These devices are connected to ducts that deliver heated air to registers throughout the house. Different types of furnaces are manufactured to burn a variety of fuels, including natural gas, propane, oil, and firewood. The most common furnace fuel in the U.S. is natural gas.
In Europe, where furnaces are almost unheard of, most homes are heated by a boiler that distributes heat through hot-water pipes. Unlike Europeans, however, most Americans insist on central air conditioning in their homes. It’s easier to provide whole-house air-conditioning in a home with a duct system. Once you have a duct system for cooling, it’s cheaper to install a furnace for winter heating than to install a boiler with a separate distribution system.
Even though the smallest available furnaces are often oversized for a high-performance home — a problem I addressed in a 2009 article, Heating a Tight, Well-Insulated House — furnaces still have virtues that are hard to ignore. They are inexpensive, widely available, and easily serviced by local HVAC(Heating, ventilation, and air conditioning). Collectively, the mechanical systems that heat, ventilate, and cool a building. contractors. For many North American homes, they are a logical way to supply space heat.
When it comes to fuel efficiency, residential furnaces in the U.S. are divided into two main categories: so-called “medium-efficiency” furnaces and “high-efficiency” furnaces.
Furnace efficiency is usually calculated using a laboratory procedure that measures an appliance’s “annual fuel utilization efficiency, ” or AFUE. This calculation accounts for heat losses up the chimney, heat losses through the appliance jacket, and heat losses due to on-and-off cycling, but it doesn’t account for electricity use (fan energy use) or heat lost through the distribution system (ductwork).
AFUE can be calculated for boilers as well as furnaces, and is used for appliances that burn many different types of fuel.
Low-efficiency and medium-efficiency furnaces
The usual definition of a “low-efficiency” furnace is one that is less than 75% efficient. The reason that you can no longer buy a low-efficiency furnace is that the federal government now requires residential gas-fired furnaces to have a minimum efficiency of 80%. (The minimum efficiency for oil-fired furnaces is 83%, except for oil-fired furnaces designed for installation in mobile homes, which have a minimum efficiency of 75%.)
Medium-efficiency furnaces have efficiencies in the range of 80% to 82%. The line between mid-efficiency and high-efficiency furnaces is not arbitrary, but marks the division between appliances with distinct operating characteristics. Mid-efficiency furnaces are designed to keep flue gases hot enough to avoid any condensation of flue-gas moisture, while high-efficiency furnaces deliberately encourage the condensation of flue-gas moisture.
It is technically difficult to manufacture a furnace with an efficiency between 83% and 89%, so none are available in that range. Furnaces with "in-between" efficiency have sporadic condensation of flue gases, and this condensation causes corrosion problems. Furnaces with an efficiency of 90% or more wring so much heat out of the flue gases that the furnace exhaust can be vented through PVC pipe, a material which is more resistant to corrosive condensate than the stainless-steel vent pipe that would have to be used for the hotter flue gases that would occur in a furnace with an efficiency in the tricky 83% to 89% range.