Monday, November 11, 2013

Vertical Stratification Limits

In their infinite wisdom (just ask them), the academics on the ASHRAE Standard 55, Thermal Comfort, are proposing that the 5°F vertical stratification limit be only for seated persons, and that standing persons should be allowed 7°F or more. I posted the following to members of the committee. 

While it is a wonderful thing that we are looking to research and make our standard as accurate as possible, we are missing the forest for the trees. An academic approach is certainly defensible, but sadly, it is of little use to the design engineer. The number one reason for occupants not renewing the lease in commercial spaces continues to be, as it has for the last 25 years, occupant dissatisfaction with the thermal environment.

The majority of commercial spaces in North America are supplied with overhead, well mixed air distribution systems, and a huge percentage of these are VAV. If air outlets are not designed to work at low flows, the space becomes stratified. Since hot air rises (I know this is obvious to all the academics on this committee, just not to many practicing mechanical engineers), it becomes cold at the floor. The only tool available to convince engineers this is an issue is Standard 55 – and you are all conspiring to take that single tool and throw it out the window.

I was asked to chair TC 2.1 back in ’78 because I was the only non-academic on the committee who was willing to express an opinion at that time. I was also involved in Standard 55 at the same time, again as a non-academic, trying to force some real world logic into the standard. As an ASHRAE DL (Possibly the only one on this committee?), I see over a thousand mechanical engineers, contractors and sales reps a year, and have done so for 20 years. The practicing engineer is woefully ignorant of the nuances of Standard 55 and has a lot of other things to worry about. But, they are most often hired by the architect, and often seeking LEED accreditation. Compliance to Standard 55 is only one point out of 100 (that’s 1% for any non-academics who might read this). Requiring analysis of a project at the design stage leaves few tools for the ME. ADPI is one that can at least be performed using software, which is available from a number of manufacturers. We on the manufacturing side have shown for many years that an ADPI of 80% of greater ensures a vertical stratification that meets the 5.4°F limitation in the present standard. The GBCI reviewers have accepted this in a number of cases, as reported to this committee in the past.

More importantly, using ADPI analysis, one can easily show that several types of air outlets do not perform well at turn down, particularly the face mounted deflector perforated outlet. This was at one time the highest selling type of air outlet in North America, due in part because it was the cheapest. Thanks in large part to Standard 55, it has, thankfully, fallen from favor, despite architects’ wishes. Standard 55 and LEED have been tools in this conversion, forcing architects to use a better performing device. The “swirl” diffuser, so highly promoted (touted?) by our European friends, also performs poorly at low airflows (VAV is seldom used in Europe).

Opening the vertical stratification limit to greater than 3°C eliminates ADPI as a limiting design factor. While Gwellen is correct in stating that an ADPI of 80% is assurance of meeting Standard 55 (and that it is not necessarily so in reverse), it is the only tool available to the design engineer. (CFD has not, to date, been shown to be a validated method of predicting air distribution performance, at least not in any peer reviewed papers readily available – I’m sure Peter has several that he can let us peek at, but cannot be circulated).

Overhead air distribution assumes a well mixed space. 3°C in a 6’ high comfort zone is a pretty well mixed space. 4 or 5°C is not. Well mixed spaces are necessary for thermostats to function properly. When air outlets “dump” at low flows (ok, exhibit “excessive drop” for you academics), not only is it cold at the floor where those not wearing socks will be uncomfortable (which applies to most women working in offices, and likely some males, especially in Berkeley), but thermostat response is significantly longer. I know of one courtroom where the thermostat response went from 55 minutes to 5 minutes by fixing the ceiling air distribution to stop it from ‘dumping’. I have repeated this many times in a number of places by simply adjusting the diverters in slot diffusers to produce a horizontal air pattern. Allowing non-well-mixed air distribution, which opening up the vertical stratification limit will permit, will have the effect of allowing the architects to again select air outlets that perform poorly at low flows, which will result in greater occupant dissatisfaction and poor thermostat response. It will also allow installing contractors to ignore the requirement (sadly seldom enforced) that adjustable slot diffusers actually be adjusted. With an allowed 6 or 7°F vertical stratification, we are in effect saying “what does it matter?”

It does matter, though. We are doing a horrible job of designing comfortable spaces. If you would actually get out there and ask folks how they feel, you might get some idea of the extent of the poor comfort conditions we have created. Almost all the complaints are “it’s too cold at the floor”, followed by “it’s too cold”, period. Resetting VAV minimums lower, eliminating code required dead bands (which force the system to control to 68°F once they go into reheat as a result of sub cooling), and using air outlets that perform well at low air flows are all strategies that need to be implemented. Opening up stratification limits, which may be technically accurate, is only going to allow us to continue our apparent practice of creating uncomfortable spaces, which will now comply with the Standard! 

The law of Unintended Consequences is about to rear it’s ugly head. Sadly, I fear I will once again be able to say “I told you so”.