Heating with Oil - New Developments
Because of new burner technologies, low sulfur fuels, and bio fuels, North Americans are finding out what Europeans have known for a while - heating with oil makes good sense.
Over the past several years, there has been an evolution in heating oil technology that will affect heating efficiency, emissions, and appliance configurations. Condensing boilers and furnaces have brought a significant increase in efficiency to natural-gas-fired appliances, and the same potential efficiency jump exists for oil-fired appliances. An oil-fired condensing boiler or furnace can achieve an efficiency rating of 90%–95%; noncondensing boilers and furnaces can be 84%–86% efficient. Condensing oil furnaces have been available for years, but concerns over corrosion, cost, maintenance, and reliability have severely limited the market for the oil-fired versions of these appliances.
That may be changing.To a large degree, this change has been supported by efforts of the U.S. DOE, the New York State Energy Research and Development Authority (NYSERDA), and the National Oilheat Research Alliance (NORA),working with research organizations, manufacturers, and service organizations.NORA,the leading industry organization involved with advancing fuel oil technology, recently sponsored the development of a new oil-fired condensing warm-air furnace that features a condensing section that is very easy to maintain.This new furnace is currently available as the HEO Series from the Dornback Furnace Division of Adams Manufacturing Company.And Monitor Products has recently introduced a condensing oil-fired boiler to the U.S. market.
Condensing boiler designs can be classified as those that are designed for condensing anywhere within the heat exchanger and those that have a primary (noncondensing) and a secondary (condensing) section. For boilers in the second category, it is important to avoid significant condensation in the primary section to avoid excessive corrosion and premature failure. Depending upon the design and control strategy, this may involve attention to water flow rates, minimum return water temperatures, or other strategies.The new NORA-sponsored furnace sports a design that prevents condensation in the primary heat exchanger, which is not designed to handle the corrosive condensate.
For a boiler to recover latent heat from the flue gas water vapor, the return water temperature to the boiler must be lower than the flue gas water vapor dew point—roughly 125ºF for oil, higher for gas. Conventional residential baseboard heating systems are designed to operate at much higher temperatures—180ºF, for example. While radiant floor systems operate at much lower temperatures and can work well with condensing boilers, these are uncommon, particularly in existing homes. Tests are ongoing in a DOE-sponsored project at Brookhaven National Laboratory (BNL) to study the effect of different control strategies with baseboard heating systems. Results to date show that by operating with an outdoor reset control on the supply water temperature, using a mixing valve, very significant levels of condensation can be realized over most of the heating season. This approach allows the boiler to provide high-temperature water—180ºF—to a domestic hot water tank and lower-temperature water—110ºF-160ºF, depending on outdoor temperature—to the baseboards simultaneously.
In Europe, the use of condensing boilers is relatively common and several floor-standing products are available. Recently,wall-hung, oil-fired condensing boilers with very compact heat exchangers have been introduced in Europe in response to market demands. Some of these products use two-stage oil burners with pumps that can deliver two fuel oil pressure levels, and variable-speed brushless DC combustion air blowers. (It is interesting to note that warm-air furnaces for home heating essentially do not exist on the European market.This is partly because they are less energy efficient and partly because residential air conditioning is very little used in Europe.) Because these newer condensing boilers have tight heat exchangers, the sulfur content of the fuel must be low;manufacturers are specifying 100–500 ppm.These products currently could not be used in the United States, as the average sulfur content of #2 fuel oil here has been in the range of 1,000–2,500 ppm (0.1–0.25 weight percent) for many years. But that too may be changing.
Recent U.S. EPA rules mandate lowering the maximum sulfur content of diesel fuel, which is now set at 500 ppm, to 50 ppm by 2006.Within the oil heat industry there is a growing voice for lowering the sulfur content of heating oil as well. One key advantage to this move would be reduced fouling of appliance heat exchangers. Low-sulfur fuel also reduces sulfur dioxide and primary emissions of fine particulate into the air.With properly operating burners, the dominant fouling deposit is iron sulfate scale, formed by a reaction between sulfuric acid and iron from the heat exchanger material. Field and lab tests have clearly demonstrated the advantages of reduced sulfur content (see “The Fate of Sulfur During Combustion”).
A two-year field test, sponsored by NYSERDA, in which 800 homes in upstate New York have been converted to low-sulfur fuel, is currently nearing completion. Results to date indicate a dramatic reduction in equipment-cleaning costs.The final report is being prepared. Several fuel oil marketers have converted to low-sulfur heating oil and now offer only this fuel to their customers. NORA is strongly supporting the transition to low-sulfur fuels.
Another fuel-related advancement in oil heat is biofuels. These domestic, renewable, sulfur-free fuels are of growing interest for partial replacement of oil. Biofuels are derived from new or used vegetable oils through a process involving reaction of methanol and vegetable oil in the presence of a catalyst, followed by separation of by-products. The fuels are miscible in heating oil.While a wide range of mixture ratios can be used, a 20% blend of bio- fuels in heating oil (B20) seems to be most popular. Relative to fuel oil, biofuels have a similar viscosity and a slightly lower heating value. One area of concern with these fuels is the low temperature flow behavior—they gel at temperatures
higher than oil does, leading to potential problems in colder regions if they are used at high concentrations in the fuel blend. There is also a concern, at high concentrations, with biofuel compatibility with the seal materials used in fuel oil pumps.
In tests to date, with 20% blends, routine use of this fuel shows no effects on service requirements or on component failure rates. A two-year field test in upstate New York is nearing completion in which 100 homes, served by one fuel oil dealer, have been converted to B20 with no equipment changes or adjustments. The purpose of this test was to identify any problems with central storage, distribution via normal commercial practices, storage in home tanks, and heating equipment service. The results have been an apparently seamless conversion to B20. This same dealer is currently conducting limited tests with 100% biofuel (B100). No seal material changes or special provisions to protect against low-temperature gelling have been made for these limited B100 tests.
There have also been some exciting recent developments in the burners used for oil combustion in home heating applications. Currently, the market is totally dominated by yellow-flame, retention head burners. Manufacturers have adopted permanent split-capacitor (PSC) motors over the past few years that reduce power consumption. Burner fans have higher static pressures that improve performance under back pressure conditions. In the case of highefficiency appliances that are sidewall vented, these burners allow installation without a draft inducer. Burner primary controls are microprocessor based and provide the option for pre- and postpurge operation (purges blow air through the combustion chamber before and/or after fuel is burned). Controls manufacturers, including Carlin Combustion Technologies and Honeywell, Incorporated, are also very actively developing low-cost concepts for monitoring the condition of the flame, run time, and other operational parameters. This information will be communicated to Web servers, processed, and presented to service organizations.
The benefits of these developments for both the service organization and the homeowner are substantial. The service organization can increase the efficiency of their service and delivery operations. By providing service to units that have developing problems but that have not yet failed to operate, they can greatly improve reliability for the customer. In addition, efficiency degradation that might be caused by extended operation under sooting conditions can be prevented.
In another new development, Heat Wise, Incorporated, has recently completed certification testing of a new oil burner that uses novel fuel pump and blower components to reduce electric power. Conventional burners draw from 160 to 230 watts.This new burner uses about 50 watts, and should be available to consumers within a year.
Perhaps the most exciting new development in oil burner technology is highly recirculating, low-nitrogenoxide (NOx) burners. Here the burner head air flow patterns are very much different from the patters of conventional yellow-flame burners. Essentially, all of the combustion air is introduced in a single stream on the burner centerline into the combustion zone that is contained in a metal flame tube. This produces a high rate of recirculation of hot combustion products back to the root of the flame, vaporizing the oil before it is burned. The effect is a “transparent” or blue flame with NOx emissions less than half of those from conventional burners. This approach also provides a burner that can operate very cleanly even in very small, cold wall combustion chambers, which have traditionally been challenging applications for yellow-flame, retention head burners.
The major burner manufacturers have completed basic development and product certifications and are entering into extended field testing. Versions of this technology have been operating in homes in Europe for several years and have proven to be reliable in their applications. Appliances and service practices in the United States, however, are considerably different, and a cautious rate of introduction is expected.This technology should be available to consumers within two years. BNL is currently conducting field tests of a German, advanced-version, low-NOx burner that is fully modulating between the firing rates of 0.35 and 0.55 gallons of oil (49,000–77,000 BTU) per hour. This unit is pressure atomized with a variable-frequency pulsing-piston fuel pump and a variable-speed blower. Modulation is also of interest for larger building applications. Heat Wise, Incorporated, is actively developing a fully modulating prototype that operates over the firing rate range of 0.75 to 4 gallons per hour.
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