The Carnes Model AB by-pass unit provides pressure dependent variable air volume to
individual zones while bypassing the unneeded air to the ceiling plenum for recirculation. Although zone air volumes
in small buildings may vary greatly, the cost of fan controls many times cannot be justified. Zone variable air
volumes are realized with the by-pass unit while the supply fan delivers a constant CFM. The AB can be ordered for
reverse or direct acting pneumatic thermostats or electric thermostats. Zone thermostats directly control the by-pass
damper assuring that only the air that is needed is delivered to the zone. Downstream ductwork pressure losses can be
matched by adjusting an integral by-pass balancing damper.