Heating System Efficiency for Greenhouses - Important Factors to Consider
Fall is a good time to have the heating system serviced in preparation for the cold winter months. The cost is usually more than offset by the 5 – 10% gain in efficiency. The following are a few of the things that may be overlooked by service personnel.
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, 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% increase in efficiency in fuel usage in a 30' x 100' greenhouse will save about 200 gallons of fuel oil during the winter. When done on a regular basis, the test can indicate when problems are beginning to occur.
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 too small a wire size that feeds the furnace. This problem is quite common in hoophouses where long wires are needed to power heaters that are located at opposite ends.
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 in the standby. A barometric damper (oil fired heating unit) or draft hood (gas fired units) should be located in the first 12" of the stack nearest the burner. Its purpose is to maintain a constant draft on the fire. Gordon Siebring, Siebring Manufacturing, George IA has reported that savings of up to 15% can be achieved with the installation and proper adjustment of a barometric damper.
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 those recommended result in over-firing and possible damage to the heating unit.
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% excess air, 20 cu ft/1000 Btu of heater input is generally considered adequate. With fuel oil, 1400 cu ft/ gallon of No. 2 fuel oil is required for proper combustion and venting.
It takes about 250,000 Btu/hr of heater capacity to heat a typical 30' x 100' hoophouse with a double poly cover to maintain a 65ºF temperature on a night when it is 0ºF outside. A 250,000 Btu/hr gas heater will require about 5000 cu ft of air/hour to supply the oxygen to the burner. Without any make-up air the 30,000 cu ft of air in the greenhouse will be used up in 6 hours. The heater will only operate at peak efficiency for part of that time and then incomplete combustion will occur and heat output will decrease. It may also result in pollutants, such as ethylene gas and sulfur dioxide getting into the greenhouse and causing damage to the plants. A motorized louver or 6" PVC pipe that provides make-up air near the burner is necessary. Make sure that the intake is located so that it is not blocked by snow.
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' above the roof of the greenhouse or 3' above a 10' 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.
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., Extension Professor Emeritus & Agricultural Engineer, Department of Natural Resources and the Environment, University of Connecticut, Storrs CT – 2013