volume of outside air admitted to the ventilator in corresponding relation to the outside air temperature so that the volume of fresh air delivered to the room is maintained substantially constant application_number US 65940076 A description referring to fig1 there is shown a heating and ventilating system constructed in accordance with the present invention wherein outside air dampers 2 are controlled by a damper motor 4 for admitting a controlled amount of outside air into a fresh air duct 6 of the system the system may be of a recirculating type that has recirculation air dampers 8 also operated by motor 4 in a manner complementary to outside air dampers 2 that is to say as dampers 2 close dampers 8 open dampers 8 admit room air into a recirculating air duct 10 the outside air and recirculating air are drawn through a heating means 12 by a blower 14 where the combined air is heated to the desired temperature and thereafter discharged to the room through a duct 15 thus outside air is drawn through dampers 2 at ambient temperature of t a while recirculating air is drawn through dampers 8 at a temperature of t r and the combined air is discharged through duct 15 at a temperature of t d damper motor 4 is adapted to receive a control signal v c for operating said motor and driving the dampers control signal v c is provided by a control circuit as shown in fig2 where there is shown a means 16 for selecting a minimum percentage or volume of fresh.