Every cubic foot of air your house breathes passes across the evaporator coil and through the blower wheel. Both are wet, cold and constantly moving — which makes them the place dust bonds and stays.

A blower wheel caked with a quarter-inch of dust on each fin can lose a large share of its air-moving capacity. An evaporator coil matted with debris can't transfer heat properly, so the system runs longer for less cooling and the compressor pays for it.

This is the work most duct cleaning companies skip because it takes real time and real disassembly. It's also where the biggest efficiency gains usually are.