Controlling the environment in a greenhouse can look fairly simple in some cases, and many of us don’t give it a lot of thought. When the temperature dips below or above certain points, we know how to turn on the heat or fans, respectively. Simple right? Well, to a certain point, but by studying a greenhouse’s environment, we realize that there is actually an intricate dance going on between many systems, even in a small hoophouse, to ensure the delivery of optimal conditions to our crops. Here are some of the principles and practices that will help maintain proper greenhouse temperatures and conditions.
Greenhouse Volume And Size Matter
The size and height of a greenhouse (think about overall volume) can have an effect on how it heats and cools. Two greenhouses with the same footprint but different gutter heights will have different air volumes inside the greenhouse, and the one with taller gutters will cost more to heat based on area (square feet). But on a volume basis (cubic feet of air), the taller greenhouse will cost less to heat. There are other advantages of having taller greenhouses too. Taller greenhouses allow for more space for lighting, shade/energy curtains and hanging baskets.
Shadows are also more dispersed in a taller greenhouse. To illustrate this
concept, try this experiment — hold your hand a couple of inches above the pages of the magazine. Notice that the shadow cast by your hand is dark and well defined. Now hold your hand 12 to 18 inches higher, and the shadow becomes less defined and lighter. The same concept holds true with hanging baskets and the superstructure of a greenhouse with taller gutter heights.
Larger volumes of air can help moderate temperature fluctuations as well. Imagine driving in your car (a small volume of air) and turn on your heater. The interior of the car warms up fairly quickly, but as soon as you turn off your heater, the air cools down just as quick. Increasing the volume of air will reduce the modulation between the temperature extremes as our heating and cooling systems cycle on and off.
Unfortunately most of us are stuck with the dimensions of the greenhouses we have, and although it is possible to raise the height of your greenhouse, in most cases it will be cost prohibitive. Keep these points in mind when you are constructing your next greenhouse:
• The length and width of a greenhouse can affect the ability to effectively cool a greenhouse, especially if you rely on exhaust fans. Fans should be 100 to 200 feet from the vents that they pull air from, on the opposite side of the greenhouse. While distances greater than this result in a large temperature gradient from one end of the greenhouse to the other, distances shorter than this cause turbulence and reduced air velocity.
• Not all fans are created equally. Fans come in a variety of horsepowers and diameters, both of which dictate the volume of air that can be exhausted. Fans need to be properly matched to each greenhouse to ensure they cool efficiently. In addition, the amount of air intake and vent needs to be properly calculated for the size of greenhouse and the size and horsepower of the fan used. Undersized air intake results in inefficient cooling and, therefore, higher greenhouse temperatures, which can shorten the life of a fan motor.
Common Cooling Problems Involve Louvers And Extra Openings
There are two common problems in greenhouses that undercut the ability of a cooling system to work correctly. First is a louver that doesn’t work properly. Ideally the louvers should be motorized and open automatically just seconds before the fans turn on. If your louvers are not motorized, then they should open easily when the fan turns on and begins to pull air. When louvers do not open fully, airflow is restricted and the greenhouse will not cool properly.
The second problem is having extra openings for air to enter other than the intended vents, like walk-through doors propped open to allow extra airflow to cool the greenhouse. Although this might indicate that the vents are not big enough, it is not necessarily a problem. About 50 percent of doors in single span hoop houses, however, are centered between the two exhaust fans at one end.
Air always travels in the path of least resistance, and if a door is open a few feet away from your fan, that has become the path of least resistance. Air will not be pulled from the opposite side to cool the entire greenhouse. The air exchange will only occur in the first few feet of the greenhouse. Closing a door to facilitate cooling can seem counterintuitive, but failing to do so in this situation could lead to a 20°F to 30°F difference from one end of the greenhouse to the other.
Use HAF Fans Properly To Reduce Temperature Variations
Horizontal Air Flow (HAF) fans are an integral part of a greenhouse heating and cooling system. HAF fans create airflow throughout the greenhouse (when the cooling system is not activated). The proper use of HAF fans will reduce variations of temperature within a greenhouse structure. When used correctly, there should be no more than a 2°F variation within the greenhouse. HAF fans should be running at all times, unless the cooling system is activated. When HAF fans and exhaust fans run at the same time, turbulence is created in the air and neither system works properly. This results in higher-than-desired temperatures in the greenhouse. In a passive cooling system like roll-up sides and ridge vents, HAF fans should also be turned off, as the horizontal air flow hinders the natural rise and
venting of warm air.
As previously mentioned, manipulating the greenhouse environment is actually an intricate dance, so it is imperative that all the parameters are “dancing” together. The most effective way to guarantee all the systems are working together is to use an integrated environmental controller.
Mechanical thermostats work, but tend to go out of calibration over time. Plus, each component (fan, vents, louvers, heater, HAF fans) requires its own dedicated mechanical thermostat. In this scenario, it is common for the greenhouse’s heating and cooling systems to be running at the same time, especially when greenhouse temperatures are near the set points.
Using a controller will consolidate these individual thermostats into a single
thermocouple. When a single controller runs all the components, the potential for running heating and cooling systems simultaneously is eliminated. The controller will also turn HAF fans on and off at the appropriate time, as well as open and close the vents. Controllers range in capabilities, complexity and price, but even low-cost controllers can be very effective and may have added benefits like programmable DIF and humidity reduction cycles.
Regardless of the use of a high-end controller, the thermocouples or thermostats need to be located in the appropriate place, not where it is most convenient for you. The center of the greenhouse at plant canopy height is the best place. Plant canopy height can change with crops and throughout the growing season, so hang your thermostats or thermocouples from a chain that can be easily adjusted up or down. Finally, be sure they are shielded from the sun. Better yet, have them aspirated. A shielded aspirated sensor gives the most accurate temperature reading for the environment.