Designing Sustainable Buildings

The USGBC has just released the latest (V4) version of the various accreditation paths, and at the same time both tightening the requirements and modifying others that were impractical. In order to meet the increased complexity of LEED credits, designers must be aware of the conflicts between first cost economics, occupant productivity, and life cycle costs. The 2009 ASHRAE Handbook of Fundamentals, Chapter 20, provides two basic rules for overhead heating and cooling:

  • In cooling mode, diffuser selection should be based the ratio of the diffuser’s throw to the length of the zone/area being supplied, at all design airflow rates, to achieve an acceptable Air Diffusion Performance Index (ADPI).

  • In heating mode, the diffuser to room temperature difference (delta-t) should not exceed 15°F, to avoid excessive temperature stratification. ASHRAE Standard 55-2004 defines the level of acceptable vertical temperature gradation at not to exceed 5°F in the occupied zone.

ASHRAE has defined a term for describing the mixing of supply and room air, replacing the somewhat ambiguous terms Ventilation Effectiveness and Ventilation Efficiency with a new term, Air Change Effectiveness, or ACE. This term is used in both ASHRAE Standard 129, the Method of Test for Air Change Effectiveness, and in Standard 62.1-2013. As the current LEED rating system has the Ventilation Rate Procedure of ASHRAE 62.1 as a prerequisite, a point is no longer gained for meeting this requirement.

The ASHRAE Ventilation Standard, 62.1, assumes ventilation mixing of 100% in setting minimum ventilation rates. If it can be shown that the ACE is less than 100%, then the amount of outside air must be increased above the required minimums. When a high ADPI is measured, the ACE is always high as well. ASHRAE 62.1 (2013) now includes Air Change Effectiveness in a table. When cooling from the ceiling, ACE is always assumed to be 100%. When heating from the ceiling with ceiling returns, the discharge to room delta-t must not exceed 15°F, and the 150fpm throw must reach to within 4.5 ft from the floor, or the outside air ventilation rate must be increased by 25%.

Utilizing the ASHRAE Handbook table for predicting ADPI is cumbersome and is seldom accomplished. It is possible, however, to simplify this analysis by combining a diffuser’s throw performance with a cfm/sqft analysis and diffuser spacing to produce an ADPI “Performance Envelope” graph.

An example ADPI chart is shown below. The x axis is flow rate/unit area, and the y axis is half the separation distance, or L, the characteristic room length. The horizontal curved lines are simply calculations of flow vs. area served, while the vertical boundaries are computed from the ASHRAE maximum and minimum T50/L ratios for this type of diffuser. Performance within the area bounded by the lines should achieve an ADPI of 80% or greater.

Krueger ADPI Chart for Architectural Diffuser

This graph (above) shows the performance of an Architectural (plaque) diffuser with a similar 4 jet pattern. As seen, this diffuser will have both good performance at low flows and will have greater turn down in a VAV situation.

Krueger’s newest release of the K-Select GRD selection program includes an output option to assist meeting the LEED comfort point using ADPI. After determining a diffuser’s ADPI in the program at design conditions, the program then allows the preparation of a graph showing the performance of this diffuser, at the set spacing and delta-t, at reduced airflows. Submission of the prepared graph (based on ASHRAE ADPI selection data) to the LEED certification officials will demonstrate proof of compliance to the vertical temperature difference item in the Standard 55 Compliance template. The following text is included for clarity, and as a guide to the LEED reviewer:

“V4 LEED-NC Credit 7.1 may be awarded for complying with ASHRAE Standard 55-2013. This Standard limits vertical air temperature difference, within the occupied zone, to be no greater than 5°F. Assuring an ADPI no less than 80% will comply with this requirement.”

Manufacturers know the proper way to apply their products and often provide excellent tools for use by engineers and architects. The use of ADPI to determine compliance to a critical part of Standard 55 is one example. Other systems use validated CFD programs to predict space temperature variations. Failure to utilize these tools, however, all too often results in spaces with excessive vertical temperatures in the occupied space, in both heating and cooling situations. The result is non-compliance to Standard 55, (and the loss of that LEED credit) but more importantly, reduced occupant productivity.

To read in more detail about how LEED, ADPI, and selecting proper air distribution solutions for occupant comfort interact with each other, click here to read Dan Int-Hout’s white paper.