A heat pipe heat exchanger is used in combination with a damper assembly to selectively control an amount of heat exchange provided. A divider defines discrete heat pipe plenums and bypass plenums within a duct, and the heat pipe system is configured so that all of the coils of one portion of the heat pipe system are received in the heat pipe plenum(s), while the bypass plenum(s) are free of any coils. The damper assembly includes adjustable heat pipe dampers aligned with the heat pipe plenums and adjustable bypass dampers aligned with the bypass plenums. The damper assembly can include a single actuator that simultaneously opens the heat pipe dampers and closes the bypass dampers and simultaneously closes the heat pipe dampers and opens the bypass dampers.