A chimney height 2 feet above the ridge, along with a cap, prevents downdrafts.
Photo: John Bartok, Jr.

The efficiency of most greenhouse heating systems can be increased 5 to 10 percent with just a few replacement parts and minor adjustments. Fall is a good time to have this service done. Here are a few uncommon things often overlooked.

Barometric dampers compensate for variations in wind to keep an even draft on the fire.
Photo: John Bartok, Jr.

Efficiency test – This 10-minute test should be done by the technician each time a furnace or boiler is serviced. It consists of measuring the flue gas temperature and carbon dioxide content, smoke level and draft reading over the fire. Adjustment of the air to the burner can improve efficiency and reduce carbon buildup on heat exchangers and lower emissions. A 2-percent increase in efficiency in fuel usage in a 30-feet x 100-feet greenhouse will save about 200 gallons of fuel oil, 300 gallons of propane or 270 ccf natural gas during the winter. When done on a regular basis, the test can indicate when problems are beginning to occur.

Fuel pressure – The normal operating pressure of a fuel oil pump is 100 psi. On atmospheric types of burners, the usual ranges of manifold gas pressure are 3 to 5 inches of water for natural gas and 10 to 12 inches for propane. The pressure should be checked and adjusted if necessary. Higher pressures than recommended result in over-firing and possible damage to the heating unit.

Barometric damper – The draft on the fire is affected by the height of the chimney, the temperature of the flue gases and the temperature and wind speed of the outside air. Increasing the height, flue gas temperature or wind speed will increase the draft. Draft is also greater in the winter due to cold temperatures. Excessive draft increases the heat loss when the heating unit is on standby. A barometric damper (oil-fired heating unit) or draft hood (gas-fired units) should be located in the first 12 inches of the stack nearest the burner. Its purpose is to maintain a constant draft on the fire.

Flue pipe and chimney – To get adequate draft and reduce the effect of wind turbulence, the top of the chimney should extend at least 2 feet above the roof of the greenhouse or 3 feet above a 10-feet horizontal distance from the roof. Install a cap on the top to prevent down drafts and possible air pollution injury to plants. Avoid decreasing the size of the flue connector from the heater to the chimney as this affects the draft and capacity.

Make-up air – The recommendations to tighten up the greenhouse for the winter may adversely affect the operation of the furnace or boiler. It is not uncommon to hear of greenhouses that freeze up during a cold night even if the heater has been operating. This is usually due to the lack of oxygen for combustion. How much air is necessary for good combustion? With most gas burners adjusted to operate with 50-percent excess air, 20 cu. ft./1,000 Btu of heater input is generally considered adequate. With fuel oil, 1,400 cu. ft./ gallon of No. 2 fuel oil is required for proper combustion and venting. A motorized louver or 6-inch PVC pipe that provides make-up air near the burner is necessary. Make sure the intake is located somewhere that it is not blocked by snow.

Adequate voltage to the heating unit – Low voltage causes motors to burn out by overheating the windings. It can also affect the operation of the transformer creating a weak spark. Voltage at the heating unit should be at least 110 volts. Two causes of low voltage are inadequate power to the electric distribution panel in the greenhouse and wire that is too small to feed the furnace. This problem is quite common in hoophouses where long wires are needed to power heaters that are located at opposite ends.

Environment control sensors – Thermostats and sensors should be cleaned several times a year. Blow off dust with compressed air. Check for accuracy by placing in an ice bath or comparing to an accurate digital thermometer. Be sure that the sun shield is in place and that the location is close to the plants. Aspiration of the sensors can reduce the differential between the high and low setting by several degrees.

Keeping the heating system in good repair and operating condition can reduce fuel use. Annual servicing and adjustment is a good starting point.

John W. Bartok Jr. 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