Furnaces with an 80% Annual Fuel Utilization Efficiency (AFUE) rating turn up to 80% of their fuel into usable heat while losing 20% through the exhaust. They have a single heat exchanger. These furnaces are commonly used to heat multifamily homes.

Furnaces with a 96% Annual Fuel Utilization Efficiency (AFUE) rating turn up to 96% of their fuel into usable heat while losing 4% through the exhaust. They have a secondary heat exchanger, which helps recover lost heat and provides better fuel efficiency than single-exchanger furnaces. They heat multifamily homes and are often used in areas with harsh winters where the furnaces' fuel efficiency significantly reduces energy usage and where local code requires higher-efficiency furnaces.

These accessories enhance the functionality of whole-house gas furnaces. They may include parts for converting the fuel source, adding venting, or providing heater controls. Whole-house gas furnaces use natural gas to produce heat for multifamily homes.