A poly cover over un-needed fans and shutters eliminates cold air infiltration.
Photos courtesy of John W. Bartok Jr.

After last year’s respite in energy price increases, they appear to be on the rise again. Fuel oil, natural gas and propane have more than doubled in cost since this time last year. Even electricity prices have gone up in many states.

Interest in an upgraded heating system, new computer controls or multiple energy screens are common. But looking around the greenhouse, large gaps around the doors and shutters, low-cost wall insulation and equipment maintenance have been overlooked as a way of achieving significant savings.

The following six energy savers cost very little to implement and have a short payback.

Reduce infiltration. Gaps under exterior doors, in shutters or vents and around the foundation can let a considerable amount of heated air escape. For example, a 48” fan shutter that fails to close properly leaving 1” gaps, allows about 23,000 Btu of heat to escape each hour, costing 46¢ with $2.00/therm natural gas and 63¢ with $3.00 fuel oil. A little time spent sealing the door cracks with weatherstripping or placing removable insulation board or poly over the shutters will reduce this to a minimum.

Dust buildup on HAF fan blades and guards can reduce air flow up to 30%.

Insulate the kneewall to bench height. Heat loss from a greenhouse is directly related to the surface area, insulation value of the glazing and the temperature difference between inside and outside. Although the sidewall surface of a free-standing greenhouse is less than 20% of the structure’s total surface, the heat loss can be significant. I have measured temperature reductions in kneewall surface temperature of 40°F when an inch of insulation board ($1/sq ft) or a piece of aluminized double bubble wrap ($0.50/sq ft) was placed on the inside wall. Payback is usually less than one heating season. For hoophouses, the flexible bubble wrap can be tucked between the plastic and hoops.

Pipe insulation has a good payback. Whether it’s the ¾” pipe that supplies the sink or the 4” line supply from the boiler, there is considerable heat loss from hot water pipes. This heat loss continues every day the system is operating. Payback for insulating pipes usually takes less than two years. At $3.00/gal of fuel oil, the approximate annual savings for installing 1” thick fiberglass or foam insulation on the ¾” pipe will be about $3.15/ft and $16.50/ft for the 4” pipe. It is about half this for a natural gas fired system.

Change to more accurate thermostats. Many older mechanical thermostats have a large differential between on and off, sometimes as much as 8°F. When the greenhouse is heated above the setpoint, the temperature difference between inside and outside is greater, increasing the heat loss. For every degree that the temperature is above the setpoint, there is an approximate 3% increase in fuel consumption. In most cases, installing a thermostat having +/- 1°F will save approximately 10% on the heating bill. Cost is about $100. Locate both the heating and cooling system thermostats together near the center of the growing area for more accurate sensing. Shade them from direct sunlight. All thermostats should be checked for calibration at least once a year.

Clean HAF and exhaust fans. An eighth of an inch of dust accumulation on blades, guards and shutters can reduce efficiency up to 30%. Worn belts increase slippage and fan output. Use a straight edge to check alignment of the pulleys and adjust the belt tension so that it has no more than ½” depression between the pulleys.

Replace old incandescent and fluorescent lighting with efficient LEDs. A 9.5-watt LED bulb gives as much light as a 60-watt incandescent bulb, has a longer life and uses 84% less electricity. Energy-efficient LED strip fixtures are available in many light options.

Small savings in energy add up over the year. Simple conservation measures are often easy to install or implement and require only a small investment of time and money.

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