For most climates, there exists at least a period of time during the year where the ambient temperatures outside are too low for crop production, or the low temperatures would result in significantly reduced crop productivity. This is the primary reason behind greenhouse-based agricultural production, whether it is for ornamental plants, forestry, or vegetable crops. Therefore, providing heat energy to maintain optimal temperatures within the greenhouse (or hotbed, growth chamber, etc.) is a critical function in greenhouse management.

Heat energy may be measured as calories, joules, and even horsepower. However, most often it is measured as British thermal units (Btu). A Btu is the amount of heat energy required to raise 1 pound of water 1° F.

In greenhouse heating, we are concerned with replacing heat at the rate that it is lost from the greenhouse. There are 3 ways that heat is lost from greenhouses:

Greenhouses may utilize central heating systems or localized heating systems. Central heating systems generate heat, most often using a large boiler, in one location, and distribute that heat to many locations. Localized 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 localized system. However, the cost of installation and maintenance of a centralized heating system can be high. For smaller operations, this expense may be hard to justify. However, the size of the boiler unit, the fuel source, size of the operation, and maintenance costs all must be considered when deciding whether to use a centralized or localized heating system.

Central Heating Systems

Central heating systems generally distribute heat as either steam or hot water. Hot water is produced in a central boiler and is usually pumped through the greenhouse at 180° F. Because only 1 Btu of heat is provided by each pound of water as it cools 1° F, large volumes of water are required. Additional disadvantages of a hot water system include the complicated plumbing and circulating pumps. An advantage of hot water systems is that if the boiler fails, the hot water in the pipes acts as a heat reservoir for a short period of time.

Steam systems require less plumbing, no circulating pumps and provide steam at 215° F. More Btu can be provided by steam than hot water. Steam systems allow for more rapid temperature adjustments but do not provide heat reservoirs. In many European, and in some U.S. greenhouses, pressurized hot water is used. This allows the water to be delivered at a higher temperature (203° F). The higher temperature allows for a reduction in the volume of water required and thus reduces the boiler size and plumbing required.

Central heating systems most commonly use wood, coal, oil, natural gas, or liquid propane as fuel sources.

Some greenhouse facilities are taking 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 (co-generation). The advantage to the greenhouse is that a supply of lower cost heat is obtained. The advantage to the factory or electrical plant is that the greenhouse 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 heat source.

Localized Heating Systems

There are many types of localized 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.

Unit heaters

These types of heaters are also called forced-air heaters (i.e. ModineÒ or ReznorÒ heaters). In these types of heaters, the fuel is combusted in a fuel box, and the hot exhaust is passed through thin-walled metal tubes (heat exchangers). Heat is transferred to the metal, and the exhaust is removed from the greenhouse through an exhaust stack. A fan behind the unit draws greenhouse air over the hot metal tubes, and the heat is transferred from the metal tubes 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. These types of heaters require oxygen in order to burn efficiently. If the greenhouse is too tight, the flame can go out. This can result in a loss of heating capacity and can cause carbon monoxide build up in the greenhouse. Carbon monoxide can cause sickness or death for people working in the greenhouse. Malfunctioning units may also generate ethylene that can be very damaging to plants. As a rule of thumb, one square inch opening near the heater should be provided for each 2500 Btu capacity. Often, this is accomplished by running an outside airline to the heater. Heaters that do not have a heat exchanger (and blow the hot exhaust directly into the greenhouse) should be avoided. It is best to work with the manufacturer of the unit to insure proper installation and maintenance. These systems may use kerosene, liquid propane, or natural gas as a fuel source.

Convection heaters

These are low cost unit that are usually used in hobby and small commercial greenhouses. They have no internal heat exchanger as do the unit heaters. The exhaust from the firebox moves through the exhaust vent or pipe that is routed through the greenhouse. As the hot exhaust moves through the pipe, the heat is transferred to the pipe and eventually to the greenhouse air. 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 to escape into the greenhouse. The same requirements for oxygen need to be followed for these units as for unit heaters. This type of unit may use wood, coal, oil, kerosene, or natural gas as a fuel source.

Radiant heaters

These heaters are composed of an aluminum tube with a reflector. 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. Horizontal airflow fans are not used to circulate air as this speeds the loss of heat from the surfaces to the atmosphere. However, low-flow fans or poly-tube fans may be used to maintain some air circulation. Radiant heaters are usually designed to burn natural gas or manufactured fuels.

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.

After heat is generated, it must be distributed throughout the greenhouse facility. Even distribution of heat, without having cold or hot spots, is an important but often neglected aspect of greenhouse heating. Uneven temperatures can result in uneven crop growth rates, variation in maturation times, and can affect substrate drying rates.

When a central heating system is used to produce hot water or steam, a system of pumps and pipes are used to distribute the heat energy throughout the greenhouse. Pipes may be made of cast iron, aluminum or copper. Hot water is usually supplied to the greenhouse at 180° F or 203° F if it is pressurized. Steam is usually supplied at 215° F. Because less resistance occurs in moving steam, smaller diameter pipes can be used for steam as compared to hot water. Also, because steam is delivered at a higher temperature, it provides more Btu's than hot water per linear foot of pipe. Therefore, fewer pipes are needed when using steam.

When using a central heating system, placement of pipes is very important in order to minimize heat loss and maximize heating efficiency. The actual arrangement of pipes depends upon the amount of pipe needed to provide enough Btu's to heat the greenhouse. This depends upon whether steam or hot water is being used and the type of pipe being used.

Stacked pipes (placed in layers) are less efficient than single pipes. If pipes are stacked, additional pipes will be required to compensate for the reduced efficiency. As is demonstrated in the table above, the higher the temperature of the pipe or the greater the diameter of the pipe, the greater the number of Btu given off per linear foot of pipe. Finned pipes are more efficient at heat transfer than smooth pipes due to their increased surface area. Finned pipes may transfer four or more times the amount of heat as a smooth pipe. The disadvantage of finned pipes is that they release more intense amounts of heat in a small area. In new greenhouses, steam or hot water pipes are usually placed under benches. This provides heat close to the plants and helps to maintain warm soil temperatures.

When steam or hot water is used for heating, the steam or hot water must first be distributed through pipes and then the heat given off by the pipes must be distributed. When unit heaters are used, heated air is directly discharged into the greenhouse. The primary concern then becomes one of evenly distributing the heated air. If unit heaters are used to heat greenhouses, a temperature gradient can occur along the length of the greenhouse. Temperatures will be warmer closer to the unit heater. Several strategies may be employed to minimize the temperature gradient along the length of the greenhouse. First, use multiple unit heaters placed at opposite ends of the greenhouse in a counter opposed position. In addition to providing heat at both ends of the greenhouse, the counter opposed airflow assists with mixing of the greenhouse air and improves temperature uniformity. Where relatively long greenhouses are used, horizontal air flow fans (HAF) may also be added to help move warm air down the length of the greenhouse and to further promote the mixing of air. In some cases, the unit heater may be mounted in the gable of the greenhouse and connected to a polyethylene jet tube. The warm air is first forced down the length of the jet tube. After the jet tube fills, the warm air evacuates the tube from holes along its side. Jet tubes may also be used in combination with HAF fans.

Often in spring and fall, heating is required at night and cooling required during the day due to solar heating. The fan of the unit heater and polyethylene tube can be used (with fire box and fuel source turned off) for cooling in these situations if outside louvers are included. The louvers located behind the unit heater are opened and the fan turned on so that cool outside air is forced into the polyethylene tube. Additionally, the louvers can be closed and the fan turned on to improve internal greenhouse air circulation.

If a central heating system is used and the pipes placed under greenhouse benches, the warm air rises, and as it does, it cools. This creates temperature stratification. Cool air in the gable area of the greenhouse descends down the inside walls of the greenhouse and creates cold pockets along the greenhouse walls. Horizontal airflow fans placed below the gable area force warm air to move down the length of the greenhouse and prevent temperature stratification.

Some specialized heating systems use polypropylene or rubber tubes to circulate hot water in close proximity to the root system of the plants. There are many variations to this system, but the most common is the BiothermÒ system. This system is most commonly used for increasing substrate temperature during propagation rather than general greenhouse heating.

There are many considerations when deciding upon a fuel source. These include availability, cost, price volatility, pollution regulations, storage requirements, equipment requirements, boiler requirements, and maintenance requirements.

The underlying principle for determining the heating requirement of a greenhouse is to replace the Btu that are lost from the structure (expressed on a per hour basis) so as to maintain the temperature within a desired range. Typically, heat loss through radiation is ignored since the amount is negligible. Therefore, only heat loss through conduction and infiltration/exfiltration need to be determined. These losses are determined for the coldest expected temperatures occurring at night. These provide maximum values for the heating capacity that should be required during the coldest time of the year. During the day when solar input provides additional heat energy or during warmer times of the year, the full heating capacity may not be utilized.

h_t = h_c + h_sa

where: h_t = total heat loss

h_c = heat loss by conduction

h_sa = sensible heat loss by mass transfer

h_c = AU(t_i-t_o)W

where: h_c = heat loss by conduction

A = surface area

U = heat transfer coefficient

t_i = desired inside air temperature

t_o = minimum outside air temperature

W = wind correction factor

h_sa = 0.02(t_i-t_o)(V)(M)(W)

where: h_sa = sensible heat loss by mass transfer

t_i = desired inside air temperature

t_o = minimum outside air temperature

V = greenhouse volume

M = air exchanges per hour

W = wind correction factor

As an example, the Btu requirement of an A-frame greenhouse is determined below. The greenhouse is a 40 ft x 100 ft, glass-glazed, metal frame greenhouse, and of tight construction. The gable is 8 ft from the eave to the peak. It has an 8 ft wall with 2 ft of the wall being a 6-inch concrete block curtain wall. The maximum expected wind velocity is 15 mph. The minimum expected low temperature is 0^oF, and the minimum desirable inside temperature is 60° F.

The surface area glazed with glass is 8000 ft², and the surface area for the curtain wall is 560 ft².

The volume of the structure is 48,000 ft³.

Therefore:

h_c = 8000(1.13)(60)(1.0) + 560(0.51)(60)(1.0) = 559,536 Btu/hr

h_sa = 0.02(60)(48,000)(1.08)(1.0) = 62,208 Btu/hr

h_t = 559,536 Btu/hr + 62,208 Btu/hr = 621,744 Btu/hr

An example is outlined below for a 30 ft x 100 ft quonset greenhouse (covering is 47 ft wide), with a double polyethylene covering. The maximum expected wind velocity is 15 mph. The minimum expected low temperature is 0^oF, and the minimum desirable inside temperature is 60° F.

The following equations can be used to estimate surface area and volumes of quonset greenhouses:

Circumference of a circle = 2πr

Area of a circle = πr²

Total surface area of a cylinder = (2πrH) + (2πr²)

Volume of a cylinder = πr²H

Therefore:

The surface area glazed with polyethylene film is 5409 ft².

The volume of the structure is 35,325 ft³.

and:

h_c = 5409(0.70)(60)(1.0) = 227,178 Btu/hr

h_sa = 0.02(60)(35,325)(0.70)(1.0) = 29,673 Btu/hr

h_t = 227,178 + 29,673 = 256,851 Btu/hr

Greenhouse design

Minimizing the exposed surface area can reduce heat loss. This is primarily accomplished through the use of gutter-connected designs.

Glazing selection

Heat loss can be reduced by selecting a glazing with low thermal conductance values.

Wall insulation

Heat loss may also be reduced by including insulated curtains walls along the lower three to four feet of the greenhouse walls.

Thermal screens

Polyester, cloth, or polyethylene screens that can be pulled closed at night reduce heat loss through the roof panels of the greenhouse.

Windbreaks

Windbreaks reduce the effect of wind on heat loss. However, windbreaks (i.e. high walls or trees) can also reduce light entering the greenhouse if placed too close to the structure.

Close air leaks

Broken panels, loose panels, poorly sealed doors, and other openings in the greenhouse structure increased the mass air flow (infiltration and exfiltration) and increases heat loss.

Equipment Maintenance

Regardless of the type of heating system utilized, proper maintenance of the entire system is critical. Not only will maintenance maximize efficiency of the heating system but will protect against a malfunction that can result in the release of ethylene and/or carbon monoxide into the greenhouse. Maintenance should include appropriate cleaning, checks of the air intake, checks of the exhaust system, checks of the fuel line, checks of fans, checks of the burner system and the heat exchanger, calibration of the thermostat, and any other maintenance items prescribed by the manufacturer.

Read the article "Maintenance Guide for Greenhouse Ventilation, Evaporative Cooling and Heating Systems"

Read the article "Greenhouse Heating Checklist"

Read the article "Heating Greenhouse"

© 2005, M.R. Evans