Better efficiency boosts profits

Efficiency testing of a furnace or boiler involves a simple 10-minute procedure, and if done on a regular basis, can indicate when problems are beginning to occur. Records of temperature and carbon dioxide levels of the flue gases taken several times during the heating season may indicate that carbon is building up on the heat exchange surfaces or air leaks are developing in the combustion chamber.

An efficiency test will also indicate when excess air is being supplied to the fire and robbing it of some of its expensive heat. Adjustment of the burner to obtain higher efficiency can make a significant reduction in fuel usage over the heating season. For example, in winter, a 2% increase in efficiency in a furnace or boiler heating a 30-foot by 150-foot greenhouse will save about 250 gallons of propane, 225 centum cubic feet of natural gas or 200 gallons of fuel oil. This increase is quite realistic based on recent tests on greenhouse heaters and furnaces in Connecticut.

If your heating units are maintained by a service person, for an efficiency test, run after the units are cleaned. If you maintain your own furnaces and boilers, it may pay to purchase a combustion analyzer that can measure oxygen (O2) and carbon dioxide (CO2) efficiency, excess air, draft and pressure.

Before the heating season begins, the furnace or boiler should be cleaned and serviced. The burner-blast tube, fan housing and blower wheel should be free of dirt. Leaks into the combustion chamber, especially joints between cast iron boiler sections and around the fire door, should be sealed. The oil filter should be replaced and carbon should also be removed from heat-transfer surfaces. Manufacturers’ recommendations should be followed for replacing the nozzle and adjusting the ignition electrodes.

For gas burners, servicing consists of cleaning the orifice, the burner, the heat-transfer surfaces and the controls. Gas valves are checked for operation and leaks. Gas pressure is adjusted for the type of fuel used. The pilot light or ignition system is cleaned, soot is removed and the fan and limit controls are checked for safety.

In the combustion process, air is mixed with fuel, which is heated. Excess air is always supplied to help with mixing and to provide carbon monoxide-free combustion.

For oil units, a smoke test is conducted to reduce pollution. A high smoke level indicates that the carbon in the fuel is not being burned and is escaping up the chimney.

When new greenhouses are built with a gutter height of 16 feet high or greater, the ratio of wall area to roof area results in greater heat loss. For example, a 20,000-square-foot greenhouse with 16-foot height to the gutter will have about 30% more wall surface area than a standard house with 12-foot-high walls.

Most of these new houses have one or more energy curtains to insulate the roof area, resulting in the heat loss through the walls being greater than through the roof. If the above house was glazed with double polycarbonate it would require about 428,000 BTU/hr for the uninsulated wall area and only 309,000 BTU/hr for the roof area with a double screen on a night with a 60° F temperature difference between inside and outside. If the walls were glass, the savings would be considerably more.

The systems for insulating the walls have improved over the last few years. A system using a center motorized curtain roller is common, faster and more efficient, and stays straighter than rolling from the top only. The gearmotor unit with a large ratio rotates the shaft. It can be located on one end of the shaft for short greenhouses or in the center for long ones. Depending on weight and design, curtains as long as 450 feet can be rolled by one gearmotor. Safety torque and limit switches protect the motor.

Wall curtains need to be installed to provide a tight seal all the way around the edges to prevent the cold air next to the wall from draining down and into the growing area. A fixed seal of polycarbonate sheets or fire-resistant screen material is frequently installed to create the seal.

On gas units, CO2 in the fuel gas can result from flame impingement on a cool firebox surface or from insufficient primary-combustion air. It also produces a yellow flame that can be detected by the eye. The pressure setting of the gas valve should be checked because excess pressure will give a smoky flame that accumulates soot on heat-exchange surfaces.

A measure of the CO2 or O2 of the flue gases indicates how much heat from the fuel has gone to heat the greenhouse and how much is escaping the chimney. Efficiency increases as the CO2 reading increases or the O2 reading decreases. The test instrument must be set to the heating-unit specifications of the manufacturer, as results will vary depending on the type of fuel, excess air, poor fuel-air mixing or poor regulation of the fuel input.

On older units, the efficiency achieved should be more than 70% for small burners and 75% for larger burners. If the efficiency recorded is much below the above level, consideration should be given to replacing the burner or, possibly, the furnace or boiler.

New unit heating efficiency is usually higher than 80% for oil burners and 90% for gas units. Condensing-type units achieve more than 95% efficiency. The increased efficiency of a new unit will usually have a short payback.

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. [email protected]

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