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Ice, Ice, Baby, Too Cold

On a recent project, an owner's engineer had decided a large supply duct running down a corridor did not need to be insulated.

“You don't need it, and we'll save lots of money,” he said.

But the corridor was freezing! When the contractor claimed the long narrow corridor was cold because of the big uninsulated duct, I wondered if he was trying to cover up for excessive duct leakage.

I knew an uninsulated duct would absorb a fair amount of heat from the space. There were no exterior walls or windows to add to the cooling load, and a lot of lights (22 60-w incandescent bulbs) were adding heat to the space. My first thought was that something else had to be making the corridor so cold.

I was not anxious to do the long, tedious calculations required to determine the duct heat gain and surface temperatures. Inside a long (more than 100 ft) duct, the temperature changes as the air traveling through absorbs heat. As a result, calculus has to be used, or duct heat gain must be calculated in a series of short sections.

My son has a computer program that can calculate in a few hours what would take me a whole day by hand. His first reaction was the same as mine: It seemed unlikely that the surface of the duct could absorb heat from all of those lights and make the corridor cold. But he also agreed that we needed to do the arithmetic.

I gave him the duct dimensions, airflows, and supply-air temperatures, and he set up a spreadsheet so I could plug in different supply-air temperatures and airflows.

When I saw the results, I was reminded once again that experience helps you figure out where to look and what questions to ask, but you have to crunch the numbers if you want to know what's really going on: At the design supply-air temperature, the duct would suck more than 11,000 Btuh out of the corridor. That was a lot more than I had expected and more than twice the amount of heat the lighting was putting in. No wonder it was so cold in there!
Dave Elovitz
Energy Economics Inc.
Natick, Mass

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