Packaged Rooftop heating-cooling units are used on almost half of all commercial buildings, so they are literally everywhere! Newer RTUs are more efficient than older units, and 90% efficient heating is coming for rooftop equipment. But what about the older, less-efficient units that are too costly to replace? Fortunately, there is a solution: several manufacturers have created and are making retrofit control systems designed to dramatically lower the cost of operation of EXISTING Rooftop units. While complex, the heart of the retrofit system is applying variable-frequency drive (VFD) technology to existing Rooftop units. This means that air movement, and associated power consumption, can be ramped up or down to match the actual need, not just blow at a fixed speed non-stop whether needed or not. Similar upgrades are available for the air conditioning compressors–the largest energy-using device in the HVAC system. This technology is new but is being beta-tested in the field right now with good success. The only catch is that the technology can only be used on Rooftop units with 3-phase motors, effectively limiting the retrofit applications to units of 6 tons or larger. However, there is no upper limit on size, and in fact, larger Rooftop units incur larger savings. Paybacks of 1-3 years are common.