Wiktionary
feedwater
n. Water added to a boiler to replace that lost during operation.
Usage examples of "feedwater".
The boiler feedwater, as had become standard practice, was preheated by being used as the cooling-water for the compression cylinders.
Trevithick's next crucial innovation was the external feedwater condensor, which permitted recycling of boiler water (1799) and the uniflow valve system, which raised fuel efficiency another order of magnitude by separating the steam entry and exhaust areas of the cylinder.