For forced-air furnaces, what standard applies to solid fuel units?

The standard that applies to solid fuel units for forced-air furnaces is UL 391. This standard specifically addresses the safety and performance requirements for solid-fuel-fired central heating appliances. It covers aspects such as design, construction, and testing, ensuring that these units operate safely and efficiently.

UL 727 pertains to the safety of factory-built chimneys and venting for residential appliances, which is relevant but not specific to solid fuel units. UL 1995 governs the safety of air-conditioning and heat pump equipment, while UL 499 covers electric heating appliances. Both of these standards focus on different types of heating technologies rather than solid fuel applications. Therefore, UL 391 is the correct standard for solid fuel units in forced-air furnace systems, as it is tailored to address the unique considerations and safety concerns associated with solid fuel sources.