Monday, June 6, 2011

A Visit with Engineers in Philly

I recently had the opportunity to visit a number of engineer’s offices in Philadelphia. We saw over 50 engineers in 7 different offices on this trip. It was great to get back to Philly where we hadn’t had a strong engineering presence for some time.

Philly is experiencing the same construction doldrums that the rest of the country is having, but I was glad they made time for me to visit and bring them up to date on the latest proposed LEED changes along with the impact of Standard 62.1 VRP (Ventilation Rate Procedure) being adopted as code by the IMC, and Pennsylvania as well.

Once again, I asked how many knew the recommended maximum delta-t when heating from the ceiling. I estimate I have asked 12,000 engineers this question over the past 10 years. The response in Philly was on par with the rest of the engineers I have questioned. Out of 57 engineers questioned, only 6 knew the answer (or were willing to guess, and guessed correctly). The answer, of course, is 15F delta-t (difference between room and discharge). Higher than this and two bad things happen:

1) ASHRAE 62.1 requires that 25% more outside air be brought into the zone being heated, to compensate for the ventilation air short circuiting that is sure to be happening as hot supply air is drawn into the ceiling returns and

2) ASHRAE 55 (Thermal Comfort) vertical stratification limitation will be compromised, and one cannot claim compliance to 55 in a LEED project.

ASHRAE 62.1 is a prerequisite for any LEED building, and code in Pennsylvania, so that is not optional. In LEED for 2012, outside air will likely require to be monitored (at least with VAV systems, so compliance can be verified.

We also discussed the impact of oversizing diffusers with VAV systems and the result at low flows. While the common term for the dropping of cold air into the occupied zone is “dumping”, I informed them, as I usually do, that diffusers may exhibit “excessive drop”. I was informed of a couple alternative terms for this phenomenon. These include:
- Premature Ceiling Separation (“PCS”)
- Failure of Coanda (“FOC”)

I have since been informed by our marketing department that the politically correct description is “horizontally challenged airflow”.

I’m glad that confusion is finally cleared up.

Authored by: Dan Int-Hout, Chief Engineer Krueger