Most U.S. homes are heated with either furnaces or boilers. Furnaces heat air and distribute the heated air through the house using ducts; boilers heat water, providing either hot water or steam for heating. Steam is distributed via pipes to steam radiators, and hot water can be distributed via baseboard radiators or radiant floor systems, or can heat air via a coil. Steam boilers operate at a higher temperature than hot water boilers, and are inherently less efficient, but high-efficiency versions of all types of furnaces and boilers are currently available.
Understanding the Efficiency Rating of Furnaces and Boilers
A central furnace or boiler's efficiency is measured by annual fuel utilization efficiency (AFUE). The Federal Trade Commission requires new furnaces or boilers to display their AFUE so consumers can compare heating efficiencies of various models. AFUE is a measure of how efficient the appliance is in the energy in its fuel over the course of a typical year.
Specifically, AFUE is the ratio of heat output of the furnace or boiler compared to the total energy consumed by a furnace or boiler. An AFUE of 90% means that 90% of the energy in the fuel becomes heat for the home and the other 10% escapes up the chimney and elsewhere. AFUE doesn't include the heat losses of the duct system or piping, which can be as much as 35% of the energy for output of the furnace when ducts are located in the attic.
大部分美國家庭都用鍋爐取暖。鍋爐加熱空氣并且把加熱的氣體通過管路輸送到房間里。蒸汽通過管道輸送到蒸汽散熱器里，而熱水可以通過散熱器和地板輻射環(huán)路把熱量傳給房間。蒸汽鍋爐和熱水鍋爐相比，它的工作溫度高，熱效率低；目前各種高效率的新型鍋爐已經(jīng)面世了。
了解鍋爐和空氣加熱器的效率
鍋爐的燃料效率通過年度燃料利用率afue來衡量。聯(lián)邦貿(mào)易委員會(huì)的要求新出廠的鍋爐標(biāo)注出年度燃料利用率，這樣消費(fèi)者可以比較各種加熱設(shè)備的熱效率年度燃料利用率是設(shè)備效率的衡量標(biāo)準(zhǔn)。具體地說，它表明了一年里燃料所貢獻(xiàn)出的有效能量。
年度燃料利用率90%意味著90%的能量變成有效熱量，而其他10%的熱量失散了。年度燃料利用率不包括管道系統(tǒng)的熱損失，當(dāng)管道設(shè)置在閣樓時(shí)，這部分熱損失高達(dá)鍋爐輸出熱的35%。全電鍋爐沒有煙氣損失。