Mechanical System Design and Installation
Design inputs the inspector can verify
The UMC does not make the inspector the designer, but it makes design assumptions visible. Check that the appliance input rating, fuel type, altitude, and category match the listing and the venting method. Ducts must be sized and installed to deliver the required airflow; flexible duct must be supported, not compressed, and protected from damage. Refrigerant systems are classified by refrigerant safety group; the installation must match the listed equipment and the occupancy limits.
Combustion air and venting
For fuel-burning appliances, the two failure modes are inadequate combustion air and improper venting. In a confined space using indoor air, the code typically requires two permanent openings, one near the top and one near the bottom, sized by appliance input. Outdoor-air openings use different sizing ratios. Vents must be listed with the appliance category, slope correctly, be supported, and terminate with proper clearance from windows, doors, and intakes.
Worked example
A 120,000 Btu/h natural-gas furnace is installed in a closet with two permanent openings directly to outdoors, one at the top and one at the bottom. Using the common UMC outdoor-air ratio of 1 square inch free area per 4,000 Btu/h, the calculated area is 120,000 divided by 4,000, or 30 square inches. But each opening has a minimum of 100 square inches. Therefore each opening must be at least 100 square inches, not 30. The inspector should also confirm that louvers and screens are accounted for when they reduce free area.
Traps
Using the indoor-air ratio for outdoor openings, or the reverse. Ignoring the 100-square-inch minimum. Sizing one opening instead of two. Accepting a vent connector that is not listed for the appliance category, or that slopes upward toward the appliance instead of toward the chimney.