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Design & Construction

Section 2: Types of Greenhouse Heating Systems

Greenhouses may utilize central heating systems or local heating systems. Central heating systems generate heat energy, most often using a large boiler, in one location, and distribute that heat energy to many locations. Local heating systems are located in the greenhouse or greenhouse section that they are responsible for heating. For large operations, a central heating system may be more efficient than a local system. However, the cost of installation and maintenance of a central heating system can be high. For smaller operations, this expense may be hard to justify. The size of the boiler unit required, the fuel source or sources available, size of the greenhouse operation, and maintenance costs all must be considered when deciding whether to use a central or local heating system.

Central Heating Systems
Central heating systems may distribute heat energy as either hot water or steam. In the former, hot water is produced in a central boiler by burning a fuel such as coal, wood chips, fuel oil, liquid propane, natural gas or various types of waste oils. Where coal and wood are used as fuels, the fuel must be mechanically or manually supplied to the boiler fire box. Where oil, propane, natural gas or other oils are used, a constant automated supply line is used to supply fuel to the boiler fire box. Boilers are designed to have a very high Btu output capacity with most boilers having heating capacities of over 1,000,000 Btu/hr. The hot water, usually at 180° F if not pressurized, is produced in the boiler and then pumped through distribution pipes or tubes around the greenhouses. Because only 1 Btu of heat is provided by each pound of water as it cools 1° F, large volumes of water are required with a hot water-based central heating system. In addition to the pipes required for distribution of the hot water throughout the greenhouse, pumps are required to circulate the hot water through the pipes. In some greenhouse operations, pressurized hot water is used. This allows the hot water to be delivered at a higher temperature (typically 203° F). The higher temperature allows the water to carry more heat energy which reduces the volume of water required (the higher temperature means a given volume of water carries more heat energy) and thus reduces the boiler size and plumbing required.

A central heating system may also use steam. In this system, steam is produced in the boiler. The steam then moves in pipes throughout the greenhouse. The steam moves through the pipes due to the pressure of the steam, and pumps are not required as with a hot water system. Because the steam is at a temperature of 215° F, it contains more heat energy than hot water and thus less water and plumbing are required as compared to a hot water-based system.

Most central heating systems use steel, copper or aluminum pipes to distribute hot water through the greenhouse. The hot water or steam heats the pipes which in turn give off the heat to the air in the greenhouse (distribution pipes are discussed in more detail under heat distribution). Others may distribute the hot water through rubber tubes placed directly in the concrete floors of the greenhouse. The hot water-filled rubber tubes heat the greenhouse floors which in turn release heat energy to the greenhouse atmosphere. This system is referred to as floor heating or sometimes as radiant heating (not to be confused with radiant heating units discussed under "Local Heating Systems". The floor-based radiant heating system is a particularly good system where plant material is being grown on the floor since it places the heat close to the substrate (soil) and helps to maintain a warmer substrate temperature.

Some greenhouse facilities are able to take advantage of being located near electrical plants or factories that produce steam or hot water. Waste steam or hot water is diverted from the factory to the greenhouses where the heat is utilized in a central heating system. This practice is referred to as co-generation. The advantage of co-generation to the greenhouse company is that a supply of lower cost heat is obtained. The advantage to the factory or electrical plant is that the greenhouse company helps to cool the waste hot water (which must be done before release or reuse), and additional income is derived from the sale of the waste heat energy.

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Local Heating Systems
There are many types of local heating systems. However, their defining characteristic is that they are located within the structure that they are designed to heat. These systems typically will not have the Btu generation capacity of a central boiler and different types of localized heating systems will utilize different types of fuels. Some of the most common local heating systems include unit heaters, convection heaters and radiant heaters.

Unit heaters
These types of heaters are also called forced-air heaters (i.e. Modine® or Reznor® heaters). Unit heaters come in various capacities but units producing 100,000 to 400,000 Btu/hr are most common. Unit heaters commonly burn natural gas or propane but may also burn kerosene or various types of waste oils that are automatically provided to the firebox through a supply line. In these types of heaters, the fuel is combusted in a fuel box, and the hot exhaust is passed through thin-walled metal tubes or hollow panels known as heat exchangers. Heat energy is transferred from the exhaust to the metal of the heat exchanger as the exhaust is vented through the heat exchanger and out of the greenhouse. A fan behind the unit draws greenhouse air over the heat exchanger and the heat is transferred from the heat exchanger to the greenhouse air. The hot air may be blown directly into the greenhouse or forced through a polyethylene tube (jet tube) that runs the length of the greenhouse. Some types of unit heaters do not have heat exchangers. These types of unit heaters are often used in animal confinements (i.e. chicken houses), garages, factories, or workshops. If operating at 100% efficiency, all of the fuel burned in the unit heater is converted to heat energy and CO2. However, if not functioning at 100% efficiency or of the units malfunction, dangerous carbon monoxide (CO) or plant-damaging ethylene gas (C2H4) maybe produced and exhausted into the greenhouse (see "Atmospheres" learning unit for more information). For this reason, it is generally recommended that unit heaters used in greenhouses have a heat exchanger.

Unit heaters require oxygen in order to burn fuel efficiently. If the greenhouse is too tight, oxygen levels may be too low for the unit heater to operate most efficiently.  As a rule of thumb, one square inch opening near the heater should be provided for each 2500 Btu/hr capacity. Often, this is accomplished by running an outside airline to the heater. Most new unit heaters have as part of their standard design such an air line. It is best to work with the manufacturer of the unit heater to insure proper installation and maintenance.

Convection heaters
These are relatively low cost heating units that are usually used in hobby and small commercial greenhouses. These types of heating units typically are used to burn wood, coal or agricultural waste products. They have no internal heat exchanger as do unit heaters. Fuel is burned in a large firebox. The exhaust from the firebox is vented into a large pipe that goes out and down the length (or partially down the length or down the length and back) of the greenhouse before being vented outside. As the hot exhaust moves through the pipe, the heat energy is transferred from the hot exhaust to the exhaust pipe and then to the greenhouse air. In this case, the exhaust pipe essentially serves as the heat exchanger.

When using a convection heating unit, it is best to have a fan at the exhaust end of the vent pulling air out (negative pressure in the pipe) of the exhaust pipe. This pulls the exhaust through faster allowing for more even heat distribution and if a leak in the pipe occurs, greenhouse air will be pulled into the pipe rather than allowing exhaust (which would contain dangerous CO and plant-damaging C2H4) to escape into the greenhouse. The same requirements for oxygen need to be followed for these heaters as for unit heaters.

A significant problem with this type of convection heater is that the firebox is at one end of the greenhouse and the exhaust is typically vented through a single pipe traversing the length of the greenhouse. This can result in significant issues with temperature uniformity and hot and cold spots in the greenhouse are common when using this type of heating unit. Horizontal air flow fans (see Heat Distribution later in this learning unit) may be used to improve temperature uniformity. Another problem with convection heating systems is that fuel (i.e. wood) must be constantly supplied to the firebox and this typically must be done manually for these systems.

Another type of heating unit is actually a hybrid between a convection unit heater and a unit heater. In this type of system, the unit is located at one end of the greenhouse on the floor, has a large firebox and burns coal or wood as in the case of a standard convection heater. However, the exhaust from this unit is vented directly to the outside of the greenhouse. The unit has internal pipes that are heated by the burning of the fuel and a fan behind the unit forces air through the pipes where it is heated before being forced out the front of the unit (similar to a unit heater). Therefore, this type of unit is essentially a large unit heater, placed on the greenhouse floor that is designed to burn wood, coal or other agricultural waste products. The problem with temperature uniformity still occurs but can be overcome to a great degree through the use of horizontal airflow fans. Additionally, as with standard convection heaters, fuel must either be continuously (usually manually) supplied on a constant basis to the firebox.

Radiant heaters
These heaters are composed of an aluminum tube with a reflector. Fuel (typically natural gas or some type of manufactured fuel) is combusted within the tube so that the tube reaches a temperature of approximately 900° F. At this temperature, the tube emits infrared radiation. The reflector directs the radiation downward. When the radiation strikes a surface (i.e. plants, benches, etc.), the surface absorbs the radiation, and it is converted to heat. After warming, these surfaces give off heat to the greenhouse atmosphere. Because the surfaces are heated first, and the air is heated by convection from surfaces, the air in a radiantly heated greenhouse can be up to 7° F colder than the surfaces. However, because less energy is wasted heating the entire air volume of the greenhouse, radiant heating units may reduce heating costs by 30 - 50%. However, the initial set up costs can be expensive, and radiant heating units must be placed in such a way that cold spots do not occur in the greenhouse. High-flow horizontal air flow fans are not used to circulate air as this speeds the loss of heat from the surfaces to the atmosphere. However, low-flow horizontal air flow fans or poly-tube fans may be used to maintain some air circulation in the greenhouse.

Solar Heating
Although some of the heat requirement of a commercial greenhouse will be met by incoming solar radiation during the day, this method is not used significantly in commercial greenhouses. The major reasons are cost, degree of control, and reliability. However, solar energy is used in some hobby greenhouses.

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