Be sure that the energy screen edge seal is tight to stop heat flow to overhead.
Photo: John W. Bartok, Jr.

Most energy prices are on the rise again. Compared to July of last year, the wholesale price of crude oil has increased from $45 to $75/barrel. This has affected propane, which has increased from $0.60 to $0.91/gallon. Fuel oil has gone up from $1.50 to $2.06/gallon. Gasoline has risen from $1.61 to $2.02/gallon. Only natural gas has decreased. It has dropped from $3.07 to $2.72/MMBtu due to a glut on the market. There is currently a lot of volatility in the fuels market due to political conflicts in the world, creating uncertainty of supply.

Conservation can help to offset some of the increased costs. Energy audits on many greenhouses show that savings from installing conservation measures can reduce fuel use from 15 percent to 35 percent. Last month’s column looked at some ideas for purchasing your fuel. This month, I will review some measures that are easy to implement and have a good payback.

Wall insulation

Insulating wall areas below benches and on endwalls has a payback of less than two years. Foil-faced, double bubble insulation is commonly used for this purpose. It works well on hoophouses where the insulations is placed between the hoops and the inflated poly. On greenhouses with permanent polycarbonate or glass walls, foam board type insulation is easy to install and lasts longer. Insulation is readily available at most home centers and lumber yards.

Furnace and boiler servicing

The efficiency of most greenhouse heating systems can be increased five percent to 10 percent with minor adjustments and a few replacement parts. Fall is a good time to have this service done. The cost of about $100 is a good investment that will be recovered in less fuel consumed. Have the technician run an efficiency test so that you can track when problems are beginning to occur.

HAF air circulation

Use a smoke bomb or fogger to check the air flow pattern on HAF systems. The air should travel the length of one side of the greenhouse without short circuiting. Clean and lubricate fans to improve efficiency.

Controls

The $100 cost of replacing an old thermostat with one having a +/- 1° F differential has a very good payback. For every degree that the greenhouse temperature is above the setpoint there is about a three percent increase in fuel consumption. For example, for a 30’ x 100’ greenhouse the savings between a 2° F differential and a 6° F differential thermostat can be as much as 500 gallons of fuel oil, 750 gallons of propane or 700 therms of natural gas over the fall to spring heating season. Be sure to locate the thermostat in the center of the greenhouse to sense plant temperature. It should be shaded to avoid influence from direct sunlight. Although a controller is a more expensive change, it can provide good savings by integrating both the heating and cooling systems.

Wall insulation reduces heat loss through the glazing by more than 75%.
Photo: John W. Bartok, Jr.

Infiltration

This is a major source of heat loss in many greenhouses. Check for weatherstripping on doors. Cover doors that are only used in warm weather with a sheet of poly. Batten roll-up sidewalls for the winter.

Lubricate fan shutters frequently so that they close tightly. A partially open louver may allow several building air changes per hour. For example, a 48-inch fan louver that fails to close properly leaving 1-inch gaps, allows 23,000 Btu/hour of heat to escape, costing $0.46 if you are burning $2.00 fuel oil. Shut off some fans during the winter and cover openings with insulation or plastic to reduce infiltration of air.

Check for tight edge and closure seals on energy screens. The chimney effect through cracks can allow considerable heat to escape to above the screen.

Because energy is the second-largest cost in producing most plants, reducing fuel use should be given a top priority before the heating season arrives.

John is an agricultural engineer, an emeritus extension professor at the University of Connecticut and a regular contributor to Greenhouse Management. He is an author, consultant and certified technical service provider doing greenhouse energy audits for USDA grant programs in New England. jbartok@rcn.com