Disclosed is a method for operating a condenser of the type having a housing inside of which is disposed a bundle of water tubes, a steam inlet for steam to flow inside the housing for contacting the tube bundle for cooling, and having a stagnant air zone during operation wherein any air in-leakage preferentially collects and condensate in the air zone becomes subcooled. A trough or drain is placed beneath the stagnant air zone for collecting subcooled condensate from the stagnant air zone. Collected subcooled condensate is transported from the trough or drain in a pipe to said steam inlet. The transported condensate is injected with an injector for contacting with steam entering the condenser, whereby the injected condensate is heated by the steam for expelling dissolved oxygen in the injected condensate. Advantageously, the condenser is fitted with an array of temperature sensors at the stagnant air zone for determination of its presence and/or size. Additionally, disclosed is a method for preventing air bound zones in the tube bundle sections of the condenser.