A Load on My Mind

Published January 14, 2009

Tags: Energy

Sometimes I wonder if it's just me.

I'm evaluating a project's HVAC system designed by an international engineering firm, neither of which shall be named, for the potential to save energy. As a first order cut of where things stand on a sizing basis, I calculated the gross floor area cooled per ton, which came out to roughly 350 square feet. This is at the high end of 1960s HVAC engineering wisdom of 250-350 square feet coverage per ton of cooling capacity. Pretty representative of standard practice (sadly!) but certainly not adequate for this project's aspirations.

Since I'm genetically programmed to want to figure out how things work, I began going through the load calculations to see how they got there.

Generally, developing a cooling load calculation is a paranoid's dream. First, you take the hottest day of the year, then you add lots of warm bodies -- it's always Black Friday in cooling load land. (For our friends outside the U.S., "Black Friday" is the day after Thanksgiving and the largest shopping day of the year … Christmas presents). Of course, ALL of the lighting and ALL of the equipment are on simultaneously.

And what lighting and equipment it is! Close to 2 watts a square foot in offices and almost 5 watts a square foot for retail spaces, which is illegal in most of the developed world. In physical terms, this is basically three three-lamp fixtures in a 130-square-foot office. For retail, think JiffyMart or about 25 MR 16s in the same amount of floor area. Also, put this power density in the corridors, emergency stairwells, closets, etc and you've got the idea of what's ass-u-me'd.

It gets better. Equipment assumptions for office and retail were 4 and 8 Watts a square foot, respectively. To hit those numbers, every 130 square feet in an office would have one laser printer, one desktop computer with a 21-inch LCD and a copier. In the corridors … in the stairwells … in the janitors' closets. According to figures in the Building Energy Data Book (BEDB), office equipment represents about the same energy consumption as lighting, which on average for offices is in the 1.2-1.3 watts per square foot, less than a third of what is assumed in the load calculation.

In the retail sector, the only occupancies that will even come close to this level of equipment density are restaurants and grocery stores, but these occupancies only represent 20 percent of retail sector floor area. Indeed, BEDB figures indicate that 8 watts per square foot for equipment is more than ALL energy use in an average retail building in real life. BEDB data shows that equipment loads represent approximately 25 percent of energy use in retail buildings, so 1.5 watts per square foot is a much more realistic figure for a mixed retail situation rather than 8 Watts.

Now, to this grossly overblown load calculation we add a "safety factor" of about 20 percent AND at least one major chiller's worth of redundancy in case of catastrophic failure of a chiller.

Wow. What a load of …

Rob Watson is the Executive Editor of GreenerBuildings.com, and the chairman, CEO and Chief Scientist of the EcoTech International Group.

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