Wednesday, July 3, 2013

40 FPM Issue Resolved

The members of ASHRAE 55 have finalized the draft of an upcoming revision to the standard. We have moved the entire “guidance” wording to informative appendices, and what remain are now all “normative”, or mandatory requirements. This will greatly improve the use of the standard when referenced in codes, standards, and of course, LEED. In addition, they have changed the name of the section which was called “Adaptive” to “Naturally Ventilated Spaces”. (I have always had issue with the term “adaptive”.)

If you have been following my blogs, you know I had a concern regarding implementation of ASHRAE Standard 55 (Comfort). There are several compliance paths available in the standard. The most used is the “graphical method”. This is the method used in Krueger’s thermal comfort program (http://www.krueger-hvac.com/tools/comfort.asp). Apparently, I, along with most users, ignored a statement that says if the airspeed at the point of interest exceeded 40fpm, one could not use the graphical method. A similar clause shows up in the second method, direct calculation of PMV using the supplied BASIC program, or the ASHRAE Comfort cool program. It all came to light when some folks submitting to LEED for the comfort point were either denied on the lack of a stated airspeed or stating an airspeed greater than 40 fpm. Since manufacturers only show throw to 50 fpm, this is a significant road block.

However, in the upcoming revision to Standard 55, which is expected to be printed in late summer, it has been taken care of in a couple of ways (subject to some final tweaking, but not likely). The Standard points to a section called “Elevated Air Speed” when air speeds are higher than 40 fpm (30 fpm if the setpoint is 72°F or below.) This section uses a methodology called ‘SET’, which calculates what the temperature would be at 30-40 fpm, providing the same thermal environment as at the elevated air speed.

UC Berkeley’s CBE has posted an on-line thermal comfort program that includes a graphical display. When one uses the step-by-step methodology for air speeds less than 70 fpm, it has the effect of moving the comfort box an almost immeasurable distance to the left! We also added a comment in the informative part of the Standard that points to the ASHRAE Application Handbook, where we will include a statement that references Standard 55. Included in the Standard is the following: “Spaces with air distribution systems which are engineered such that HVAC system supplied air streams do not enter the occupied zone will seldom have averaged air speeds that exceed 40fpm. See the ASHRAE Applications Handbook Chapter 57 on Air Distribution for guidance on selecting air distribution systems.” 

It is very likely that with any air distribution system designed with an ADPI >80%, there will be points in the occupied zone where the air speed will be greater than 40 fpm at air flow rates of 2.0 cfm/sf or less. The average in the comfort zone, however, will likely never exceed 40 fpm. It is also safe to say that at an ADPI >80%, the vertical stratification in the occupied zone will not exceed Standard 55’s limit of 5.4°F. Between the Standard (prepared by the Standard 55 committee) and the Handbook (prepared by ASHRAE TC 5.3), both the GBCI reviewers and the engineers submitting designs can point to these references to prove compliance to the Standard at the design stage.

As final note, I have finally served 4 years on the USGBC’s Indoor Environmental Task Group and will roll off. LEED V4 just passed all hurdles, including internal and external reviews, and will be released in October, as promised last year. I was pleased to get AHRI Standard 885 included by reference in both the 40 dBA HVAC sound requirement prerequisite in LEED for schools, and as a path for a point when meeting the acoustical requirements in all other sections. The AHRI 885 spreadsheet is available on the Krueger website and includes a room sound calculator for calculation NC, RC, and dBA combining discharge, radiated, and air outlet octave band room sound pressure levels.    

Authored by: Dan Int-Hout, Chief Engineer Krueger