Abstract: A method and apparatus to recover energy from used or foul air and to return the energy into fresh air to reduce the energy input to a ventilation system. Ventilator apparatus is adapted to maintain fluid flow through a first heat exchanger, positioned in a fresh air passage, and through a second heat exchanger, positioned in an exhaust passage, at a controlled rate to afford maximum or controlled heat transfer efficiency for pre-cooling or pre-heating fresh air drawn into the system. Control apparatus is provided to maintain flow of fluid through the heat exchangers such that the sum of the mass flow of fresh air times the specific heat of the fresh air, plus the mass flow of the exhaust air times the apparent specific heat of exhaust air, is equal to two times the mass flow of fluid times the specific heat of the fluid.