I've submitted this credit to USGBC and it was rejected. The reviewer stated that the design air speed values exceed 40 fpm and therefore do not meet ASHRAE 55-2004 (I have a range from 50 - 70 fpm). However, i've used two tools that show the scenarios do comply. Climate Consultant 5.0 and the Berkely tool referenced on this site. Air temp of 75F, Mean radiant temp of 85F (large single pane historic windows), air speed of 70 fpm, Humidity of 50%, MET of 1.1, and 0.57 clo. What am i missing? The Climate Consultant graph is hard to follow (this is the one I submitted), perhaps if I submit the Berkely tool it will be more clear to the reviewer? I had a narrative explaining all the values I used as well. Any help is appreciated.
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Barbara McCrary
Mechanical Engineer, PE, LEED AP BD&CHHB Engineers, P.C., Prattville, AL
119 thumbs up
February 5, 2013 - 12:28 pm
I have researched this more, and have found that air speed values above 40 fpm are allowed but only if Section 5.2.3 are met, which requires occupants to control the air speed. We have VAV boxes, so the occupant can slow the air speed by raising the room setpoint. But every occupant does not have their own thermostat, therefore it sounds like we do not meet this credit. anyone have any other thoughts?
Tristan Roberts
RepresentativeVermont House of Representatives
LEEDuser Expert
11478 thumbs up
February 6, 2013 - 7:41 pm
Barbara, this credit is not a particular expertise of mine, but at a glance, I wouldn't have thought that controls for individual occupants factor in here. Isn't that under IEQc6?
Barbara McCrary
Mechanical Engineer, PE, LEED AP BD&CHHB Engineers, P.C., Prattville, AL
119 thumbs up
February 6, 2013 - 8:43 pm
I thought so too, but ASHRAE 55 mentions allowing "elevated air speeds" above 40 fpm at the occupant level if the occupant has control over the air speed. I am very ready for ASHRAE to publish the User's Guide for ASHRAE 55. I'm wondering if anyone has ever achieved this credit with air speeds above 40 fpm?
Jeremy Rapoza
EngineerTighe & Bond
8 thumbs up
February 27, 2013 - 3:44 pm
Barbara, I've been getting this feedback from LEED as well but I think the LEED reviewers (as well as others below on this forum) are incorrect. Section 5.2.3. is specifically to show the designer that you can maintain thermal comfort with higher ambient temperatures than shown in figure 5.2.1.1. if air speeds are increased above 40 fpm. Their chart shows how high the temperature limit can increase for corresponding air speed increases (and given air and radiant temperatures). I've had to explain this to LEED consultants as well with varying degrees of success.
I believe ASHRAE 55 includes this to provide a guideline to those who would like to design for less mechanical cooling while keeping comfort at the same time. I do not read this to be an absolute that 40 fpm is the comfort limit and any speed over that is uncomfortable.
Discomfort due to draft is covered later in 5.2.4.2. You can see there may be certain circumstances where 40 fpm would be the limit, but not always.
We'll see if LEED agrees when I get a review of my response.
Cam Fitzgerald
Energy Engineer7group
25 thumbs up
February 28, 2013 - 9:33 am
Jeremy is correct that Section 5.2.3 includes provisions for increasing airspeed to offset elevated space temperatures for individual comfort. However, this option is only applicable if the affected occupants can control the airspeed.