Heat pumps are HVAC devices that transfer heat from a lower-temperature fluid to a higher temperature fluid. In the 1970s, heat pumps were touted as the energy efficient HVAC systems of the future. Everyone, including electrical utilities, rigorously promoted heat pumps, birthing a new market. New markets typically have new problems, and for heat pumps the nature of the problem related to odors. A particular foul smell was so prevalent that the moniker Dirty Sock Syndrome was coined and universally accepted.
A thorough research of this subject, including that which is published by ASHRAE (2008, 2011, and 2012 Handbooks), suggests that the Dirty Sock Syndrome odor emanating from heat pumps stems from the decay of accumulated organic debris by microorganism activity — similar to sewers, plumbing traps, and other rotting, decomposing organic material.