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"Stack pressure and wind pressure both influence net building or space pressure flow. Some walls can show a positive flow, while others could be negative (because of) wind. Trying to overcome anything but a light wind's effects usually is not possible or may damage window seals, windows, or doors. The objective should be to maintain a positive net flow from the area of lower dew-point temperature to higher dew-point temperature. In most climates and seasons, this is inside to outside. In cold climates with higher inside humidity, you can't run the space negative, and you don't want to push humid air into the wall cavity to freeze. Net neutral appears to be the best alternative.

"If not DP direct measurement, then what? Volumetric tracking by zone or AHU (air-handling unit) is much more stable (and) controllable and can manipulate very small increments of DP using differential volume control."

In response, author Dave Moser writes:
"Indoor- and outdoor-sensor port terminations should be located in as representative a location as possible, with minimal bias to one side of the building or the other, to minimize wind effects. Shielding from wind effects, regular calibration of static-pressure sensors, and proper tuning of control loops are necessary for an active building-pressurization-control strategy to work properly.

"Pressure set point can be adjusted to suit project conditions. If the climate is cold and there are concerns about humid air condensing and freezing in a wall cavity, 0 in. may be the best set point, as 'papaD' suggests.

"Volumetric tracking is another method of pressurization control, but may be difficult to add to an existing system because of the space requirements of airflow-measuring stations."

"javed_rizvi" wrote:
"The article does not address issues with reduced OA (outside air) with (a) reduction (in) VFD (variable-frequency-drive) speed, especially during cold weather. Normally, exhaust fans and kitchen exhaust fans are constant speed, and (a) reduction in OA with (a) reduction (in) VFD speed ... reduces load in cold weather. This and stack effect causes negative pressure in (a) building."

In response, author Dave Moser writes:
"The variation of outside-airflow rate with fan speed/supply-air volume is indeed an issue with air-handling systems that use fixed-position minimum-outside-air dampers. The intent of the article was to discuss building pressurization. When outside-airflow rates change because of variations in supply-fan speed and air-side economizer operation, active building-pressurization systems will respond directly because they measure building pressure directly. If building pressure is not measured, varying outside- and exhaust-airflow rates could indeed be an issue."

Print vs. the Web

I liked Associate Editor Megan White's Views From My Cube editorial ("The Print-vs.-Web Conundrum") in the April 2010 issue of HPAC Engineering. I don't like online magazines. I rarely read them and never read anywhere near every page. I do look forward to my print versions, though. Some I thumb through all the pages looking for an article to read; some I read cover to cover. I can say I have purchased much more equipment reading about it in a magazine than discovering it online. My magazines get Post-it notes stuck through them waiting for me to pass them along to a colleague or the need to conduct research for a particular project to arise.
Jeffrey Mahaney
Maine Public Broadcasting Network
Bangor, Maine

I'm not ashamed to say I got my PE license in 1977 and have seen many publications come and go. The Web is a good place for a quick fix, but I need to look at something other than my monitor. Give me the news on the screen, but keep the articles in print. I like to carry my magazine with me, even to lunch.
Ernest D. Yonkers, PE
Harrison French & Associates Ltd.
Bentonville, Ark.

I fully agree with Megan White's optimistic view of the long-term survival of the media we've come to know and love. Print lives!

Encouraged by certain manufacturer clients, my firm has become fully immersed in the digital age, including Facebook and Twitter. Am I enamored with or passionate about all of these newfangled forms of communication? To be honest, the jury's still out. But when a stack of magazines or the newspaper arrives, it's a wee slice of heaven. I want to see and feel what I read. I want to be able to easily mark, stash, or set aside something of value when the phone rings.

Sure, I'm the first one to hit the Web when buying that hard-to-find something or other, credit card in hand. And I appreciate robust, easy-to-navigate magazine Websites when I need to find an article fast or see breaking news. But there's nothing that guides me and fills my personal and professional interests like print media.

Magazine Publishers of America just launched a campaign that states in part:

"Barely noticed amidst the thunderous Internet clamor is the simple fact that magazine readership has risen over the past five years. ... Even among the groups one would assume are most singularly hooked on digital media, the appeal of magazines is growing.

"... During the 12-year life of Google, magazine readership actually increased 11 percent.

"What it proves ... is that a new medium doesn't necessarily displace an existing one. Just as movies didn't kill radio. Just as TV didn’t kill movies. An established medium will flourish so long as it continues to offer a unique experience. And as reader loyalty and growth demonstrate, magazines do."

Thanks again for the affirming editorial.
John Vastyan
Common Ground
Manheim, Pa.

Letters on HPAC Engineering editorial content and issues affecting the HVACR industry are welcome. Please address them to Scott Arnold, executive editor, at scott.arnold@penton.com.

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