Greenhouse Condensation Control- Bottom Heating and Between-row Heating, ...

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Ohio State University Fact Sheet
Food, Agricultural and Biological Engineering
1680 Madison Ave., Wooster, OH 44691

Greenhouse Condensation Control
Bottom Heating and Between-row Heating
AEX-801-00
Jessica J. Prenger
Research Engineer
Peter P. Ling
Assistant Professor
The methods for heating greenhouses are constantly re-evaluated due to pressures for energy
conservation, pollution prevention, and most importantly, efficient production. Heating
distribution systems that are lower in the greenhouse and closer to plants achieve substantial
energy savings and are more effective at maintaining warm canopy temperatures. Keeping thetemperature of leaf surfaces above the dew point is an excellent way to prevent condensation andthus fight common greenhouse diseases. Further, new approaches to disease and pest management
require careful attention to the microclimate of crops, using heating and humidity to treat diseases
and even affect insect behavior. The following discussion lists recommendations for bottom andbetween-row heating, which provide a better environment for plant production with energy
savings as well.
Pipe Heating
One of the most common greenhouse heating systems is a network of metal or plastic pipes
distributing steam or hot water. Pipe heating is an effective means of warming crops both byconvectively heating the greenhouse air and by radiating heat directly to the leaves. Nevertheless,there are several ways to increase efficiency. The key to improving climate control with pipe
heating systems is locating the pipes nearer or below the crop. First, lowering overhead pipes canresult in up to 20% energy savings because of more direct heating of the plants and less radiation
loss through the transparent cover (Bakker et al. 1995). Placing the pipes lower to the ground also
stimulates air movement through the crop canopy. Warm air rising through the canopy removes
moisture and creates a uniform microclimate around the plants. The most successful radiative
heating from pipes occurs at the mid-height of the crop (Figure 1), while the least effective
horizontal placement is directly between benches. Thus, the best position for pipes is between themid-height and bottom of the crop, with pipes closer to the benches rather than between them

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Greenhouse Condensation Control- Bottom Heating and Between-row Heating, ...

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The disadvantages of low pipe position are that the pipes need to be resistant to horticulturalchemicals, and they must not hinder cultivation. Some systems are now adjustable to allow
growers to move pipes out of the way during cultivation. Lowering the pipe system does require
careful attention so that crops are not scorched by adjacent pipes.
Warm Floor
A widely accepted improvement of pipe heating systems is warm floor heating. Heat is provided
through a hot water or steam pipe system below the soil or concrete floor surface. The main
advantage is homogenous heat delivery throughout the greenhouse from the large heat transfer
surface of the floor. Heating from the bottom of the greenhouse allows warm air to rise through theplant canopy, providing better circulation of the heat. In addition, heating the root zone positively
affects plant health and growth (Bakker et al. 1995). However, the system does havedisadvantages, most notably the slow response time to control actions, and the relatively highexpense of the installation. The expense is significantly offset by the improved heating efficiencyof the system. In some cases, systems can pay for themselves within a year (Kelly, 1994). Often
supplemental forced air or pipe heating is used to provide a quicker response time.

Figure 1. Efficient position of heating pipes given by Teitel et al. 1999.
Bench Heat
Bench heating is another technique to maximize the surface area for heat transfer. Some growers
place pipes directly under or on the benches to conduct heat to the root zone of the plants, andimprove air movement through the canopy (Figure 2).

Figure 2. Below bench heating using hot water pipes.
Air Heating
Forced air heaters offer a quick-response system for greenhouse heating. These heaters are often
overhead, with heat distributed by fans or perforated polyethylene tubes. As with pipe and floor

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heating, air heating systems can be improved by lowering the heating ducts. Under-bench airheating ducts have been shown to significantly decrease Botrytis infections (Kelly, 1994). The
main disadvantage of using air heat is the expense of running a fan to move the heated air, which
can add 10% to energy costs (Bakker et al. 1995). Another problem is that air heating is aconvective heat process, meaning it requires air to transfer heat; thus, the leaf temperatures cannot
reach any higher than the air temperature. Alternatively, radiative heating such as solar radiation
and radiative heat from pipes or heated floors can heat the leaves higher than the temperature of
the air. Further, the on-off periods of air heating can leave the canopy temperature cooler than air
temperature for much of the heating cycle, allowing condensation to form. The following tableshows estimates of energy efficiency for four heating systems. Note that a "near-floor" system isalmost as efficient as the best "warm floor" system.
Estimated
HeatingHeating System
SystemEfficiency
Perfect Heating System
100%
Warm Floor-Insulated*
90%
Warm Floor-Uninsulated* 75%
Hot water pipes, Near85%
Floor
Hot Air: Unit Heater, Fan
60%
jets and Tube
*Supplemental pipe heating needed
approximately 15% of the time. From The
Ohio State University, Department of Food,Agricultural, and Biological Engineering,
1984.
Estimated for a densely cropped flower orvegetable production, night temperature 6065°F.
Conclusion
Moving the heating distribution system lower in the greenhouse improves heat transfer to theplants and helps prevent low canopy temperatures that lead to condensation problems. This results
in a better climate around the plant canopy and significant energy savings. Moreover, theefficiency of heating systems can be significantly improved by locating heating surfaces nearer tothe crops. Additional considerations for better climate control such as using thermal blankets toconserve heat, and circulating air to better distribute heat are discussed in fact sheets AEX-802 andAEX-803.
References
Aldrich, R.A. and J.W. Bartok, Jr. 1989. Greenhouse Engineering. NRAES-33. Northeast
Regional Agricultural Engineering Service. Cornell University, Ithaca, NY.
Bakker, J.C. et al. (Eds.) Greenhouse Climate Control-An integrated approach. Wageningen, The
Netherlands: Wageningen Pers. 1995.
Kelly, M.K. "The Well-Ventilated Greenhouse." GrowerTalks. April 1994: 54-57.

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Teitel, M., I. Segal, A. Shklyar, and M. Barak. 1999. A Comparison of Pipe and Air Heating
Methods for Greenhouses. J. Agric. Eng. Res. 72:259-273.

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