A draft hood is installed on standing-pilot __________ furnaces.

A draft hood is a critical component specifically designed for standing-pilot gas furnaces. It plays an essential role in ensuring proper ventilation and safe operation of the furnace by allowing combustion gases to escape while also preventing backdrafts from entering the system. In contrast, electric furnaces do not utilize combustible fuel and therefore do not require a draft hood. Oil and coal furnaces operate differently and typically employ different venting and combustion systems that do not necessitate the use of a draft hood as found in gas systems. Thus, gas is the correct choice as it accurately reflects the application of a draft hood in relation to standing-pilot systems.