heat output you need.

You should also consider the following:

  • Size: By this we mean the heat output in Btu/h (British thermal units per hour). Calculate the required size by multiplying the surface area inside the greenhouse by the heat loss value of the covering material. The lower this value is, the better the material insulates. Multiply these by the difference between the outside temperature and the target temperature in the greenhouse. A size calculator like the one at ACF Greenhouses┬álists heat loss values of common covering materials and makes this calculation for you.
  • Fuel source: If you want to use natural gas or electricity, your greenhouse must to equipped for it. Otherwise, your options are propane or paraffin. Paraffin isn’t as easy to get as it used to be, so paraffin heaters aren’t common.
  • Ventilation: Some propane and natural gas heaters need good ventilation. Many burn so efficiently that ventilation isn’t an issue, although the small possibility of carbon monoxide off-gassing means you should install a corbon monoxide detector. Electric heaters have no emissions and don’t need ventilation.
  • Static or forced-air: A static heater distributes heat by convection, which is fine for a small greenhouse. A large greenhouse in a cold climate may need a forced-air heater.
  • Thermostat controls: A heater with a thermostat can self-adjust to maintain a steady temperature. If you make frequent trips to the greenhouse, you may not need this feature, but it’s nice to have.
  • Fire safety: Greenhouse heaters are sometimes left unattended for long periods, so most electric ones have a breaker that trips when the unit begins to overheat. Some gas heaters monitor oxygen levels and switch off when those levels fall too low. And most heaters, whether gas or electric, switch off automatically when they tip over.
  • Splash protection: Many electric heaters are splash-proof, so they won’t be affected if you spray them while watering plants.
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Bio Green Pal Ecomm Via Amazon
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Best All-Around Greenhouse Heater

Designed for small greenhouses, the Palma Bio Green Greenhouse Heater puts out 5,200 Btu, enough to heat 120 square feet. This electric space heater has a built-in circulation fan and draws 1,500 watts of electricity on a 120-volt circuit.

Lightweight and sturdy, this splash-proof unit features an auto-shutoff in case of overheating. A detachable 0 to 90 degree thermostat can be removed from the heater housing and mounted at eye level. It earns top marks from Amazon raters and reviewers.

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Dr Heater Greenhouse Garage Workshop Ecomm Via Amazon
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Best Budget Electric Greenhouse Heater

The Dr. Heater DR218 Infrared Heater offers a single 1,500-watt setting, temperature sensor and a thermostat to cycle the unit off and on and maintain a constant temperature. It heats up to 150 square feet. This bare-bones, no-nonsense heater can also be used in fan-only mode on warmer days to keep air circulating around tender plants.

It’s sturdy and extremely portable, so you can move it around to direct radiant heat to plants that need it most. It automatically shuts off to prevent overheating.

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Comfort Zone Fan Forced Ceiling Mount Heater Ecomm Via Amazon
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Best Large Electric Greenhouse Heater

It needs to be hardwired into a 240-volt circuit, but the Comfort Zone Ceiling-Mount Heater delivers 17,065 Btu of heating power while drawing 5,000 watts of electricity.

This unit hangs from the ceiling, from where the powerful blower circulates heat throughout a large area. The adjustable thermostat control offers settings of 3,000, 4,000 and 5,000 watts, and the adjustable vents let you direct the heat where it’s needed. It switches off automatically to prevent overheating.

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Mr Heater Indoor Safe Heater Ecomm Via Amazon
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Best Propane Greenhouse Heater

The Mr. Heater F232000 Buddy Heater offers two heat settings that deliver 4,000 and 9,000 Btu. It the higher setting, it can warm a 225-square-foot area. It works with a one-quart or five-gallon propane bottle, though the latter requires an optional hose. The smaller bottle provides heat for a few hours, and the larger one a few days.

This heater is rated for indoor use and doesn’t require ventilation. It switches off automatically if it tips over or detects low oxygen levels.

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Pelonis Oil Filled Radiator Heater Ecomm Via Amazon
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Best Greenhouse Heater for Mild Climates

Oil-filled heaters are mostly for indoor space heating. But they can provide enough ambient heat to protect plants from frost in a 120-square-foot greenhouse when temperatures dip below 32 degrees.

The Pelonis Portable Space Heater features three power settings and uses electricity to heat an oil-filled radiator. It switches off when the oil warms enough to radiate heat to the surroundings, which saves energy. Unlike many oil heaters, this one has an onboard fan that circulates the warm air throughout the space.

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Bluegrass Living Propane Gas Space Heater Ecomm Via Homedepot
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Best Natural Gas Greenhouse Heater

If you have natural gas piped into your greenhouse, the Blugrass Vent-Free Space Heater can take advantage of a cheaper energy source than electricity or propane.

This heater delivers 10,000 Btu, enough to warm 500 square feet, and can be mounted to a wall or the floor. It features a push-button starter and a built-in thermostat that cycles the heater on and off to maintain a steady temperature. It switches off automatically when oxygen falls below a safe level.

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Chris Deziel
Chris Deziel has been active in the building trades for more than 30 years. He helped build a small city in the Oregon desert from the ground up and helped establish two landscaping companies. He has worked as a carpenter, plumber and furniture refinisher. Deziel has been writing DIY articles since 2010 and has worked as an online consultant, most recently with Home Depot's Pro Referral service. His work has been published on Landlordology, Apartments.com and Hunker. Deziel has also published science content and is an avid musician.