Greenhouse heaters are needed when you plan to grow year-round in your greenhouse, and when spring and fall temperatures are cooler than your crop can tolerate. Choosing a greenhouse heating system is a fairly simple task. The most common greenhouse heating systems include forced air, radiant heat, and under-bench heating.

Heaters can generate three different types of heat:

Heaters are rated by BTUs (British Thermal Units), and forced-air heaters will also be rated for CFMs (cubic feet per minute). Higher BTU rated forced-air heaters also have higher CFM ratings. BTUs and CFMs needed should be calculated based on the greenhouse area. Ceiling fans, vent tubing, and thermostats are often used to accessorize and improve the performance of heating systems.

Forced hot air systems involve a heating coil and a fan behind it that turns on when the coils heat up to temperature and push the heat out into the greenhouse. Several fuel types may be used, including liquid propane, natural gas, or hot water heated by solar or wood boilers.

These systems are used for many reasons. First, for effective humidity control, the greenhouse air must be heated quickly, then purged with an exhaust fan. Second, forced hot air systems heat up the greenhouse very fast, so in instances with air leaks or other poor insulation factors, this is the only system that can keep up.

Forced hot air is not the most efficient way to heat the greenhouse, however, because you need to heat the air (which is the worst conductor of heat) in order to heat the objects within.

This Modine PTP 200 greenhouse heater is just one of their models. Other models are available for different greenhouse sizes. These heaters are great for winter greenhouse heating needs and are also very affordable.

Commercial greenhouses typically use propane gas combustion heaters. Forced air propane heaters are one of the least expensive ways to heat a greenhouse. However, if they are not vented they will create humidity inside your facility. Another downside to inexpensive propane heaters is that the light from the open flame is not contained which can be problematic for crops that require light deprivation periods.

High-end propane models that use tubular heat exchangers address this problem. They contain the light from the heat source, also acting as a conductor of heat. A fan mounted behind these tubes blows through the tubes pushing hot air into the greenhouse. These heaters are also vented which helps prevent humidity increases.

Under-bench heating is a very effective way to accomplish root zone heating. Bench heating is very similar to a traditional radiant heat floor system mentioned above with the exception that it is installed on the underside of a greenhouse bench system rather than inside a thermal mass such as a concrete slab.

For obvious reasons, this means that greenhouse benches are needed in order to install the system. Rolling greenhouse benches are an incredible way to increase the amount of canopy space. Rolling benches can increase cultivation space by up to 40% by eliminating the aisles.

With the addition of added cultivation space, under-bench greenhouse heating is a good option to consider. Under-bench heating should be used in conjunction with a forced hot air system but allows for a forced-air thermostat reduction of 5-10 degrees!

A radiant floor heating system consists of 3 major parts

Radiant heating in greenhouse floors works fairly simple. First, the boiler heats up water and pumps it through the tubing buried inside a thermal mass such as a concrete slab. Next, the heat then rises out of the thermal mass and heats the greenhouse.

Radiant heat floors provide many benefits besides just heat. They guarantee the heat is distributed to every part of the plant because it starts from the bottom and rises naturally. Radiant heat floors also help keep the floor dry by evaporating any water or puddles on the floor (beneficial for pest control and safety). Another great benefit to greenhouse heating is the long term cost savings from the efficiency of heating a thermal mass such as a concrete slab.

Since the thermal mass is heated by water rather than air, the heat exchange is much more effective. Multiple fuel types may be used in conjunction with radiant heat floors, or as a stand-alone source to heat the water such as liquid propane, natural gas, wood, or solar.

Electric heaters are a good option for small spaces. They use electric current to heat elements and move the hot air with a fan, creating convection heat. Electric heaters are convenient since they can just be plugged into electricity, but they require a significant amount of power to operate and are generally not very efficient.

Fun Fact: A BTU (British Thermal Unit) is the approximate amount of energy needed to heat one pound of water one degree Fahrenheit.