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I'm standing in a cramped attic in a one-year-old house, trying to discover the problem with the ground-source heat pump in front of me. As an engineer who tests dozens of heat pumps each year, I often get called in to troubleshoot problems no one else is able to solve. Within a few minutes, I determine that one of the two compressors in this two-speed heat pump has shut down. I also discover that the return duct is undersized, causing extremely low air flow, and the supply duct to one particularly cold room is losing more than half its air through leaks into the attic. These discoveries explain