|How to Buy an Energy-Efficient Commercial Boiler|
|Written by Federal Energy Management Program|
|Tuesday, 08 June 2010 15:17|
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How to Buy an Energy-Efficient Commercial Boiler
Why Agencies Should Buy Efficient Products
■ Executive Order 13123 and FAR section 23.704 direct agencies to purchase products in the upper 25% of energy efficiency, including all models that qualify for the EPA/DOE ENERGY STAR® product labeling program.
■ Agencies that use these guidelines to buy efficient products can realize substantial operating cost savings and help prevent pollution.
■ As the world's largest consumer, the federal government can help "pull" the entire U.S. market towards greater energy efficiency, while saving taxpayer dollars.
a) This Recommendation covers low- and medium-pressure boilers used primarily in commercial space heating applications. It does not apply to high-pressure boilers used in industrial processing and cogeneration applications.
b) These “Best Available” efficiencies do not consider condensing boilers, which are generally more efficient, but are not readily ratable with ANSI Z21.13.
How to Select Energy-Efficient Boilers
Specify boilers with efficiency levels that meet this Recommendation. Select only boilers rated under the certification program run by The Hydronics Institute (see “For More Information”) of the Gas Appliances Manufacturers Association (GAMA). Although the HI directory reports only combustion efficiencies, thermal efficiencies can be calculated for model series listed without a pound sign by dividing gross output by input (using 140,000 Btu/gal. for #2 oil models).
Sizing and Part Load Performance
A boiler system should be capable of meeting the building’s peak heating demand while also operating efficiently at the more common part-load conditions. Sizing and selecting a boiler system properly, therefore, requires a knowledge of the peak heating load, as well as an understanding of the load profile. If building loads are highly variable, as is common in commercial buildings, designers should consider installing multiple, smaller (modular) boilers. Modular systems are more efficient because they allow each boiler to operate at or close to full rated load most of the time, with reduced standby losses. Other efficient options for handling variable loads are modulating boilers, which can run at partial capacity (instead of cycling on and off), and condensing boilers.
While both water and steam models are covered in this Recommendation, water boilers and distribution systems tend to have lower maintenance requirements.
There is a broad array of options in boiler equipment and controls that can enhance energy performance: stack gas heat recovery equipment, such as air preheaters and economizers; condensing heat exchangers, which also utilize stack gas waste heat; turbulators (fin enhancers) to improve heat transfer and balance of gas flows between tube banks; water recovery equipment, to re-use heat from blowdown and water return condensate; outdoor temperature controls, which control the system loop temperature in accordance with outside temperatures; electronic ignition devices; increased boiler and piping insulation; and high performance (including “power”) burners.
Maintenance and Operations Tips
Several diagnostic and preventive procedures are important to maintaining efficient operation. Flue gas temperature monitoring is useful in detecting efficiency and operation problems. Maintaining steady excess air levels (with an oxygen “trim” system) ensures that burners will mix air and fuel efficiently. The Boiler Efficiency Institute provides maintenance and operation manuals for boilers and boiler controls (see “For More Information”). Low water levels can damage boiler vessels, so water levels must be checked frequently. Water treatment prolongs the life of boilers, while increasing efficiency. Waterside and fireside surfaces should be cleaned annually. Steam boilers should be blowndown daily to remove sludge and sediment.
Annual energy use in this example is based on 1,500 equivalent full-load hours per year. The assumed gas price is 40¢/therm, the federal average gas price in the U.S.
Understanding the Cost-Effectiveness Table
In the example shown above, a 5,000,000 Btu/h gas-fired water boiler with a thermal efficiency of 80.0% is cost-effective if its purchase price is no more than $16,000 above the price of the Base Model. The Best Available model, with an efficiency of 83.2%, is cost-effective if its price is no more than $41,000 above the price of the Base Model.
How Do I Perform a Life-Cycle Cost Analysis for My Situation?
FEMP provides a Web-based boiler “cost calculator.” Go to www.eren.doe.gov/femp/ procurement/boiler.html, and click on the “Cost-Effectiveness Example.”