10 tips for better summer cooling

  John W. Bartok Jr.An efficient cooling system can keep your greenhouses within the desired temperature range during warm weather. My observations have shown that after energy and labor, ventilation is the area where significant improvement can be made in most greenhouses. Here are a few things to check before hot weather sets in.

1. Shading is a good starting point. External shade blocks the heat from getting into the greenhouse. Good spray-on shade materials are available that will give all season coverage with one application. If two levels of shade are desired, a light coating applied in spring can be added to during the summer. Cleaners to remove the shade material in the fall are available for glass, polycarbonate and polyethylene.

External shade cloth is the common material used on the outside of hoop houses. It is available in many levels of shading and is easy to apply and remove. With the material lasting five to seven years, the cost is only 2-3 cents per square foot per year.

Internal shade screens with aluminized material are common in gutter-connected greenhouses. They keep direct radiation off the plants and reflect the radiation back out through the glazing before it turns to heat. Screens also act as energy blankets during the winter to lower heating bills. Motorized systems cost from $1.75 to $2.50 per square foot depending on the type of system, the material used and the complexity of installation.

2. Roll-up sidewalls are a good choice. Roll-up sidewalls are a good choice for hoop houses for spring and fall crops. A location with a good breeze and plenty of space between houses increases effectiveness.
 Although the roll-up operation can be motorized, most growers use a manual system as it usually has to be tended only once a day. Be sure restraints are in place to keep the sidewall from billowing and provide a good seal at the ends and bottom to prevent heat loss at night.
 
A reinforced woven polyethylene material is a better choice for the roll-up sidewall section because of its durability. This material is about twice the cost of standard polyethylene and will double its life.
 
Side vents enhance cooling in gutter-connected greenhouses with natural ventilation. Tests at Ohio State University have shown that in wide gutter-connected greenhouses a combination of a windward side vent and leeward roof vents provided the most effective cooling.
 
The recent development of the guillotine vent eliminates the rack and pinion arms that tend to get in the way of plant handling. Cool air that is introduced at the top of the vent has a chance to moderate before reaching the plants.

3. Fan maintenance improves efficiency. Fan capacity for warm season ventilation is normally installed at 8 to 10 cubic feet per minute per square foot of floor area.
 
The use of two-speed motors with a controller will deliver multiple levels of cooling.
Shutters and fan bearings need to be lubricated a couple of times a year.  Check belts for wear or looseness and clean the fan blades frequently.
 
Intake shutters should have an area at least 1.25 times the fan area. Check to see that they close tightly as this is one area that can add significantly to infiltration losses during the winter.

4. Avoid short circuiting of ventilation air. I’m surprised at the number of times I see fans operating next to an open door, vent or roll-up side. Fans create a vacuum that exhausts heated air. The makeup air takes the easiest path from outside. If there is an opening next to the fan, this is where the makeup air will come from. The other end of the greenhouse does not receive any cool air.

5. Purchase efficient replacement fans. Replacement fans should have a ventilation efficiency rating (VER) of at least 15. VER is a standard testing system that rates fans on the number of cubic feet per minute per watt of energy input. For example, a fan with a VER of 15 will provide 15 cubic feet per minute for each watt of electricity usage.
 
The higher the VER the greater the efficiency. Fan VERs are usually listed in manufacturer’s literature and can be from 8 to 22.

6. Keep insect screening clean. Screening is commonly a source of poor ventilation system performance. Frequently there is not enough screen area to overcome the resistance of the tiny holes.
 
It usually takes three to five times the vent or shutter area covered with screen to get adequate intake. Also the very small openings in the screen can be blocked by insects, dust and pollen. Vacuum or wash the screen frequently.

7. Calibrate controls yearly. If your cooling system uses thermostats for sensing, make sure they are located at plant height, near the center of the growing area, adjacent to heating thermostats.
 
Location on an outside wall, an endwall or attached to an overhead truss does not give accurate sensing for plant requiremens. Installing each thermostat in an aspirated box will give more accurate control.
 
Check the accuracy of a thermostat by placing an accurate digital thermometer next to it to compare readings. Thermometer accuracy can be checked by placing it in a container of ice water.

8. Install a controller to integrate systems. Compared to a mechanical thermostat that has a four to six degree differential between on and off, a controller with a two degree differential will give much better environmental control.
If the cooling and heating systems are incorporated, it avoids having both systems operating at the same time. The controller eliminates several thermostats. Installed cost depends on the equipment that is connected, but usually is around $1,000 with the relays and motor starters.

9. Evaporative cooling can improve summer environment. Although fan and evaporative pad cooling systems work best in dry climates, growers in more humid northern areas have found benefit from these systems where crops are grown through the summer. The greenhouse can be cooled to several degrees below the outside temperature.
 
Evaporative pad area and water supply need to be carefully sized for the area to be cooled. Modular units come with all of the components assembled and ready to be installed. Some growers have had success with portable evaporative cooler units that can be moved at the end of the summer.

10. Keep greenhouses full of plants. Transpiration from plants aid in cooling the greenhouse. Each gallon of water that is transpired by a plant and evaporated, can remove 9,400 Btu of heat. Exhausting this moist, warm air helps with cooling.