For central warm-air furnaces, which are in 85% of homes heated with natural gas or propane, less than half have a rated efficiency of 90% or greater, as an Annual Fuel Utilization Efficiency (AFUE) (DOE SNOPR, 2016).
homes have forced-air heating distribution, with 73% of single family homes with central A/C and of those with gas heating, 88% have central warm-air furnaces. As a result, the focus of this study is combi systems with forced-air distribution.
It is designed for Category II and IV gas burning appliances, including tankless and storage water heaters, high-efficiency water heaters, condensing boilers and warm-air furnaces
The manufacture of air conditioning and refrigeration equipment and of warm-air furnaces involves the production of heat transfer apparatus for residential, commercial, and industrial applications, as well as for hospitals, marine vessels, freight and passenger vehicles, and many specialized applications.
thus, the production of heat transfer equipment other than unitary or room air conditioners or warm-air furnaces increased at a rate nearly 10 times higher over the 1967-73 period than during the 1973-82 span.
Increases in the size of homes and other structures generated the shift in demand from room air conditioners to central systems and spurred the demand for warm-air furnaces, which function through the same air circulation system as central air conditioners.
Warm-air furnaces built by the industry are mostly gas-fueled forced-air devices.
Advances in the manufacture of air conditioners, refrigeration equipment, and warm-air furnaces have been linked chiefly to technological progress in metalworking machinery, welding, methods of storage and transfer of parts, and assembly.
The production of air conditioners, refrigeration equipment, and warm-air furnaces basically involves the cutting and forming of metal, as well as welding, brazing, and soldering of components.