Gas Furnaces

Question	Answer
Categories of Furnaces	I-IV based on vent gas temperature and pressure(positive or negative)
Category I	vent gas is around 300º; uses stack effect to vent exhaust. Low and Mid efficiency and draft hood furnaces
Category II	vents at 140º; difficult to create stack effect so very uncommon(not covered on NHIE)
Category III	gas is hotter than Category I usually; uses factory components rather than conventional venting ducts. Can have condensate issues; look for collection system issues.
Category IV	vents at 140º, using high efficiency heat exchangers. Draws heat from exhaust, cooling enough to use PVC as vent; positive pressure. Must have appropriate condensation collection system.
draft hood type furnace	Category I production ended in the 70s. Low efficiency ribbon type burners(65%)
induced draft type furnace	Category 1 2-6 in shot burners; medium efficiency(80%)
Category I inspection key points	scorching or staining(flame rollout) bypassed controls undamaged wires(grommets) vent system intact
Draft hood ignition sequence	click of gas valve burner ignition fan activation
induced draft ignition sequence	inducer fan activation ignition device activation click of gas valve burner ignition fan activation
gas flame is blue	properly burning; yellow tip ok too.
gas flame is yellow/orange	improperly adjust burner or valve; inadequate combustion air
gas flame distorts or flickers	may indicate cracked heat exchanger

Resource summary

	Created by Andrew Adams over 6 years ago