High-efficiency, condensing-gas heaters will operate at higher than 90%.
Photos courtesy of John W. Bartok Jr.

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. Some instruments come with a printer such as manufacturers Bacharach Inc, Testo North America and E-Instruments International. Prices for these electronic all-in-one instruments start at $800.

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.

Efficiency tests are usually done by sampling the flue gases to determine the amount of unburnt carbon.

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.

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.

For gas burners, servicing consists of cleaning the orifice, the burner, the heat-transfer surfaces and the controls.

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. jbartok@rcn.com