once a year to ensure that you are achieving the desired 

 set points and that heating and cooling equipment wori< 

 together. Position the temperature sensors from all ther- 

 mostats in one place so that calibration is easier. 



More sophisticated thermostats are able to work in 

 stages. An example is a cooling fan that works at a low 

 speed when the air is 3F above the desired temperature 

 and at a high speed when it is 6F too warm. Staging al- 

 lows more precise control, keeping temperatures close 

 to the set point target without large swings as the green- 

 house is overheated, then overcooled. 



5. INTEGRATED TEMPERATURE CONTROLLERS 



An option that ensures that individual components work 

 together is an integrated temperature controller that can 

 control multiple pieces of heating and cooling equip- 

 ment (e.g. ridge vents, side vents, fans, and multiple 

 heating valves). These units cost from $400 to $2500. 

 Cost depends on the number of heating and cooling 

 stages, the number of environmental factors that are 

 monitored (typically, at least light and temperature), the 

 number of greenhouse zones that can be controlled, and 

 special features (e.g., an exhaust/dehumidify function 

 that can dehumidify the greenhouse on command by 

 raising temperature and then opening vents). 



Integrated controllers allow multiple staging: the first 

 stage of cooling might be opening the ridge vent; the 

 second, opening the side and ridge vents; the third 

 stage is side and ridge vents open and exhaust fans on 

 More sophisticated controllers allow programming of 

 separate set points for day and for night; at least one 

 also allows a dip of temperature near dawn. Many units 

 also display 24-hour, day, and night temperature averages. 



6. ENVIRONMENTAL CONTROL COMPUTERS 

 Environmental control computers provide the highest 

 level of control and integration of multiple pieces of 

 equipment. In these systems, sensors (e.g., light and 

 temperature sensors) and equipment (e.g., vent controls) 

 are linked to a central computer. Environmental control 

 computers generally cost $10,000 upward, but are de- 

 signed in a modular fashion that allows you to purchase 

 only the equipment you need for the number of zones 

 in your greenhouse and also allows easy expansion to 

 new zones. With some systems, temperature control, 

 misting, lighting, carbon dioxide, irrigation, and fertiliza- 

 tion can all be controlled by the same computer. 



Environmental control computers have several advan- 

 tages in addition to precise control and equipment inte- 

 gration. They have a built-in ability to display tempera- 

 ture or other factors on the computer screen, and to 

 store historical data so that you can have a complete 

 crop record. All environmental control systems have the 

 ability to activate alarms when temperatures are too 

 high or low and most can automatically inform you over 

 a modem and telephone line. With some systems, you 

 can also change control settings (e.g., vent temperature) 

 remotely from your home or office via a modem. 



Factors to Consider When Choosing Between Options 



The decision of which control and monitoring system is 

 best for your business should be made on the basis of 

 many factors, and not just on up-front price. 



First, you need to list all of the heating, cooling, and 

 other greenhouse equipment you need to control (e.g., 

 heating valves, vents, fans, pads, curtains, HID lamps, 

 carbon dioxide supply). As the list per greenhouse zone 

 increases, this argues in favor of a more sophisticated 

 system. As the number of greenhouse zones increases, 

 this argues in favor of a centralized system rather than 

 separate units for each house. Consider not just the cur- 

 rent stage of your business, but also future plans. 



Decide how precise a level of control is necessary for 

 the crops you grow and for your need to have sophisti- 

 cated regimes, such as DIF temperature with a dip near 

 dawn for height control. More precision and complexity 

 again argues in favor of a sophisticated control system. 



When choosing between companies and products, try 

 to see the systems in action either at a trade show or in 

 a neighboring greenhouse. It is especially important to 

 talk with growers who are using the system because 

 products vary considerably in their ease-of-use. Your 

 new system will also need a trained employee (perhaps 

 you) to operate it. You need to assess how much new 

 employment or training will be necessary and to find 

 out how much training the company will provide. If the 

 intended operator is not comfortable with the technol- 

 ogy or is not willing to work through the inevitable ini- 

 tial problems, you may have a top-of-the-line system 

 that is underutilized. Workers may even ignore the sys- 

 tem in favor of manual operation. If possible, involve 

 the intended operators in the process of choosing the 

 system so that they have a sense of ownership. 



If you are buying a sophisticated system, have more 

 than one environmental control company visit your op- 

 eration and provide them in advance with a list of your 

 needs. Find out about up-front costs, installation fees, 

 their level of technical support, and costs of updates. 

 Once installed, some companies can diagnose and solve 

 problems in your environmental control computer via a 

 modem and telephone without needing to actually visit 

 your operation. Ask other growers about response times 

 to problems and attitudes to service. 



Finally, as Don Josko said in a recent Greenhouse Busi- 

 ness article, after buying a new system, use it! If you buy 

 a sophisticated system, set the unit to automatic and 

 learn to use it as just another piece of equipment. Do 

 not buy a system that is more complex than you need 

 or want to deal with because it will not pay for itself in 

 improved crop quality and productivity. 



Paul Fisher is assistant professor in the Department of Plant 

 Biology, University of New Hampshire, Durham. He can be 

 reached by phone at 603-862-4525 or fax at 603-862-4757; 

 his e-mail address is prf@hopper.unh.edu 



THE PLANTSIMAN 



