Rochester, New York
Few indeed are the drivers of the steam engines who have not heard a similar statement upon firing-up a cold boiler. Luckily the boiler was generally only 'sweating' not leaking, so the only lamentable aspect was the lack of adequacy of the explanation.
Two conditions are necessary to make a boiler 'sweat' when fired. There must be water vapor present in the flue gases and the flue gases must be exposed to surfaces sufficiently cool to cause some condensation of that vapor. The vapor may come from moisture in the fuel or from the burning of hydrogen, and hydrogen is present in the combustible gases given of by burning wood or coal. In burning, the hydrogen unites directly with the oxygen of the air to form water vapor as an end product. Water vapor is invisible. In passing through a boiler too hot to condense it, it escapes undetected in the stack gases . Whereas in a cold boiler, the vapor is condensed to form liquid water. Hence, 'The boiler is leaking, Dad.'