Heat distribution in the greenhouse depends on
furnace type and location, growing system used,
and crop produced. Direct-fired units in individual
greenhouses generally use air to move heat through
ducts of either galvanized steel or polyethylene
film. Ducts are designed to provide acceptable
distribution and temperature control. Polyethylene
film tubes are available with holes sized and spaced
to provide required distribution. The tubes are
placed overhead for floor crop systems and on the
ground for bench systems. The tubes can also be
used with unit heaters supplied with hot water or
steam from a central boiler.
Bare pipe or finned tube can be used for perimeter,
under bench, beside raised bed, or under floor
heating. A combination of perimeter and under
bench piping in either bare pipe or finned tube can
provide even distribution of heat.
It is best to provide at least two stages of heat
supply to permit closer control of temperatures.
Motorized proportioning valves on water or steam