ASHRAE Journal - February 2011 - 8

letters
INSIDE: Q&A on New HFO Refrigerant

December 2010

Net Zero Acceptability
In December’s Engineer’s Notebook, Dan Int-Hout states a truism that energy managers of the 1970s and ’80s also faced: energy costs are about 1% of the cost of doing business, causing some to say why bother. The answer is another truism: avoided energy costs drop to profit so a corporation can define an energy conservation team as a profit center. An example of that thinking was given in the June 9, 1980, issue of Energy User News where a corporation was supporting a team of 30 engineers and technicians to reduce energy consumption, resulting in a total accumulated energy cost avoidance of about $850 million, from 1973 through 1986, with an investment of about $50 million. When the oil embargo occurred in the fourth quarter of 1973, some believed the living standard of the U.S. would have to decline if major reductions in energy consumption were to occur. The reasoning was based on the ratio of U.S. energy consumption divided by (corrected for inflation) GDP dollars, a ratio that had been about constant since 1950. U.S. GDP dollars continued to increase since 1973, and energy conservation occurred as evidenced by the fact that the ratio of energy consumed to GDP dollars (kBtu/$GDP) is about half of the 1973 value. It’s an American success story that is not well known. However, much is yet to be done as America continues its quest for energy independence. Kirby Nelson, P.E., Life Member ASHRAE, Springfield, Mo.

The magazine of the American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc.

Commissioning
The Memorial Hermann Project
®

A L S O

· Radiant Slab Cooling · Data Center Retrofit · New Product Preview · Confusion About Diffusion · Technical Retrocommissioning

AJcover.indd 2

11/12/2010 3:49:12 PM

AJcover.indd 2

10/15/2010 3:25:40 PM

Energy Modeling and Commissioning
December’s column on commissioning was well done, and I enjoyed the read. Something that was lacking, however, was mention of how the energy model was adjusted/“tuned” to match the actual energy bills. I am convinced that modeling of existing buildings is an important but emerging science. Most modeling has been done for new construction and has used many assumptions about operation, since there is no history to compare to. When you are modeling an existing building, assumptions must be minimized. Mr. McFarlane mentioned that many measurements were made of existing conditions; good for him and his company to recognize the importance of doing so! My experience, however, is that measurements alone do not guarantee a good alignment of predicted vs. actual energy use. Other factors such as U-factor degradation due to bridging effects, actual weather patterns vs. TMY data and actual vs. catalog efficiencies can have a significant impact. The accuracy of the prediction model (its match to actual history) largely determines the solidity of the investment. At the end of the day, we engineers will be judged by how well our payback calculations are met in real life! In future columns, I’d love to hear more about how energy savings predictions are made more robust to ensure accuracy in a real building setting. James V. Dirkes, II, P.E., Member ASHRAE, Grand Rapids, Mich.

The Author Responds
As I hope I indicated, saving energy is both an admirable goal, and likely has a real and measurable payback. I am certainly not surprised at a corporation being formed to manage this opportunity. The potential for a trade-off between energy and comfort still exists, and if they are disconnected (such as behind a corporation), there may not be a good feedback loop. We should probably find a way to dial in “occupant satisfaction with the environment” as a dollar figure, and make it count in the big picture. As always, if you want to figure out why it happened, follow the money. Dan Int-Hout, Fellow ASHRAE, Richardson, Texas

The Author Responds
I wish I had a really scientific answer for this question but I don’t. We basically take the actual energy usage as shown on actual energy bills and then compare that to the model we use to predict the energy usage. We then start playing with the variables that Mr. Dirkes mentioned. We change the U-values, the infiltration assumptions, the equipment efficiencies, etc., until we get a model that as closely as possible predicts the actual energy usage. It isn’t scientific, but it gets us closer than ignoring all the possible elements that affect energy usage. We then use this model to predict the future. Dave McFarlane, Grand Forks, N.D.
8 ASHRAE Journal

Confusion on Diffusion
Joe Lstiburek’s “Confusion on Diffusion” article in the December issue hit on some nice condensation issues, but he left out a couple crawlspace issues. His Figure 3 shows two condensing surfaces when there are really three (or four). The underside of the floor sheathing is a critical one when the indoor temperature is lower than the crawlspace dew point. Condensation happens above the insulation. You can adapt his
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ASHRAE Journal - February 2011

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