: Low-Rise Residential Ventilation
Plain English: Plain English Highlights
403.3.2: For Group R-2/R-3/R-4, ≤3 stories above grade plane → design outdoor air (OA) + local exhaust per 403.3.2.1-.5.
403.3.2.1 (Dwelling units): Whole-house mechanical OA required (exhaust, supply, or balanced; OA duct to AHU return allowed). Must run continuously when occupied, or at least 1 hour of every 4 hours with equivalent average OA. Minimum continuous OA by Eq. 4-9: QOA (cfm) = 0.01×Afloor (ft²) + 7.5×(Nbr + 1). Two conditions allow 30% reduction: (1) OA is ducted directly to each bedroom and to the living/dining/kitchen, and (2) the system is balanced.
403.3.2.2 (Common areas): Corridors/other common conditioned spaces: OA ≥ 0.06 cfm/ft².
403.3.2.3 (Local exhaust): Kitchens: 100 cfm intermittent or 25 cfm continuous. Baths/toilets: 50 cfm intermittent or 20 cfm continuous.
403.3.2.4 (Controls): Provide unit controls with clear text/symbol indicating the OA system function.
403.3.2.5 (Fans): OA/exhaust fans must be listed & labeled for airflow per ANSI/AMCA 210 - ANSI/ASHRAE 51.
Calculation Check: Step-by-Step (mirror on M-601)
1. Inputs: Afloor (ft²), Nbr (min 1).
2. Eq. 4-9: QOA = 0.01×Afloor + 7.5×(Nbr+1).
3. Intermittent option: If cycling, ensure average over 4 hours ≥ QOA (document duty cycle and fan cfm).
4. 30% reduction check:
Duct OA directly to each bedroom + to living/dining/kitchen, and
Whole-house system is balanced → QOA×0.70.
5. Common areas: OA = 0.06 cfm/ft².
6. Local exhaust: Kitchen/bath fans meet table rates (INT or CONT).
7. Fans: Verify listing to ANSI/AMCA 210-ASHRAE 51 and schedule rated cfm @ external static.
Code Path: Where to Show It (sheet map)
M-001: "R-2/R-3/R-4 ≤3 stories per IMC 403.3.2; dwelling OA by Eq. 4-9; intermittent ≥1 hr/4 hrs with equivalent average; 30% reduction only if bedrooms + living/dining/kitchen ducted and system balanced."
M-101+: OA routes, bedroom supply points, transfer paths; tag kitchen/bath exhaust cfm and INT/CONT.
M-601: Afloor, Nbr, QOA, reduction (Y/N & basis), fan/model, cfm@s.p, sone, balance method.
M-002/M-603: Duty cycle logic; "balanced" verification/alarms.
M-501: Terminations, backdraft dampers, short/straight duct details; list standards.
Coordination Check: Frequent Redlines
Claiming 30% reduction without bedroom OA or without a truly balanced system.
Missing 0.06 cfm/ft² OA for corridors/common areas.
Field Tip: Field Tips
Balanced = proven: Commission and document supply vs exhaust within tight tolerance.
Design for delivered cfm: Short, smooth ducts; few elbows; verify cfm at the terminal.
Label controls: Clear "Whole-House Ventilation" text/symbol reduces disablement and inspection fails.
If you want a fillable Eq. 4-9/duty-cycle calculator and a 1-page "Balanced System Proof" checklist, comment IMC403.
Masterbuild QA Lens
Ventilation posts should translate code language into a permit-ready airflow story: occupancy, outdoor air quantity, intake location, exhaust relationship, controls, and any exception being used.
Drawing / Submittal Check
Verify the drawings show occupancy basis, ventilation method, outdoor air path, intake and exhaust separation, transfer air assumptions, controls, and schedule values that match the calculation.
Common Review Risk
The common review risk is an airflow value without a traceable basis. Another risk is showing outdoor air on a schedule while the plan, intake location, or control sequence tells a different story.
When To Escalate
Escalate for healthcare spaces, high-occupant-load areas, unusual contaminants, existing air handlers with unknown outside air capacity, or any project where pressure relationships matter.
Rated Assembly Coordination
When rated construction is involved, the drawings should identify the assembly, damper type, access location, actuator/control basis, fire alarm interface if applicable, and who coordinates the opening.
Duct System Coordination
For duct and plenum items, check material limits, insulation continuity, vapor control, access, listed products, and whether the surrounding space changes the requirement.