Residential gas furnaces may be vented with single wall vent piping under all conditions except?

Single wall vent piping is a common method of venting residential gas furnaces; however, there are specific conditions under which its use is not permitted. In the case presented, the correct assertion is that single wall vent piping is not allowed when the vent runs through an interior concealed space.

This restriction exists primarily due to safety concerns. When venting runs through concealed spaces, it is at greater risk of being exposed to combustible materials or other conditions that may lead to deterioration or compromise the integrity of the vent system. Concealed spaces can also lead to inadequate ventilation and the potential for hazardous combustion gases to build up, thereby creating unsafe living conditions. Hence, using a more robust venting system designed for these situations helps ensure that gas fumes are safely expelled from the home.

In contrast, the other scenarios present situations where single wall venting might be acceptable. For example, a direct outdoor vent or an outdoor furnace installation can often utilize single wall piping as they do not involve the same risk of trapping combustion gases or exposing combustible materials to heat. Age of the furnace itself does not inherently dictate the venting requirements, as they are based more on installation conditions rather than the age of the equipment.