The Economiser: The Heat Still Going Up the Chimney

Even a good boiler sends its flue gas up the chimney hot. Edward Green's economiser of 1845 put a bank of cast-iron tubes in the flue and ran the cold feed water through them on its way to the boiler, so the gas that was leaving anyway warmed the water that was arriving anyway. His tubes carried mechanical scrapers that travelled up and down them to keep the soot off, because a sooted tube is an insulated tube. The same idea is in every modern condensing boiler, which cools its flue gas until the water vapour in it condenses and gives up its latent heat too. This rung works out how much heat the stack carries away, measures a real flue, and finds the dew point below which an economiser starts to run wet.