EQUIPMENT NOTEBOOK DESIGN Selecting Small Condensing Boilers Considerations for single- and multiple-unit applications By PETER C. D’ANTONIO, PE, CEM, LEED AP not require burners. Gas and air are INSTALLATION PCD Engineering Services Inc. drawn into a combustion chamber and Longmont, Colo. ignited by an electronic spark. Between boiler can consume several 60 and 70 pulses of the gas/air mixture times its initial cost in fuel in are ignited per second, producing A a year. That, coupled with rapid burning and a pressurized pulse. MAINTENANCE OPERATION & escalating utility rates and an increasing Once started, the process becomes self- focus on energy and the environment, perpetuating, no longer requiring the has engineers and contractors turning spark ignition. to an ever-expanding array of super- Pulse-combustion boilers are a bit efficient condensing hot-water boilers. Company Inc. of Viessmann Manufacturing Courtesy louder than their more traditional Choosing a condensing hot-water counterparts. Mufflers and insulation boiler involves consideration of HVAC- reduce the extra noise, but do not system type, life-cycle cost, and owner eliminate it; therefore, noise should COMMISSIONING needs. This article will examine factors be taken into account in decisions to to be weighed in the application of single purchase and/or locate a unit. and multiple condensing hot-water Although condensing boilers cost boilers. Its focus will be on the selection more than non-condensing ones, they and sizing of both floor-standing and can last for 25 or more years when wall-hung condensing boilers with properly applied, installed, and main- inputs of up to 500,000 Btuh. tained. Consider a case in which a The greatest consideration in deter- condensing boiler costs $2,000 more mining whether a gas-fired condensing than its non-condensing counterpart, boiler is right for a particular project but results in additional energy savings concerns return-water temperature. Cutaway of a condensing boiler. of $400 per year. That equates to a Condensing boilers achieve high effi- five-year simple payback ($2,000 divided ciency by capturing the additional heat 140 F. For this reason, condensing boilers by $400) and a 20-percent return on released from condensing flue gas (up to are most suitable for low-temperature investment ($400 divided by $2,000). 12 or 13 percent of the original energy systems, such as radiant floor heating, Condensing boilers are environmen- content of the gas). For flue gas to con- swimming pool or spa, dedicated water tally friendlier than non-condensing dense, return water must be below about heating, snow melt, and water-source ones. Through their advanced controls heat pump. The application of a con- and high-grade burners, they typically densing boiler to a heating system with produce less nitrogen oxide (NOx) while Peter C. D’Antonio, PE, CEM, LEED AP, a return-water temperature above 140 F, exceeding all current environmental is founder and president of PCD Engineer- such as a baseboard heating system, standards. In California, look for boilers ing Services Inc., an award-winning would result in a peak thermal efficiency provider of sustainable mechanical/ of 88 percent and represent a squandered electrical-design, energy-management, investment. and integrated-building-system solutions. Condensing boilers are state of the Additionally, he serves as a board member art, with advanced heat exchangers, and committee chair for several professional burners, and controls. There are two and community organizations, including basic types of these boilers: one using of Weil-McLain Photo courtesy the U.S. Green Building Council and traditional gas-fired combustion and the Association of Energy Engineers. In the other using the pulse-combustion September, he will present at HPAC Engi- principle. Both use sealed combustion neering’s third annual Engineering air supplied through intake piping Green Buildings Conference and Expo and direct vent products of combustion. High-grade stainless-steel burner with (www.engineeringgreenbuildings.com). Pulse-combustion boilers, however, do woven steel-fiber mesh. HPAC Engineering • May 2006 63 EQUIPMENT NOTEBOOK certified by the South Coast Air Quality Heaters and Small Boilers. Rule 1146.2 Management District, which looks to applies to units with rated heat inputs A boiler can consume reduce NOx emissions from large from 75,000 to 2 million Btuh. (commercial) water heaters, small With high-efficiency equipment, several times its initial cost (industrial) boilers, and process heaters return on investment is justified mainly as defined in Rule 1146.2, Emissions of through the energy savings generated in fuel in a year. Oxides of Nitrogen from Large Water over the life of the equipment. Energy savings, in turn, are dependent on the maximization of operating efficiency over all operating conditions. Many efficiency terms are used in the market- place. Steady-state efficiency, combus- tion efficiency, cycle efficiency, and annual fuel-utilization efficiency (AFUE) are but a few. AFUE is good for compar- ing boilers, as all manufacturers publish AFUE ratings. For condensing boilers, AFUE typically ranges from 90 to 94 percent. Maximum efficiency is achieved when a heat source (boiler) precisely tracks system load, and cycling is mini- mized. Designers should select and size equipment with maximizing efficiency Do not oversize.Be sure to perform an accurate heating-load estimate before selecting a boiler. Do not size a boiler based on estimates of building thermal,ventilation,or domestic-water loads. through load tracking and maximizing boiler run time in mind. Small condens- ing boilers typically have a 4-to-1 (25- to 100-percent modulation) or 5-to-1 (20- to 100-percent modulation) turndown. When attempting to maximize boiler efficiency, keep the following in mind: Do not oversize. Be sure to perform an accurate heating-load estimate before selecting a boiler. Do not size a boiler Circle 172 64 May 2006 • HPAC Engineering EQUIPMENT NOTEBOOK based on estimates of building thermal, in a commercial application. Many DESIGN ventilation, or domestic-water loads. smaller condensing boilers are designed When selecting a boiler, discuss all for residential and light-commercial factors, including performance expecta- applications. tions, life-cycle cost, and sizing consider- Condensing boilers offer a number of ations, with the design team and owner benefits, including efficiency, compact early in the design process. When sizing size, and low emissions. We have taken a a boiler system, the number of boilers look at important considerations, includ- INSTALLATION is an important consideration. Even if ing application, life-cycle economics, not dictated by law, full or partial redun- and owners’ needs. The field of available dancy may be necessitated by the owner’s products is growing at a fast rate as needs or the nature of the operation. Company Inc. Viessmann Manufacturing Photo courtesy of technology advances. Be cautious when For instance, a 911 call center may need applying first-generation equipment, to be online at all times. or risk getting burned. Stick with equip- Besides providing backup, a multiple- Stainless-steel radial-heat-exchanger ment with a track record of successful condensing-boiler system has several surface. performance and documented mainte- OPERATION & MAINTENANCE OPERATION & advantages over a single-condensing- nance. You will be glad you did. boiler system, including higher seasonal loads above 200,000 Btuh. efficiency, partial heat delivery during Lastly, when selecting boilers, always Coming in September: HPAC Engi- downtime, consolidation of heat sources, keep safety in mind. Specifically, look neering’s third annual Engi- and ease of installation attributable to for the American Society of Mechanical neering Green Buildings Con- COMMISSIONING EGB reduced unit size. Generally, multiple- Engineers stamp. This is especially im- ference and Expo. To learn Engineering Green Buildings boiler systems are appropriate for system portant when using a residential product more, go to www.egbregistration.com. Circle 178 Circle 154 66 May 2006 • HPAC Engineering.