It started with a call from the homeowner saying the child's bedroom had been "freezing cold" that morning. But Bill, our heating subcontractor, said the bedroom was "fine"—within three degrees of the thermostat setting. The problem was that, by the time we drove out to the renovated house, it was 9:30 in the morning, 10 degrees warmer outside, and the bedroom door had been open for hours. The next day, we got the same call: The bedroom was too cold at 7 a.m. with the door closed. Our heating company was stuck in the rush of repairs that accompanies the first cold weather in fall, and once again couldn't get to the house first thing in the morning.

That's when we decided to buy some data loggers so we could see exactly what was happening. There are certain problems that are hard to catch because you may not be at the house when they happen. We've found data loggers to be invaluable for evaluating HVAC performance, condensation, and other temperature problems, especially when they are...

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