Greenhouse Heating Options

What are the different options to heat my greenhouse?

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.

Quick Tips on Greenhouse Heating

  • Heaters should be placed so the airflow is above nearby plants. In some cases, this may mean hanging the heater from the ceiling or placing it in an open central location.
  • Growers should only heat the area that they need and can partition off the greenhouse if the full space is not needed.
  • Bubble insulation is a good tool for preventing heat loss. It is best to insulate the north and west walls, any walls that do not have glazing should be insulated.
  • Special precautions need to be taken using heaters at an elevation at or over 2000 ft above sea level.

How does greenhouse heating work?

Heaters can generate three different types of heat:

  1. Conduction heat
    Examples: touching something hot and being burnt, ice cooling your skin
  2. Convection heat
    Examples: hot air rising, cooling, and falling (convection currents), radiator heating in old houses
  3. Radiant (infrared) heat
    Examples: heat from a fire, heat from a light bulb, heat from the sun

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.

Greenhouse Forced Hot Air Heating

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.

Product Feature: Modine PTP 200

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.

Modine PTP 200 Greenhouse Heater

  • Power vented heater
  • Thermal efficiency of 80% 
  • Stainless steel heat exchanger resists corrosion
  • BTU/hr input of 200,000 BTUs and output of 160,000 BTUs
  • 10-year heat exchanger warranty, even in greenhouse applications
  • Permanently-lubricated motor for trouble-free dependability
  • Made in the USA

Propane Heating Pros and Cons

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.

  • Forced air gas furnaces are very common and can be wired into a thermostat. 
  • They create warm air using combustion and push the air through ducts and into the greenhouse.
  • Propane heating burns gas to create heat but also releases CO2 which can help increase yields.
  • Propane is also a very accessible fuel source in most areas.

Using Under-Bench Heating in Greenhouses

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!

Radiant Heat Floors for Greenhouse Heating

A radiant floor heating system consists of 3 major parts 

  1. The boiler and pump
  2. The thermal mass 
  3. The tubing 

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.

Greenhouse Electric Heaters

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.

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