Abstract: A method of operating a heat exchanging system seasonally which is mounted in an air duct is disclosed. The system has means for support and movement between a first position where the heat exchanger is generally normal to the air flow in the duct, and a second position where the heat exchanger is generally parallel to the air flow in the duct. During periods of the year when the heat exchanger is not used, it is moved to the second position which is parallel to the flow of air. The static pressure caused by the presence of the apparatus in the duct is then substantially reduced when in this second position and results in reduced power consumption of a fan used to generate the air flow. The fan is automatically controlled by a volume controller with a probe which senses changes in static pressure and air flow rate in the duct. The volume controller adjusts the fan motor speed and inlet guide vanes to save more than 25% of the energy used to operate the fan over a whole year.