IMC 508.1
What it means on the job
IMC 508.1 requires makeup air to be provided at a rate approximately equal to the exhaust rate. At 1,200 CFM of makeup against 2,000 CFM of exhaust, the kitchen runs an 800 CFM deficit — nowhere near 'approximately equal' — and the symptoms prove it: doors that fight you and drafting problems at the gas appliances. Severe negative pressure can pull combustion products back down flues and starve burners for air. Makeup air isn't optional equipment; it's how the exhaust system is allowed to work.
Tested in 1 CraftIQ puzzle
- code-checkUnion Training: Kitchen Hood Makeup Air
Related IMC sections
IMC 501.3.1 / NFPA 110 Section 7.8
IMC Section 501.3.1 requires engine exhaust outlets to be located at least 10 feet from any building opening (door, window, or air intake). In high-rise buildings with rooftop generators, exhaust must be positioned to prevent re-entrainment of diesel exhaust into the building HVAC system. Carbon monoxide and diesel particulate entering the building during a fire emergency creates a secondary life-safety hazard.
IMC 506.3
Kitchen exhaust ducts must be steel (min 16 gauge) or stainless steel. PVC will melt from grease-laden exhaust heat and is a severe fire hazard.
IMC 508
Kitchen exhaust systems require makeup air to replace exhausted air. Without it, the kitchen goes negative pressure causing back-drafting of gas appliances (CO hazard) and door operation issues.
IMC 603.10 (flexible duct support)
Flexible duct must be supported at intervals not exceeding 4 feet (IMC 603.10), with no more than 1/2 inch of sag per foot between supports. Supporting this run every 5 feet exceeds that maximum, so it fails. Note the 8-foot length itself is not the violation — there is no universal 5-foot length cap in the IMC, IRC, or SMACNA for flexible duct; the genuine defect here is the support spacing. Fix: add hangers so spacing never exceeds 4 feet.
IMC 603.6
Flexible air duct must be supported at intervals not exceeding the manufacturer’s spacing (typically 5 ft) with sag no more than 1/2 inch per foot of spacing. Excess sag chokes airflow and traps condensate.
IMC 605 (Air Filters)
Return air grille has no filter rack. All return air must be filtered before entering the air handler per mechanical code.
CraftIQ explanations are original educational commentary written in trade language and verified against the cited standards. Always confirm requirements against the code edition adopted by your jurisdiction — this is training material, not a substitute for the code book or your AHJ.