Low-Rise Residential Ventilation
Plain English:
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) = 0.01xAfloor (ft^2) + 7.5x(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/ft^2.
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 (ft^2), Nbr (min 1).
2. Eq. 4-9: QOA = 0.01xAfloor + 7.5x(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 -> QOAx0.70.
5. Common areas: OA = 0.06 cfm/ft^2.
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/ft^2 OA for corridors/common areas.
Check: Local Exhaust Schedule and Makeup Air
Local exhaust schedule must show the exhaust rate for each exhausted space, the system serving it, and the basis (IMC table rate by occupancy type and space function).
Confirm each schedule entry identifies the space by name, not just floor or zone - grouping spaces without individual rates generates a plan review comment.
Show the makeup air path for each exhausted space on M-101.
Review Risk:
Local exhaust rates at the code table minimum without confirmation against actual contaminant generation are a common performance gap - verify the rate meets the process requirements, especially for lab fume hoods, spray booths, and cooking equipment.
Missing makeup air path for exhausted spaces is a consistent plan review comment - every exhaust source needs a confirmed makeup air route on the duct plans.
Local exhaust schedules that group spaces by floor without individual exhaust rates will get a comment.
Field Tip:
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.
Masterbuild QA Lens
Multi-zone ventilation rate procedure calculations are often prepared in a spreadsheet and not reproduced on the drawings. The permit set must include either the calculation or a clear reference to the calculation on file. If the design uses zone-level OA variation, show zone OA min and max on the VAV schedule - not just system-level totals.
Drawing / Submittal Check
For IMC 403 Part 4 (403.4-403.5 - exhaust ventilation and local exhaust): the drawings must show the local exhaust rate for each exhausted space, the exhaust system serving each space, and the makeup air path. Confirm the exhaust rate in each space matches the IMC table rate for the occupancy type and space function. Local exhaust schedules that group spaces by floor without identifying the rate per space will get a comment.
Common Review Risk
Local exhaust rates that were designed to the code minimum but not confirmed against the actual contaminant generation rate are a common performance problem. The code table establishes a minimum - for some processes (lab fume hoods, spray booths, cooking), the actual required rate is higher. Confirm the local exhaust rate is based on the actual process requirements, not just the code table minimum.
When To Escalate
Escalate when the local exhaust system for a laboratory or process space cannot maintain the required capture velocity at all operating conditions - when the exhaust rate varies with the process but the makeup air system cannot track the variation fast enough to prevent pressure fluctuations. Variable-process exhaust systems require controls engineering that goes beyond standard mechanical design.
Rated Assembly Coordination
Parking garage exhaust systems penetrate fire-rated separation walls between the garage and occupied spaces. Confirm damper type, rating, and actuation method at each penetration - fire dampers alone are not always sufficient at smoke barriers.
Duct System Coordination
Parking garage ventilation requires dedicated exhaust duct systems with corrosion-resistant materials and CO-sensor interlock. Confirm duct material specification and sensor location before drawing coordination.