The energy recovery wheel inside the ERV unit transfers energy from the warmer to the cooler air stream through counter-flowing supply and exhaust air streams through the slowly rotating wheel (less than 60 revolutions per minute). The large energy transfer surface arranged to provide laminar air flow through the wheel causes the constant flow of recovered energy to represent up to 70% of the difference in total energy contained within the two air streams. The Rx (ERV) Series is designed to be mated to the RE Series rooftop unit for maximum energy efficiency in a make-up air unit (100% outside air).