Heating System Selection and Operation
The ideal greenhouse heating system would operate continuously and supply heat at the exact rate at which it is being lost from the greenhouse. This would provide the highest efficiency. Since it is neither practical nor possible to design such a system, choices have to be made from conventional components that approximate such an ideal.
Factors that need to be considered include the design heat loss from the greenhouse, type of fuel, heat transfer medium, type of heating unit, method of heat distribution and the controls. Below are some of the available options.
Determine the Heat Loss
Heat loss from a greenhouse due to conduction depends on the amount of surface area, the heat transfer coefficient and the temperature difference between the desired inside temperature and the coldest outside temperature. Multiplying these three factors together will give a design value for the output needed from the furnace or boiler. To this should be added heat loss due to infiltration of cold air through cracks around the doors and vents. Perimeter heat loss, the loss of heat to the ground underneath and beside the greenhouse, should also be added.
The method of making the above calculations is explained in textbooks on greenhouse management or engineering. Greenhouse manufacturers or suppliers can also provide this calculation.
Which Fuel to use?
With the recent volatility in fuel supply and pricing, fuel selection is important. Natural gas, propane and fuel oil are the most popular fuels. Their availability, ease of handling and high efficiency give them advantages over solid fuels such as wood or coal. Most manufacturers make units that can be fitted with either gas or oil burners. Dual fuel capability is an advantage, should one fuel become unavailable or priced very high in relation to other fuels. Solid fuels should be considered only if they are readily available at a significantly lower cost, as labor and maintenance are higher.
Fuel cost is best compared on a million Btu basis (MBtu). This varies with the heat content of the fuel, efficiency of the heating system and the price of the fuel. Cost comparisons can be made using the following formulas:
The above assumes 80 percent efficiency for fuel oil, 75 percent efficiency for natural gas and propane, 60 percent for coal and 75 percent for woodchips.
Recently, there has been considerable instability in the price and availability of fuels. Some growers have saved money by contracting for their winter fuel needs during late summer when prices are usually the lowest. Others have saved money by switching fuels at peak pricing.
Furnace or Boiler
In both furnaces and boilers, the fuel is burned in a firebox. The heat generated is transferred to the heat exchanger. Distribution throughout the greenhouse is by ducts, fans, pipes or radiators. The exhaust gases, including some heat, is lost up the flue. In a new heating unit, efficiency of the burner is about 85 percent. The efficiency of the total heating system is lower as there are losses in the distribution system.
Greater efficiency can be achieved on a unit that has an induced draft blower rather than the normal gravity stack. Flue gases carrying heat only escape when the blower is on. Increased efficiency can also be obtained by purchasing a unit that uses outside combustion air. This eliminates the use of the moist, warm air from inside the greenhouse and increases the life of the heat exchanger.
In a furnace or heater system, air is the heat transfer medium. A blower or fan moves the air past the heat exchange surfaces. Advantages to using a furnace or heater include the following.
• Heat recovery time is rapid. The air gets heated quickly, but once the furnace shuts off, the greenhouse starts to cool down.
• A good choice where the greenhouse is not operated during the cold weather, as there is no water to drain.
• Lower system cost as heat distribution is by low cost circulating fans or poly ducts.
• Usually takes up less space in the greenhouse.
• Heaters are easy to install, as only a fuel supply and electricity are needed.
• Units with heat output as low as 30,000 Btu/hr are available for small greenhouses.
Radiant heating systems are
used by some growers. They consist of a gas-fired burner, an exhaust pipe and reflector, and a power exhauster. The combustion gases heat the pipe, which radiates heat to the plants below. Although Á there is a fuel savings compared to a conventional system, heat distribution may not be as uniform, especially with tall crops.
In a boiler, water is the transfer medium. The heat exchanger, pipes and radiators are always full of water. Provisions are needed for adding water should small leaks develop and for the expansion of water when it is heated. Steam boilers, another option, are not generally installed in a new greenhouse because of the difficulty of obtaining good heat control and the lack of installation and service people that understand their operation. Advantages of using a boiler system include the following.
• More uniform temperature, as the water in the radiators gives off heat after the burner is shut off.
• Water temperature can be lowered for spring and fall operation, saving energy. Filling the radiators with 180° F water when only a little heat is needed will overshoot the setpoint, wasting heat.
• Providing a different temperature in different sections of the greenhouse (zoning) is easier as pumps are used to move the heated water in the pipes and each pump can be controlled by a separate thermostat.
• Heated air can be provided with a water to air heat exchanger.
• Low temperature water for root zone heating can be provided by the same boiler that provides high temperature water for air heat.
• In larger, gutter-connected greenhouses, a central boiler system requires less maintenance than multiple unit heaters.
• An installation of two smaller boilers will be more efficient to operate and provide a back-up should one unit fail.
There are many systems for distributing the heat in a greenhouse. With a furnace, the simplest and one of the most effective is the use of Horizontal Air Flow (HAF) circulation fans. This system uses 16-20 inch diameter, 1/15th horsepower fans to move the air down one side of the greenhouse and back on the other side, creating a horizontal air circulation pattern. The fans, which operate continuously, mix the air and provide uniform temperature throughout the greenhouse. Heat from one or more furnaces or unit heaters can be added to the air stream anywhere.
Another common system uses perforated poly tubes located in the ridge or under the benches. Heat from the furnace is added before the fan or blower that inflates the tubes.
Distribution of heat with a boiler system can be just as effective. A popular system being installed by many growers today utilizes plastic pipe or rubber tubing placed in the floor or on the benches. Water heated to about 100º F is circulated to maintain a uniform temperature in the plant root zone. As this will not provide all the heat needed by the greenhouse on cold nights, a second heat distribution system is needed.
Unit heat exchangers, similar to the radiator on a car, work well as a primary heat source or as a second zone to a floor heating system. Their low cost, varied sizes and out of the way location fit into most greenhouse set-ups.
In older houses and in many Dutch greenhouses, bare metal pipe (1 1/4-2 inch diameter) is used as a radiator. Its advantage is the uniformity of heat distribution it provides, but because of the large volume of water in the pipes, it can be slow to react to sudden changes in weather conditions.
More frequently, finned pipe is used. Heat transfer from fin is related to the number and size of fins and the diameter of the pipe. One line of fin around the perimeter is equal to 4-6 rows of bare metal pipe.
When selecting a heating system, be sure that both the furnace or boiler and the distribution system are sized to cover your heat needs on the coldest night. On the other hand, there is no advantage in oversizing, as this can reduce efficiency.
Also be sure that the system is installed to meet local codes and recommended practices. Locate the heat sensors at plant height away from wall, doors or other obstructions. Finally, check the uniformity of heat distribution using maximum-minimum thermometers or by observing plant growth.
Poor maintenance can affect the operation of the heating unit and the uniformity of heating. A competent service person should clean and adjust all furnaces and boilers at least once a year, preferably in the fall before the winter heating season begins.
Maintenance should include replacement of the fuel filter, lubrication of pumps and motor bearings, tensioning blower and fan belts, checking gas valves and ignition mechanisms and cleaning heat transfer surfaces. A combustion efficiency test should be run.
In the heat distribution system, radiators and heating pipes should be vacuumed to remove dust that can affect heat transfer. Radiator valves and steam traps should be maintained to eliminate hot and cold spots. Blowers and fans that distribute heat should be cleaned and lubricated to maintain efficiency.
Distribution pipes in unheated areas and underground should be insulated to reduce heat loss. Insulating a 2-inch supply pipe in an unheated area can save up to $4 of heat/linear foot/year.
The stove pipe should have tight joints to prevent leaks of flue gases that could cause sulfur dioxide and ethylene gas injury to plants. The top of the pipe should be at least two feet above the ridge of the greenhouse and have a weather cap to prevent downdrafts.
To get the desired temperature, thermostats should be located at plant height where they are not influenced by drafts or sidewall cooling. It is best if both heating and cooling thermostats are located together in an aspirated box that is painted white to shade it from the sun. A small muffing fan in one end draws air over the sensors.
The use of solid state electronic thermostats or controllers can reduce heating costs. These are not much more expensive than an equivalent number of mechanical thermostats. The savings comes from the smaller differential between the on and off mode, usually 1° F rather than 4-6° F. All temperature sensors should be checked occasionally against a good thermometer for accuracy. Dust, moisture and mechanical damage can affect the setting.