Duct Systems and Combustion Air
Duct Systems
Duct questions test materials, support, sealing, and fire/smoke considerations. Know that ducts must be constructed of approved materials, supported at defined intervals, and sealed to limit leakage, especially where they pass through unconditioned space. Understand duct clearance to combustibles, protection where ducts penetrate fire-rated assemblies, and the prohibition on using duct interiors for wiring or piping. Flexible duct has tighter limits on length, sag, and bending radius than rigid duct.
Combustion Air
Combustion air is a perennial exam favorite. Fuel-burning appliances need a permanent source of air sized to the appliance input. The two classic methods are the interior volume method (relying on the volume of the space, often with infiltration assumptions) and the outdoor air method using openings or ducts sized to the appliance input. Know that the outdoor air approach typically requires two permanent openings, one near the top and one near the bottom of the enclosure, and that the free area of louvers and grilles must be accounted for when calculating.
Traps
The biggest trap is mixing methods - counting interior volume while also installing a single outdoor opening. Another is ignoring the free-area reduction for louvers (metal louvers are commonly derated, wood louvers more so). Also watch for appliances installed in tight, weather-stripped enclosures where the interior volume method is not permitted without additional openings.
Rule of Thumb
When in doubt, provide two permanent openings to outdoors, one high and one low, each sized per the appliance input, and derate for louver free area. Never seal a combustion appliance zone without verifying the air path.
Worked example: A furnace in a small closet with a tight door cannot rely on interior volume alone. Adding one low outdoor opening is insufficient under the two-opening method; provide both a high and a low opening sized to the input, and reduce the louver free area when selecting grille sizes.