A condensate storage tank assembly comprises a tank section having a condensate outlet at a lower end thereof. An upwardly enlarged steam chamber is connected to the upper end of the tank section and receives a condensate drain line which extends from the drain pot of a condenser. The upwardly enlarged steam chamber surrounds a portion of the condensate drain line at least up to the highest level of condensate in the drain line. A steam line from the source of steam which also feeds the condenser, is connected to the upper end of the tank section for supplying steam above the condensate level in the tank section. The steam heats the condensate drain line in the steam chamber to avoid freezing the condensate in the drain line. An upwardly enlarged deaerator may also be provided on the upper end of the tank section. The drain pot is also supplied with steam from the condenser for avoiding freezing of condensate in the condensate drain lines which supply condensate from the condenser to the drain pot.