With an aging building stock, and owners reacting to increasing energy costs, Retro-commissioning (RCx) or Existing Building Commissioning has come into being as a stand-alone engineering service. RCx is a systematic, documented process that identifies low-cost operational and maintenance improvements in existing buildings.   RCx optimizes existing system performance, rather than relying on major equipment replacement. The objective is to obtain improved indoor air quality, comfort, better control performance, and energy and resource efficiency.