6 Best Greenhouse Heaters

MAYNAGASHEV/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 lists 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.