Mechanical Systems: Heating, Cooling, Ventilation, and Ducts
Core concepts
Mechanical requirements are mandatory on every compliance path, which is the single most tested idea here. Know these buckets:
- Sizing and controls - equipment sized per an approved method (ACCA Manual J or an approved equivalent), with required controls and thermostats; some editions require verification of airflow and refrigerant charge.
- Ducts - duct insulation R-value depends on climate zone and duct location; ducts and air handlers located inside the conditioned envelope are strongly favored. Duct leakage is limited in cfm per 100 ft² of conditioned floor area, with a stricter limit when the air handler or ducts are outside the envelope; leakage is verified by a pressure test, often at rough-in.
- Ventilation - whole-house mechanical ventilation per ASHRAE 62.2, plus local exhaust in kitchens and bathrooms. Existing exhaust-only devices (dryer, range hood) generally may not be counted as the required whole-house system.
- Building cavities - using framing cavities as supply or return ducts is tightly restricted.
Traps
- Believing a high-performance envelope lets you skip duct sealing. It does not.
- Mixing up total leakage and leakage to outdoors; the limits differ.
- Testing after the air handler is installed when the adopted edition requires a rough-in test, or vice versa - the test condition changes the allowable limit.
- Forgetting that kitchen/bath exhaust must be ducted to outdoors, not into the attic.
- Assuming oversized equipment is fine because the table minimum efficiency is met.
Worked example
If the adopted limit is 4 cfm per 100 ft² of conditioned floor area, a 2,500 ft² house is allowed 4 × 25 = 100 cfm of measured leakage under that condition. If the system's air handler sits in the garage, the stricter second limit may apply as well - you must satisfy both. Always pull the actual constant from the adopted table.