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Henry Reents of Boise, Idaho, asks:

“The box of our top-loading refrigerator has a large lower compartment that is separated from the main upper section by a three-piece plastic shelf. We don’t use the lower compartment very much. Would our compressor run less if we put large blocks of foam in that lower space? This would reduce the size of the refrigerator box by about a third.”

Don Casey replies:

You’re on the right track; heat does leak into a refrigerator box. When the temperature inside the box reaches a certain level, the compressor turns itself on. The smaller the interior volume of the box, the less heat it contains, which means the compressor needs to run less to remove it. If you completely fill the unused space with foam, you reduce the interior surface area of the box, which in turn reduces the amount of heat that can leak into the interior.

However, this relationship is not linear; a 30 percent reduction in surface area will not reduce compressor running time by 30 percent. But if your box is well insulated and you fit the foam in well, I’d guess the combination of reduced surface area and interior volume would reduce your compressor run time by almost 20 percent. Of course, the benefits will be less if your box is less well insulated.

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