nu.gif (4545 bytes)

Greenhouse Cooling Equipment

In an outdoor Nepenthes greenhouse, cooling equipment will be mandatory to keep the temperatures from becoming excessive. Even on the coldest of days, sunlight entering a sealed greenhouse can make the temperatures rise to lethal temperatures. During the summer time, the temperatures inside a greenhouse can exceed 100 degrees farenheit within minutes in full sun.

About 2/3's of the Nepenthes species are truly mountain plants, which experience a drop in temperature at night. About 10% of the species are ultra-highland plants, which experience cool daytime temps, and cold night time drops. Very few places in the world could utilize an outdoor greenhouse in which to grow Nepenthes without cooling equipment.

There are three primary cooling devices in most greenhouses. These are the vent system, exhaust fan, and swamp cooler. Some greenhouses may make use of air conditioners and/or misting systems as well.

The vent system is usually the first device enacted in the cooling system. Vents are hinged or track connected panels in the roof or sides of greenhouses. They open up the greenhouse to outside natural air. Hot air that builds up in the greenhouse can escape, and fresh air can enter the house. In smaller greenhouses, the vents are usually located in the roof so that the hot air can rise straight out of the house. In larger houses the vents are located in the sides od the house, as the large exhaust fans systems pull air from one side of the house through the other. It is imperative that the vents are automatically controlled. In smaller greenhouses, the roof vents can be automatically set to open using chemical vent openers. These openers contain a chemical in the cylindar arm which expands as the temperatures rise, which push open the roof vents. Larger greenhouses usually make use of computer controlled vent openers.

The picture below shows one of two many types of vents used in a large greenhouse. This vent is located over the door. It is controlled by the main greenhouse computer. The aluminum shutter opens when a voltage is sent to an electric motor connected to the shutter arm.  (This assembly isn't shown in the picture, as it is located on the interior part of the shutter. This picture shows the exterior shutter.) When the voltage is removed from the motor, the motor arm goes free, and a spring pulls the shutter closed. The picture shows the shutter vent closed. This shutter is used in conjunction with the main vent system.
vent1.jpg (107206 bytes)

The picture below shows the main vent in the back of the greenhouse. A Velero vent motor opens and closes the vent, which is controlled by the main computer. The vent door is connected by a rack and pinion gear system. The vent in this picture is closed.

vent2.jpg (91900 bytes)

Greenhouses can also have roof/ridge vents installed. These vents are very capable of cooling a greenhouse when the outside temps are  cool. The hot air rises out of these vents, and fresh cooler air is usually pulled in through a shutter or side vent. If your greenhouse is located in a location where the outside air is quite cool for most of the year, then a roof vent is with the expense. If your temperature is quite warm for most of spring, summer, and fall, then it may not be worth the expense to have a roof vent. You will need a side vent for running an exhaust fan, so you cant replace a side vent with a roof vent. You have to have a side vent, and aadditionally you can have a roof vent. So to get the bang for your buck, you need to be located in an environment that allows you to use  roof vent frequently for cooling.  It doesnt cost any money to operate a roof vent when its not moving. So the mnore you can use a roof vent versus an exhaust fan, the more money you save, making it more worthwhile to purchase a roof vent in the first place.

One of the newer forms of venting is by rolling up the sides of a poly greenhouse. This allows maximum venting to occur.  It can also allow rodents and other varmin easy access into your greenhouse, so additional control measure have to be taken in order to keep them out.

During cold winter days venting may be all that's required to adequately cool the interior of the greenhouse. But during hot summer days, venting alone will not get the job done. Eventually the temperatures continue to rise even with the vents open. An exhaust fan is the next stage of cooling. Exhaust fans can move a large volume of the hot greenhouse air out and pull fresh air in through the rear vent. Exhaust fans are very powerful, and opening the greenhouse door while one is running at high speed can be a challenge. They're powerful for a reason, as full sun on a hot summer day can cause temperatures inside the greenhouse to superheat. An exhaust fan must be able to pull this air out, or the temperatures will continue to rise.

fan1.jpg (102433 bytes)

The picture above shows an exhaust fan in the front wall of the greenhouse. The metal shutter can be seen on the interior of the fan. The shutter closes by gravity. When the fan turns on, the force of air being pulled through the fan causes the shutters to open. Exhaust fans usually come in two speeds, low and high. This saves energy costs, as low speed is normally sufficient. On the hottest of days, and the brightest of summer suns, high speed may be necessary. Some greenhouses are large enough that a single exhaust fan either can not move enough air, or can not pull air adequately through the entire width of the greenhouse.  In this case, multiple exhaust fans are utilized. Some greenhouses are so large, being hundreds of yards in width,  that dozens of exhaust fans are used. 

One problem with exhaust fans is that when they move out hot air, they also move out any moisture in the air.  When the exhaust fan runs, the moisture in the air of the greenhouse is removed with the hot air. So even though the temperature is reduced, it is done by replacing the moist hot air inside the greenhouse with air from the outside, which may not be very moist. This causes the humidity level to plummet. To keep this from happening, the swamp cooler can be turned on when the exhaust fan is turned on.


swamp1.jpg (44820 bytes)

The picture above shows the swamp cooler (also known as evaporative coolers) made by Carolina Cooler. (It's the long brownish piece of equipment located behind the Reznor heater.) The swamp cooler is made up of cooling pads. Cool water from an outside reservoir is pumped into the top of the swamp cooler, and the water disburses through the pad. The swamp cooler is located in front of the rear vent, and air is pulled through the cool wet pads by the exhaust fan located at the other end of the greenhouse. When the air is pulled through, some of the water evaporates, which cools the air, and is pulled into the greenhouse. Excess water runs into a drain pipe and is returned to the reservoir outside. The air being pulled into the greenhouse is moisturized, which keeps the humidity high. Basically the hot air is removed by the exhaust fan, and it is replaced with cool moist air. The swamp cooler is capable of cooling the air to nearly the temperature of the water running through it.

Swamp coolers come in different widths and lengths. Your greenhouse supplier will configure the appropriate size cooler for your greenhouse, as this varies depending on the length and width of the greenhouse, location where you live, and type of plants you wish to grow. With a correctly sized exhaust fan, the swamp cooler can lower the temperature of the greenhouse to the temperature of the water that runs through it. On a hot summer day, standing in front of the operating swamp cooler can feel like your standing in front of a refrigerator.

The computer controls heating and cooling in stages. Eaxh stage can turn equipment on and off. Stage 0 is the heating stage. Stage 1 is known as the setpoint. This is the temperature range that we do not require any heating or cooling. Stages 2 through 5 set the different levels of cooling. The stage chart is shown below.

Equipment                 Stage 0   Stage 1    Stage 2   Stage 3   Stage 4   Stage 5
HAF Fans                  On            On             On             Off            Off            Off
Main Vent                  Closed     Closed   Open       Open        Open      Open  
Front Vent                 Closed     Closed   Open       Closed    Closed   Closed
Exhaust Fan Low   Off             Off           Off             On             On           Off
Swamp Cooler        Off             Off           Off             Off             On           On
Exhaust Fan High  Off             Off           Off             Off             Off           Off

Now that we know how the computer has all the equipment staged, we then program the greenhouse computer when the stages are to turn on and off. The following chart shows the cooling and heating equipment schedule during the daytime in the summer months for my highland Nepenthes greenhouse.   Although the heater is always in the schedule, the unit is turned off, the pilot light is out, and the gas is turned off from the unit during the summer months.

Stage 0   Heater On Below 60 Degrees
Stage 1   Setpoint                                      
Stage 2   On-78 Degrees Off 70 Degrees
Stage 3   On-88 Degrees  Off 82 Degrees
Stage 4   On-88 Degrees  Off 86 Degrees
Stage 5   On-92 Degrees  Off 88 Degrees

So let's see what happens during a typical summer day. Early in the morning, the temperature is normally in the low to mid sixties, which means we're at setpoint, or stage 1. The only equipment operating is the HAF circulation fans. (See the Greenhouse Circulation Section for details) Once the sun rises, the temperatures start increasing. As soon as it reaches 78 degrees, we enter stage 2 is enacted. Stage 2 opens both the shutter vent over the front door, and the main vent in the back of the greenhouse. During the early morning hours, the temperatures outside may still be cool, therefore venting may drop the temperature below 70 degrees. If this happens, then both vents will close and we fall back to stage 1. If the temperature exceeds 88 degrees, then we enter stage 3, and stage 4 simultaneously. Stage 3 turns on the exhaust fan. But because we dont want the exhaust fan to pull in dry air from the outside, we also turn on the swamp cooler. Therefore, at 88 degrees, we go straight to stage 4. The exhaust fan and swamp cooler turn on, and the circulation fans turn off. The front vent over the door also shuts. This is because we want the exhaust air to pull in air only through the swamp cooler, and not dry air from the door vent. Once the temperature lowers to 86, we fall back to stage 3. This causes the swamp cooler to turn off, but leaves the exhaust fan on. The reason I do this is because the cooler pads are now wet, and I dont need to continue to use electricity running the powerful pad pump. The exhaust fan will continue to pull air through the wet pads, which will cause them to start to dry out. And as the temperature drops to 82 degrees, we fall back to stage 2 and the exhaust fan shuts off. At stage 2, the door vent re-opens and the circulation fans turn back on. During the rest of the day, we will constantly see the temperature rise to 88, the exhaust fan and swamp cooler turn on, and the temperatures drop back to 82. This is repeated until the sun begins to set. Eventually, the temperature drops back to setpoint, and the two vents shut. There are occasions on very hot and sunny days that even with the low speed exhaust fan and swamp cooler running, the temperatures keep climbing. As soon as the temperatures reach 92 degrees, the exhaust fan switches to high speed. This causes the neighbors lights to dim, as the exhaust fan produces an equivelant thrust to that of an airliners jet engine. Ok, not quite, but at high speed he exhaust fan can produce enough suction to make it difficult to open the greenhouse door. It moves volumes of air through the swamp cooler, causing extensive evaopration, thereby cooling the greenhouse down. Once the temperatures drop to 88 degrees, the exhaust fan switches back to low speed, as we enter back into stage 4.

Now that you see how I schedule cooling equipment during challenging summer days, the following chart shows how I schedule winter days.

Stage 0   Heater On Below 70 Degrees Off 80 Degrees
Stage 1   Setpoint                                      
Stage 2   On-88 Degrees Off 86 Degrees
Stage 3   On-90 Degrees  Off 88 Degrees
Stage 4   On-90 Degrees  Off 89 Degrees
Stage 5   On-92 Degrees  Off 90 Degrees

The main difference here is that the heater is operational. On cold winter days with no sun, the heater will have to run. Otherwise the sunlight will keep the greenhouse heated. If the temperaturesd exceed 88 degrees, the vents open. This will let in air that is usually substantailly colder (30-60 degrees normally) and is usually all thats required. If the temps continue to rise, the exhaust fan and swamp cooler will switch on. If you think standing in front of the swamp cooler is cold during the summer, try doing it with the water and ouitside air temperature at 50 degrees. You might as well walk into a freezer. The temperatures usually drop so fast that these devices only run for about a minute. In fact, the vents alone may only be open for minutes. I try to reatin as much heat as possible into the nighttime, thereby reducing heating requirements.

The computer has separate settings for nights. Highland Nepenthes need cool nights, so a different schedule is needed. The below chart shows my winter night schedule.

Stage 0   Heater On Below 50 Degrees Off 64 Degrees
Stage 1   Setpoint                                      
Stage 2   On-88 Degrees Off 86 Degrees
Stage 3   On-90 Degrees  Off 88 Degrees
Stage 4   On-90 Degrees  Off 89 Degrees
Stage 5   On-92 Degrees  Off 90 Degrees

The change between day and night is the heating schedule. With no sun at night, winters will never see temperatures above 80 degrees, so cooling isnt needed. The heater is set to turn on at 50, and shuts off at 64 degrees.

By looking at these schedules and all the equipment involved, you can see why its important to have your cooling system automated. The greenhouse computer (see details in the Greenhouse computer section) can keep your system cooled to the optimum temperatures continuously, which is needed for optimum growth.


Misting systems can also be used to lower the temperatures, as cool water mist in the air will cool the greenhouse while raising the humidity level at the same time. Misting eqipment can be controlled by the main computer, and can be set to turn on at a certain temperature, or when the humidity drops below a certain level. Misters can also be set to turn on at certain times during the day. Again, computer automation makes this operation much easier.