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Methods for Effective Room Air Distribution: Part 1


©2012 This excerpt taken from the article of the same name which appeared in ASHRAE Journal, vol. 54, no. 11, November 2012.

By Dan Int-Hout, Fellow ASHRAE

About the Author
Dan Int-Hout is a chief engineer at Krueger in Richardson, Texas.

Architects and owners desiring LEED certification for their buildings are challenging engineers to design HVAC systems that are at least 30% better than the base system in ASHRAE/IES Standard 90.1-2010, Energy Standard for New Buildings Except Low-Rise Residential Buildings. There is talk of raising the bar to 40% or even 50%.

In addition, Standard 90.1-2010 lowered the maximum energy use of the base system, which is overhead delivery of VAV controlled air. Improving on this base system is a real challenge. The requirements in ASHRAE/USGBC/IES Standard 189-2011, Standard for the Design of High-Performance Green Buildings Except Low Rise Residential Buildings, are similarly challenging for designers.

As a result, some architects are considering alternate means of air delivery as they assume one cannot meet the requirements with a variable air volume (VAV) system. These alternates include underfloor air distribution, displacement ventilation and chilled beams (low inlet pressure ceiling induction devices).

However, the energy calculation programs used to predict the energy use of these systems are largely unverified, at least in the form of available published data. Overhead VAV (which includes fan-powered terminals) has been in widespread use in North America for more than 40 years, and the energy use calculations have been well vetted. (Note that ASHRAE Research Project 1292 suggests the leakage of parallel fan-powered terminals has likely been underestimated in the effect on energy use.) The validation has to account for long-term weather variations involving the continually updated bin data for most of North America, as well as many locations worldwide.

The newer systems haven’t been in use long enough (at least in North America) to have a robust volume of data to measure the accuracy of energy use (or savings) predictions. Engineers, therefore, use best known data. The result is that a great deal of creativity is possible in establishing performance parameters, and no one can prove them wrong.

Moreover, while we have been designing with overhead VAV for more than 40 years, BOMA reports that a big reason tenants fail to renew a lease is occupant dissatisfaction with the thermal environment. So we are moving into new territories of air distribution, but we apparently still don’t know how to properly design or apply the method with which we have the most experience.

Part One of this article discusses essential requirements (in standards, guidelines and rating systems) that cover thermal comfort, acoustics and ventilation, and what’s involved in properly applying them to the typical main delivery systems.

Part Two will deal with the air delivery systems. Overhead fully mixed forced air systems are by far the most prevalent HVAC system in North America, and include air supplied from almost every imaginable source, including central station air handlers, water source heat pumps, variable refrigerant volume, variable air volume, ducted fan coils and unit ventilators, and chilled beams. Partially and fully stratified systems, including displacement ventilation and underfloor air distribution systems, are becoming more popular, and have different considerations for proper air delivery and occupant comfort.


Thermal Comfort

ANSI/ASHRAE Standard 55-2010, Thermal Environmental Conditions for Human Occupancy, establishes the thermal conditions most people will find acceptable. There is an understanding that thermal acceptability is defined by a bell-shaped curve, and not everyone will find a given set of conditions acceptable.

There are ranges of temperatures and air speeds predicted to keep dissatisfaction levels under 20%. These typically are based on a predicted mean vote/predicted percentage dissatisfied (PMV/PPD) calculation developed in the early 1970s (sometimes with other calculations) that account for occupant metabolic rate and clothing ensemble, as well as environmental variables of air and radiant temperatures, local air speed and humidity levels.

An ASHRAE Standard 55-2010 appendix includes the PMV calculation algorithm in BASIC programming language. The ASHRAE Thermal Comfort Tool CD, Version 2, is available to do these calculations. An older graphical version is also available from several sources, which has the same graphical limits as the current standard (Figure 1). 

Citation: ASHRAE Journal, vol. 54, no. 11, November 2012

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