Soil temperature is more critical for plant growth than air temperature. For example, growth of tomatoes will stall when the soil temperature is 56 degrees F or less, but begin to grow when the soil temperature hits 58 degrees F. Therefore, releasing heat energy at ground level is usually more efficient than heating the air above the foliage, especially during cloudy periods when the beds are not being heated by solar energy. The convection tube heating system shown in the photos below delivers warmed air from a biomass furnace outside of the greenhouse to the ground near the crop's root system.

Note that furnaces that have a blower fan are recommended when heat is dischared into a convection tube. A furnace with a propeller fan needs an extra 'assist' fan, such as the 16" tube fan shown in Picture 3 below. Also, do not undersize the diameter of the heat discharge tube from the furnace because it can cause the furnace to overheat.

Picture 1

Two rows of tomatoes on December 1st on each side of a 12" diameter convection tube placed at ground level.

Picture 2

The heat distribution network is held up by a horizontal 1/4" cable. The 4-outlet junction box (in center of picture) is not a requirement and was difficult to construct; the heat could have entered the header line from one side of the tunnel and then downward through 2 tees and 1 ell to the-6 mil poly tubes.

Picture 3

16" variable speed fan provides an air boost to insure adequate inflation of the long convection tubing.

Picture 4

Three inflated convection tubes laying on the ground beds after the fall tomato crop was removed. It was a mistake to use white on black plastic; we will only use black plastic mulch in the future in the tunnel.

Picture 5

A side view of the heat distribution network is shown here. Note the insulated ducting. The larger tube is from a biomass furnace to the 24" fan that inflates the tubes.

Picture 6

This is a homemade 12" diameter convection tube under propagation pallets in the greenhouse. It was simple to build.