Tuesday, July 22, 2014

Air Leakage

Not too long ago, I attended my first ASHRAE meeting as a member of the Board of Directors. My time at the meetings has, for the most part, been focused around participating in the Tech Council and being a board representative on RAC (at least for one year). It’s a busy job, but someone has to do it!

As for recent happenings, I have received quite a few questions about air leakage. Ironically, there has also been a push from the 90.1 committee for VAV box manufacturers to “come clean” about air leakage rates. Just to give some history, a couple years ago the AHRI section that covers VAV terminals was asked to come up with leakage numbers. Several manufacturers put numbers on the table for single duct and dual duct boxes, as a function of size and type of heat. This data was presented to those requesting it. They then asked, “So when will you start certifying the data?” The answer is never. Not only is the required test setup expensive to run, but there are a number of uncertainties involved. With regard to cost, if for some reason a test failed to be certified, the penalties are so expensive that it would most certainly warrant an increase in cost, which we know would not be received well by customers. (Then, to make matters worse, there is little payback for the slight reduction in leakage that would result for single or double duct units.) So, when we informed the interested parties that there would not be a certification program, their response – to our surprise – was “then, never mind”. However, there have been rumors that the DOE is pushing the 90.1 committee to bring it up again.

Fan boxes, on the other hand, are a different matter. For starters, a both series and parallel fan boxes, for the most part, are under negative pressure, so leaks would be in the unit, not out. This is definitely the case for series boxes. For parallel boxes, half the unit is at a slight positive pressure, but it is probably no more than 0.5”, which is just enough to feed the downstream diffusers and ductwork, and possibly a water coil. The backdraft damper on parallel boxes, however, is seldom robust enough to effectively provide a tight seal. The ASHRAE/AHRI project evaluating series and parallel boxes at Texas A&M University found that a typical parallel box will leak between 10% and 15% at typical pressures. Of course, this could be significantly reduced, but as mentioned earlier it would add significantly to the cost of the unit and require an actuator and actuated damper. We have not (yet) seen any specifications calling for this degree of air tightness. The A&M report also stated that a parallel box would have to leak less than 7% to be more cost effective than a series box, from a total system standpoint.

So that’s where the industry is at this point with regard to air leakage. I expect that we will hear from the DOE in the future and that certain the rules will be tightened up, but sadly, the efforts are likely to generate little actual energy savings.

Authored by: Dan Int-Hout, Chief Engineer Krueger