Wednesday, October 10, 2012

Talk to the Occupants!

Last week I spent 4 days in Southern Ontario, Canada. I spoke at three different ASHRAE chapters (London, Hamilton, and Toronto) and called on 15 design engineering firms with our local reps and the regional sales manager. In total, I saw nearly 200 folks. I asked all the same question. “What is ASHRAE’s recommended maximum delta-t when heating from the ceiling”. In spite of Standard 62’s Ventilation Rate Procedure being code in Ontario Province (they have the IMC 2009 in their code, which references the 2007 VRP), there were few who knew the answer. Sadly, several of those who knew the “rule” said they ignore it because “no one is checking”. It is a major problem if there are things in codes that no one checks.

One firm did have a design leaving air of no more than 90°F, but no one in the room knew why. They were glad to understand why this rule was in place.

Checking is surprisingly easy. Every project has a schedule that lists the design discharge temperature of all devices with heating coils. All a code official has to do is compare this data with the stated design outdoor air delivery rate. ASHRAE 62.1 (Table 6.2) clearly states that if the delivered air is more than 15°F above room temperature, they must divide the room ventilation rate by 1.2, which is a 25% increase in the outdoor air quantity, when heating. In Canada, this is likely very expensive in their cold climate. If the outdoor air damper is fixed, this increases the dehumidification and cooling demand in the summer.

In addition to Standard 62.1 compliance, when the discharge temperature is high, it is extremely unlikely that the space will meet ASHRAE Standard 55’s 5.4°F vertical temperature stratification requirement. The ASHRAE Fundamentals chapter 20 on Air Distribution states: “when the room to discharge differential exceeds 15°F, it is unlikely that the vertical temperature limitation of ASHRAE Standard 55 will be met.” When we did the overhead heating analysis in the late 70’s, we ran over 900 different perimeter conditions, and in none of the cases was there compliance to Standard 55 when the delta-t exceed 15°F.

Again, however, many in the design engineering community seem to be unaware of the fact that hot air rises and that releasing it at the ceiling will result in occupant dissatisfaction. When I bring this to the attention of many engineers, too often their response is “I’ve been discharging hot air into spaces for years and no one is complaining”. I question whether they ever actually asked anyone living in such an environment. As BOMA continues to report that the #1 reason for tenants not renewing their lease is “occupant dissatisfaction with their thermal environment,” I would conclude no one is bothering to ask the occupants.

It also appears that there is poor understanding of the need to adjust diffusers prior to balancing. One Canadian air balancer told me that they had to call the engineer to find out how to adjust the linear slots on his project, as he could find nothing in the design documentation. At least one balancer is taking the time to do it right. Sadly, he is in a minority.

It appears we have a ways to go before we manage to design comfortable spaces.

Authored by: Dan Int-Hout, Chief Engineer Krueger