Greenhouse Heat Distribution
Heat is one of the several factors that control plant growth. Getting the heat from the furnace or boiler firebox where it is generated so that each plant is uniformly warm is a challenge that faces the heating system designer. With many systems to choose from the decision can be difficult.
Hot air systems
Furnaces are usually less expensive than boilers. They directly heat the air in the greenhouse. Distributing the heated air is largely dependent upon some means of air circulation. Systems include unit heaters, perforated inflated tubes and horizontal air flow circulation.
Unit heaters, either floor mounted or suspended from the frame, have a fan or blower that moves the greenhouse past the heat exchanger. A single unit will work well in a small free-standing greenhouse up to about 60' long. In larger greenhouses either two heaters or some other means of moving the air is required.
Poly tube - By connecting a perforated poly tube to the outlet of the furnace heat can be moved and distributed over a larger area. This generally requires a blower rather than a fan to get the higher pressure needed to overcome the friction loss in the tubing. Tube diameter, hole size and spacing are critical to get uniform distribution from the tube. The tubes can be placed on the ground between rows of plants, located under benches or suspended above the crop.
Horizontal air flow (HAF) utilizes circulating fans to create a horizontal air pattern of air movement within the greenhouse. The system uses 12" – 20" diameter, 1/15th horsepower fans to move the air down one side of the greenhouse and back the other. The fans, which operate continuously, mix the air from roof to floor and provide uniform temperature throughout the growing space. They are usually located above head height and spaced from 40' to 50' apart down each side of the greenhouse. The heat source can inject the air anywhere within the air stream. The air moves at 50 to 100 feet per minute. Fan cost is about $200 each installed.
Infrared systems, properly installed, can transfer heat energy to crops without the necessity of air circulation. The system is usually located near the peak of the greenhouse so that it can radiate heat to the crop. Sizing the system should be done by the supplier. Air circulation may be needed to get uniform heat on tall or dense crops.
Furnaces have the advantage that the greenhouse can be closed during the winter without having to drain a series of water pipes as is the case with a boiler system.
Hot water systems
In larger greenhouses, hot water is a better medium of heat distribution than air. Its advantages include energy saving temperature modulation for different seasons of the year, different water temperatures for air and root zone systems and more constant and uniform heat. Providing a different temperature in adjacent bays or sections of the greenhouse is easier as circulating pumps controlled by separate thermostats supply heat as needed. Heat from water can be distributed by bare steel pipe, fin radiation, root zone heat or water to air unit heaters.
Bare steel pipe is rarely used in U.S. designed greenhouse heating. Its low heat output, large volume of water and space requirements are disadvantages. It has been replaced by fin pipe.
Fin pipe is best located along the perimeter walls. The walls behind the pipes should be insulated with an inch or two of board type insulation with an aluminum foil facing. Fin pipe can also be installed under gutters where it can be controlled manually to melt snow. Fin pipe with numerous thin plates radiate heat away from the pipe and increase heat output by 5 to 10 times. The diameter of pipe and size and number of plates determine the heat output. A 1-1/4" pipe with 38 – 4-1/4" square fins/foot has an output of about 1200 Btu/hr per linear foot with 180ºF water.
Low output fin pipe (two or five linear fins) with 100 to 200 Btu/hr per linear foot is a good choice for under bench heating. It is easy to install as sections are connected with rubber gasketed couplings or high temperature rubber hose. Low output fin is also used overhead instead of bare steel pipe and for gutter heat.
Root zone heat is popular as it warms the root zone rather than the air. It can be installed on or under the benches or in the floor. Commercially available systems use EPDM rubber or cross-linked polyethylene (PEX) tubing either as single tubes or as multiple tubes attached to a web. The tubing is connected to plastic or copper headers. Hot water can be provided by a hot water heater or boiler. Most growers that used this system operate with a water temperature of 100ºF to provide a75ºF soil temperature. A remote bulb thermostat or sensor located in the soil controls the supply water circulating pump.
Water to air unit heaters convert hot water to hot air. Circulation within the greenhouse is by HAF fans. Available in outputs from 12,000 to 700,000 Btu/hr, unit heaters are low cost and easy to install. Supply pipes should be insulated to reduce heat loss. Unit heaters work well as a supplement to root zone heat by providing the extra heat needed on cold nights.
Achieving uniform temperature throughout the greenhouse will allow the thermostat to be set lower. For each one degree that the thermostat is lowered, there is an approximate three percent savings in fuel.
John W. Bartok, Jr., Extension Professor Emeritus & Agricultural Engineer, Department of Natural Resources and the Environment, University of Connecticut, Storrs CT - 2013