ASHRAE Journal - November 2009 - 8

Letter From the Editor –0.02 in. w.c. R.A. Shaft –0.07 in. w.c. R.A. Ceiling Plenum +0.03 in. w.c. +0.05 in. w.c. The author’s response to a recent letter to the editor challenged the credentials of the letter writer. The letter was written by Vernon Peppers, P.E., Member ASHRAE, regarding the article, “Equal Friction Solution for Duct Sizing,” in the May 2009 edition. To Set the Record Straight… Figure 1: Typical pressures—plenum return. +0.05 in. w.c. –1.0 in. w.c. –0.9 in. w.c. Return Duct +0.05 in. w.c. Figure 2: Ducted return pressures—plenum return. Mr. Peppers is a practicing engineer, with more than 20 years of experience. He started his HVAC career working in installation for his father’s HVAC contracting firm. He has designed many duct systems. Mr. Peppers is chair of TC 9.2., Industrial Air Conditioning, and a voting member of TC 5.2, Duct Design. He is also a member of TC 1.1, Thermodynamics and Psychrometrics; TC 1.5, Computer Applications, and TC 5.8, Industrial Ventilation Systems. Mr. Peppers has written many computer programs, including a program to calculate pipe sizes in flashing flow, ventilation requirements of a paper machine hood, pump head calculations, both for DOS-based computers and Windows-based computers, along with several accounting programs. To Correct the Article… R.A. Duct Re: 5 Fundamental Changes In the Last 50 Years I enjoy Joe Lstiburek’s “Building Sciences” column very much—it is the first article I read in each issue of ASHRAE Journal. I find it informative as well as entertaining. But I do not always agree with Joe, and I especially take issue with his position against the use of architectural plenums for return air, most recently discussed in his July 2009 article, “5 Fundamental Changes in the Last 50 Years.” The benefits of using the ceiling cavity as a return air plenum (in lieu of a fully ducted return system) include: • Reduced HVAC system costs of about $3 to $5 per square foot, about 10% to 20% of the total HVAC system cost; • Reduced costs to other trades to accommodate the congestion caused by the added return air ductwork, such as raising the floor-to-floor height or adding additional offsets in plumbing and sprinkler piping; • Reduced fan energy costs of about 20% to 30% due to the much lower pressure drop of the plenum return system; and • Reduced fan energy in systems with outdoor air economizers due to the ability to use nonpowered relief or relief fans in lieu of less efficient return fans, which are generally required when return air is fully ducted. Countering these benefits, which are 100% ensured, is the possibility of indoor air quality problems, such as the moisture and subsequent mold problems Joe points out in his articles. So, how big is the risk of indoor air quality problems? Not much, at least in my climate. From my desk I can see 8 ASHRAE Journal Please visit www.ashrae.org/journal/equalfrictionsolution, where you will find: • The article. • A letter from Alex London, P.E., and the author’s response. • A review by Patrick Brooks, P.E., of the article and the London letter and response. Fred Turner, Journal Editor high-rise office buildings in both San Francisco and Oakland, Calif. I designed the HVAC systems for many of these buildings, and I have done work in perhaps 100. None of them—not one—has a ducted return system. I would confidently wager that more than 90% (and I would not be surprised if 100%) of the high-rise buildings in both cities use ceiling plenum returns. Of the several hundred buildings my firm has designed, every one of them has ceiling plenum returns except where not allowed by code (e.g., combustible structure) or where there were strict pressurization requirements (e.g., labs, hospitals). Surely, if Joe were right about the negative impact of unducted returns, more than a few of these buildings would be “sick” buildings, yet I do not know of one that is. I think at least one reason why these buildings work is because properly designed return air plenum systems result in very similar building pressures as ducted return air systems. Buildings are typically slightly pressurized to about 0.05 in. w.c. (12.5 Pa). A ceiling return air grille typically will have a pressure drop of only about 0.02 in. w.c. (5 Pa). So the space and the ceiling plenum have Letters Continue on Page 10 ashrae.org November 2009
ASHRAE Journal - November 2009

Table of Contents for the Digital Edition of ASHRAE Journal - November 2009
