The direct-fired heater (DFH) system has the potential to provide high heating efficiency in the greenhouse. A DFH works by discharging all combustion products directly into a greenhouse. DFHs do not lose efficiency due to energy transfer through heat exchangers, and some DFHs claim nearly 100% heating efficiency.
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Direct Fired Heater
The direct-fired heater (right) discharges combustion products directly into the greenhouse. The traditional indirect-fired heater (left) uses a heat exchanger to deliver heat to the greenhouse while venting the combustion products to the outside. The efficiency of the traditional indirect-fired heater is limited by the efficiency of the heat exchanger and is usually between 80% and 94% efficient. With no heat exchanger, it is claimed that direct-fired heaters are near 100% efficient. This claim however, requires further investigation.
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Water Vapor in a Greenhouse
Combustion of fossil fuels creates heat and carbon dioxide as well as water. A significant amount of water vapor could be added to the greenhouse by a direct-fired heater. High humidity in the greenhouse reduces plant transpiration and nutrient uptake. High humidity can also lead to condensation on leaf surfaces, creating a favorable environment for plant diseases. When a direct-fired heater is in use, it is necessary to increase ventilation to lower humidity and keep plants healthy. Increased ventilation discharges heat energy and lowers the effective efficiency of the direct-fired heater.
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Direct Fired Heating Ventilation Chart
The efficiency of the direct-fired heater is dependent on the ventilation rate. With crops requiring a lower humidity, the greenhouse will be vented more often, and net efficiency will decrease. Conversely, crops tolerant of higher humidity levels require less ventilation and enable a grower to take advantage of the direct-fired heater’s efficiency. The graph shows how heating efficiency of a direct-fired heater is affected by air exchange rate.
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Direct Fired Heating Impact of Outdoor Conditions Chart
Outdoor conditions will affect the heating efficiency of the direct-fired heater. While colder outside conditions will lower the net efficiency of any greenhouse heating system, greenhouses using direct-fired heaters will be affected to a greater extent due to a higher need to purge water vapor. When the outside has very low relative humidity, each ventilation event will be more effective. Less ventilation, and therefore less energy, will be required to maintain an appropriate humidity in the greenhouse.
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Tomatoes in a Direct Fired Heating System
A comparison between a direct-fired and indirect-fired heater took place in Wooster, OH, in two identical experimental greenhouses. One house was heated with two direct-fired heaters and the other was heated with two indirect-fired heaters. Here you can see plants grown in a greenhouse with a direct-fired heater (DFH) and in another greenhouse with an indirect-fired heater (IFH). In general, plants grown with a DFH were shorter and more compact. The tomato is obviously shorter with a thicker stem.
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Bedding Plants Response to Ethylene
A properly functioning direct-fired heater should undergo complete combustion, producing only water vapor and carbon dioxide. However, during startup, or if the heater is damaged, incomplete combustion can occur resulting in dangerous gases such as ethylene and carbon monoxide. Ethylene, for example, can have serious effects on plants even in low doses. This chart shows how some bedding plant species respond to ethylene. Greenhouses using direct-fired heaters should be monitored for these gases.
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Direct Fired Heater
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Water Vapor in a Greenhouse
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Direct Fired Heating Ventilation Chart
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Direct Fired Heating Impact of Outdoor Conditions Chart
The conventional greenhouse unit heater is, by comparison, an indirect-fired heater (IFH). An IFH’s combustion chamber is isolated from the indoor environment. Heat generated is delivered to the indoor environment through heat exchangers. After passing through heat exchangers, flu gas produced from the combustion process is vented outdoors. The efficiency of this system is determined by the ability of the heat exchanger. IFHs typically have efficiency between 80% and 94%.
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A comparison between DFH and IFH must consider more than just fuel used and heat provided. The big picture of heater performance includes the quality of environment the heater provides for plant production. Through the process of combustion, heaters have influence on environmental parameters such as carbon dioxide, oxygen, and humidity.
Does the need for extra ventilation offset the efficiency of the direct fired heater? To answer this question, a study was completed comparing a DFH to an IFH. The results of this study will allow growers to make more informed decisions about heating equipment.
The comparison took place in Wooster, OH, using two identical experimental greenhouses. One of the greenhouses used two IFHs rated at 80% efficiency, while the other used two DFHs. DFH efficiency was found to be dependent on ventilation rate, outdoor humidity, and outdoor temperature.
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Plants produced in both DFH- and IFH-heated greenhouses were of very good quality and there were no differences in crop timing. For flowering bedding plants, which included marigolds, impatiens, petunias, snapdragons, and zinnias, the time to first flower was not affected by the heating source. In general, plants grown in the DFH greenhouse were shorter and more compact. Tomatoes in the DFH greenhouse were shorter and had thicker stems. The air around the crops was monitored monthly and ethylene was not detected in either greenhouse at any time in the experiment.
Should a grower use DFHs or IFHs? The answer is not universal. The DFH will have an advantage in warmer climates with lower relative humidity. Growers in cooler or higher humidity climates may want to stick with conventional IFHs.
If high humidity and ethylene gas are not concerns for the crop being grown, the DFH may be the better option. Greenhouses with systems to remove humidity without ventilation, for example by draining condensate from glazing, may also realize the higher efficiency of DFHs.
Growers who decide to use DFHs should practice good management of parameters such as greenhouse air exchange rate, passive infiltration, active heater air intake, and active ventilation.
Editor’s Note: This article was written by a team of authors at The Ohio State University: Wee Fong Lee, Luke Power, Michelle Jones, Claudio Passian, and Peter Ling.
015Researchers Examine Efficacy of Direct-Fired Greenhouse Heaters
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Brian Sparks is senior editor of Greenhouse Grower and editor of Greenhouse Grower Technology. See all author stories here.
March 14, 2017 
