Monday, October 31, 2011

People Still Don't Get It!!

I spent last week in Boston calling on engineers with our rep there. As I always do, I asked the question “What does ASHRAE recommend as the maximum delta-t between room and discharge, when heating from the ceiling?” Out of 60 engineers in 7 engineering offices, only one individual knew the answer. Sadly, this has been the pattern for as long as I have been asking the question.

The answer is, of course, 15 degrees-F. Discharges greater than 90F in a 75F room (87 in a 72F room) are going to result in significant temperature stratification in the room. ASHRAE Standard 62.1 requires that when this delta-t is exceeded, the system must increase the minimum ventilation air to the space to account for the inevitable loss of ventilation to the room due to short circuiting into the return air plenum. In Mass., as in many states, this is code, as the Ventilation Rate Procedure of 62.1 is referenced in the 2009 International Mechanical Code, which has been adopted by the state.

More importantly, ASHRAE Standard 55 (Occupant Comfort) cannot be met with high discharge temperatures. The 55 Standard only allows 5.4F vertical temperature difference between 6in and 6ft. I have never witnessed a test where this requirement was met with greater than 15 degree supply-room differentials, and I have conducted over a thousand such tests over the past 35 years.

While not code, ASHRAE 55 should be a concern to everyone. Occupant salaries in many buildings are at least $200/square foot/year. A building that uses more than $2/square foot/year in energy is a poor performer. BOMA has continually stated that the number one reason for not renewing the lease in a high rise office building is “occupant dissatisfaction with the environment”. Considering how few engineers apparently understand the very basic idea that hot air rises, I suppose this is not surprising. We continue to see VAV box schedules with design discharge temperatures in excess of 120F.

I will continue to pound on “the rules” as I call on engineers around the country. The ASHRAE Journal article on overhead heating was recently reposted as a link in the HVAC news weekly e-mail, and a version can be found at

Authored by: Dan Int-Hout, Chief Engineer Krueger