Structurally Sound

Special Advertising Section - Greenhouse Efficiency Guide

10 elements to check to keep your greenhouse in good operating condition.

July 27, 2018

kvdkz | Adobe Stock

Over time, daily wear and tear, and factors such as weather can damage your greenhouse’s integral parts. Keeping your structure up to date requires attention to many elements and all their details.

A good maintenance program anticipates problems and takes action to minimize their occurrence. Maintenance should include a thorough inspection and any corrective action needed to keep the structures and equipment in good operating condition. Cleaning and tightening up the greenhouse can save money and increase energy efficiency. Upgrading systems with new technology can improve performance. The following checklist identifies the major areas:

1. Greenhouse frame

  • Inspect diagonal frame bracing to see that it is in place and tight. This bracing is important to prevent racking of the frame and the loosening of the glazing in heavy wind and snow.
  • Tighten collar tie and truss bolts. These loosen from expansion and contraction due to heat and movement of the structure in the wind.
  • Check for rusted frame parts, especially support posts at ground or foundation level.
  • Bent structural frame members from heavy wind and rain storms are difficult to repair and usually have to be replaced.
  • Inspect the gutters. Clean leaves and debris from gutters and downspouts. Caulk any joints that may be leaking water with a gutter caulk. Inspect for rust. Wire brush and coat these areas with a cold galvanizing spray or asphalt coating.

2. Foundation

Support for a conventional greenhouse frame needs to be below the frost line. The lighter frame hoophouse should have steel posts driven into soil 30 inches or more.

  • Repairs to a cracked concrete foundation wall can sometimes be made with an epoxy adhesive.
  • Broken concrete piers usually have to be replaced.
  • Steel tubing posts that are damaged or rusted due to poor galvanizing should be replaced.

3. Doors

Doors get damaged from constant use.

  • Close small gaps with weatherstripping to reduce infiltration.
  • A door with large gaps should be replaced with an insulated steel door and frame.
  • Brush type astragal weatherstripping works well on sliding doors.

4. Glazing

Light is usually the limiting factor in plant growth during the winter. The glazing should be clean.

  • Repair any small holes or tears in the poly cover with a polyethylene tape or Poly Patch II, which is available in widths up to 10 inches. If the poly has stretched or is loose, it may have to be tightened by removing the lock strip inserts and pulling the plastic taut. Inspect for soundness at the attachment edge. Replace plastic if it is older than four years. Clean and inspect the inflation blower. Inflation pressure should be about ¼-inch water static pressure or about the same as an inflated balloon. Too much pressure will stretch the plastic and increase the space between the layers. Too little pressure will allow the plastic to ripple in the wind and tear where it is attached.
  • Polycarbonate and acrylic panels should be washed with a mild detergent and lukewarm water using a soft sponge to remove dirt, smog and shading compound. Check to see that the attachment extrusions are tight.
  • For glass-covered greenhouses, check the bar caps to see that they have not become loose. Remove spray-on shading with a cleaning compound or a shade remover made from 1 quart of hot water, 1 pound of washing soda and ¼ pound of trisodium phosphate mixed in 5 gallons of cold water. To this, add 1 quart of hydrofluoric acid (52 percent). Spray this on, let it stand for a few minutes and then flush with clean water.
  • Old glass greenhouses with broken or slipped glass can be covered with a layer of film poly to reduce heat loss. An air inflated poly tube located halfway between the ridge and the eave will keep the plastic rigid and help to shed snow.

5. Insulation

Significant savings in heat can be achieved by insulating the walls to bench height.

  • Board type insulation (1-inch thick) applied to permanent walls works well.
  • On hoophouses, double bubble wrap with aluminum foil on both sides placed between the hoop and inner layer of poly will give a four- to five-year service life. With roll-up sides, the insulation is removed and rolled up for storage once the sides are open for ventilation.
  • Repair or replace motorized shade/energy blanket system material. Check to see that the limit switches are working properly and that the blanket closes tight around the edges.

6. Benches

Wire mesh and some molded plastic bench tops often show some sag after a few years.

  • Replacement with expanded metal or molded trays is a good choice.
  • The bench frame should be supported on the edge of concrete walkways or concrete piers.
  • Untreated lumber should be replaced with pressure-treated material or steel tubing.
mulderphoto | Adobe Stock

7. Vents and louvers

These allow considerable heat to escape during the winter if they do not close tight.

  • Roof and sidewall vents get considerable use and need to be adjusted so that they close evenly and tightly. This involves lubricating bearings, rack and pinions, and vent arm hinge points and checking fluid in gearbox drives. In houses with vents that don’t close tight, adding weather stripping may be the only way to stop excess infiltration.
  • Check minimum and maximum vent position limit switches to see that they will stop vent travel at the correct position.
  • On roll-up sidewalls, adding a permanent polycarbonate closure panel between the two end hoops on each corner of the hoophouse provides a better heat seal. The plastic should be sealed during the winter with wire lock or a furring strip.

8. Fans and shutters

Fan systems have a depreciated life of about five years but can give longer service if properly maintained.

  • Clean the fan blades, motor and shutter. Check the fan belt for wear. Adjust the belt to achieve ½ inch to ¾ inch deflection in the center between the pulleys.
  • Lubricate the shutter hinge points so that they close tight. A 48-inch shutter that fails to close properly leaving 1-inch gaps allows about 23,000 BTUs/hour of heat to escape.
  • Clean evaporative cooling pads and tanks. Replace deteriorated pads.
  • Service horizontal air flow (HAF) fans by cleaning the blades and guards. Oil bearings if they are not sealed.

9. Heating systems

Consideration should be given to upgrading existing heating equipment.

  • Before the heating season begins, have all equipment serviced.
  • Check limit controls (high heat and fan or blower operation) and the barometric draft control on the flue pipe. Check the safety pressure relief valves on boilers.
  • Have a combustion test done on the heating unit to check efficiency, draft and smoke level.
  • Inspect heat exchangers for leaks and corrosion, and clean heat-exchanger surfaces including vacuuming and wire brushing radiators and fin pipes.

10. Controls

Significant energy savings can be obtained by installing more accurate thermostats or electronic controllers.

  • Check the accuracy of all thermostats and sensors by using an ice bath or an accurate laboratory thermometer. Clean the sensor coil with compressed air. Check that all wire connections are tight and not broken.
  • Check the electric supply for low voltage to prevent damage to motors.
  • Run a test mode on the temperature alarm system to see that it is operating properly.
  • Test run the back-up generator and check its fuel supply.

Assigning an employee to do routine maintenance will avoid many annoying problems and save on the heating bill this winter. To save time, set up a file with the operating manuals and the procedures to follow.

This article originally appeared in the December 2015 issue of Greenhouse Management, a sister publication of Cannabis Business Times, and has been edited for length.

John W. Bartok, Jr. is an agricultural engineer, an emeritus extension professor at the University of Connecticut and a regular contributor to Greenhouse Management, a GIE Media Horticulture Group publication. He is an author, consultant and certified technical service provider doing greenhouse energy audits for USDA grant programs in New England.