2021 IMC Chapter 4
Every occupied room needs fresh air, either through windows people can open or from a mechanical system that brings in outdoor air and removes stale air. Chapter 4 says how much. For offices, classrooms, and most other commercial spaces the answer comes from one table and one short formula. This lesson teaches both, with a full calculation. Houston follows the Uniform Mechanical Code (Lesson 1), which has its own ventilation provisions, so confirm local values.
Every occupied space is ventilated naturally under Section 402 or mechanically under Section 403 (401.2), and the air must be supplied whenever the space is occupied (401.3). Dwelling units that meet the energy code's air leakage requirements must use mechanical ventilation. Natural ventilation works through windows, doors, louvers, or other openings to the outdoors, with controls occupants can reach (402.1). The openable area must be at least 4 percent of the floor area ventilated (402.2). A room with no outdoor opening can borrow air through an adjoining room if the unobstructed opening between them is at least 8 percent of the interior room's floor area and not less than 25 square feet (402.3). That is 8 percent, not the 10 percent used for borrowed light in the residential code. Where the required opening is below grade, the clear outdoor horizontal space in front of it must be 1 1/2 times the opening depth (402.4).
Intakes must be at least 10 feet from lot lines and from other buildings on the same lot, and at least 10 feet horizontally from hazardous or noxious sources such as vents, streets, alleys, parking lots, and loading docks. An intake may be closer to streets, alleys, parking lots, and loading docks only if it is at least 25 feet vertically above them. Where a contaminant source lies within 10 feet, the intake must be at least 3 feet below it. In flood hazard areas the intake must be at or above the required flood elevation (401.4). Screens, louvers, and grilles on intakes must be corrosion resistant, with openings larger than 1/4 inch and not larger than 1 inch outside residential occupancies (1/4 to 1/2 inch in residential), and louvers in hurricane-prone regions must meet AMCA 550 (401.5, Table 401.5). Stationary sources of fumes, vapors, or heat need their own exhaust discharging outdoors (401.6).
Mechanical ventilation is a supply system paired with return or exhaust, with supply approximately equal to return plus exhaust (403.1). The minimum outdoor airflow comes from Section 403.3 (403.2). Required outdoor air may not be recirculated, though extra air above the requirement may be, and four limits apply: no recirculation of ventilation air between dwellings or into dissimilar occupancies, limits on pool and deck air, no recirculation from spaces where table note b demands exhaust, and, for note g spaces, no recirculation where more than 10 percent of the supply would be air from those spaces (403.2.1). Air transferred from occupied spaces may make up the exhaust in kitchens, baths, and toilet rooms unless the table prohibits recirculation (403.2.2). Groups R-2, R-3, and R-4 three stories or less follow Section 403.3.2. Everything else follows 403.3.1, which is the part this lesson works through.
Each row gives an occupant density (people per 1,000 square feet), a people rate Rp in cfm per person, an area rate Ra in cfm per square foot, and sometimes an exhaust rate. The design occupant count may not be less than the table density gives, unless approved statistical data show a different density. For a use not listed, pick the most similar row (403.3.1.1). A few rows we use in this course:
| Space | People per 1,000 sq ft | Rp (cfm per person) | Ra (cfm per sq ft) |
|---|---|---|---|
| Office spaces | 5 | 5 | 0.06 |
| Conference rooms | 50 | 5 | 0.06 |
| Classrooms, age 9 plus | 35 | 10 | 0.12 |
| Classrooms, ages 5 to 8 | 25 | 10 | 0.12 |
| Dining rooms | 70 | 7.5 | 0.18 |
| Places of religious worship | 120 | 5 | 0.06 |
| Libraries | 10 | 5 | 0.12 |
| Corridors | none | none | 0.06 |
Rows marked with note b or g (kitchens for cooking, science labs, beauty salons, and similar) require mechanical exhaust with no recirculation. Public toilet rooms are exhaust-only: 50 or 70 cfm per water closet or urinal, and the higher rate applies where the exhaust runs intermittently (note e).
Equation 4-1 gives the outdoor air needed in the breathing zone: Vbz = Rp x Pz + Ra x Az. Az is the net occupiable floor area, Pz is the number of people, and Rp and Ra come from the table (403.3.1.1.1.1). The first term covers what people breathe out, the second covers what the room and its contents give off.
Worked example. A community center classroom for ages 9 plus is 35 feet by 24 feet, so Az is 840 square feet. The table density of 35 per 1,000 square feet gives 29.4, so design for 30 people. Vbz = 10 x 30 + 0.12 x 840 = 300 + 100.8 = 400.8 cfm. The figure shows the two parts as bars.
Not all supplied air reaches the people. The zone air distribution effectiveness Ez (Table 403.3.1.1.1.2) is 1.0 for cool air from the ceiling or floor, 1.0 for warm air with floor return, 0.8 for warm air from the ceiling with a ceiling return, 0.7 for warm air from the floor with a ceiling return, 0.8 for makeup air drawn from the opposite side of the room from the exhaust, and 0.5 for makeup air drawn near the exhaust. The zone outdoor airflow divides by it: Voz = Vbz / Ez (403.3.1.1.1.3). For the classroom, Voz is 400.8 / 1.0 = 401 cfm in cooling and 400.8 / 0.8 = 501 cfm when warm air comes from the ceiling. A system must be able to deliver the larger number when that mode is designed. Table footnote f allows 1.0 for ceiling warm supply only when the supply air is less than 15 degrees F above room temperature and the 150 fpm jet reaches within 4 1/2 feet of the floor.
Section 403.3.1.1.2 turns zone numbers into what the air handler intake must deliver. With one air handler serving one zone, the intake equals that zone's Voz (Equation 4-3). A system that supplies only outdoor air to one or more zones sums the zones (Equation 4-4). A multiple-zone recirculating system works from each zone's primary outdoor air fraction Zp and a system ventilation efficiency Ev from Table 403.3.1.1.2.3.2: Ev is 1 when the largest Zp is 0.15 or less, drops through 0.9 (0.25 or less), 0.8 (0.35 or less), and 0.7 (0.45 or less), and falls to 0.3 above 0.75, so the intake grows as one zone dominates. Interpolation is allowed, and ASHRAE 62.1 Appendix A is an alternative. This course stops at the single-zone case.
Exhaust flow follows the table, and outdoor air brought in by an exhaust system counts toward the outdoor air requirement (403.3.1.2). The ventilation system runs continuously while the building is occupied. With demand-controlled ventilation the outdoor air may follow the actual head count, but it may never drop below the area rate, Ra x Az, whenever the space is expected to be occupied (403.3.1.3). For the classroom that floor is 100.8 cfm. Variable air volume systems other than 100 percent outdoor air systems need controls that keep outdoor air at or above the required rate across the full operating range (403.3.1.4).
In the field
A common rehab finding is a converted space whose use changed but whose air did not. A former storage room becomes a training classroom, the old exhaust fan stays, and no outdoor air is ever introduced. GCHS plan review asks three things before approving the conversion: what table row matches the new use, how many people the design count says, and where the outdoor air enters. Then in the field the inspector looks for an intake louver with a screen, the required distance from the dumpster or loading dock, and a balancer's report that shows the airflow that Vbz called for.
Group R-2, R-3, and R-4 buildings up to three stories skip Table 403.3.1.1 and use Section 403.3.2: continuous outdoor air for each dwelling unit, corridors at 0.06 cfm per square foot, and local exhaust of 100 cfm intermittent or 25 continuous in kitchens and 50 intermittent or 20 continuous in bathrooms (403.3.2.1 to 403.3.2.3). A fourth story pushes the building back to 403.3.1. Enclosed parking garages run continuously or use carbon monoxide and nitrogen dioxide detectors listed to UL 2075, cycling between full-on at 0.75 cfm per square foot and standby at 0.05 (404.1).
1. A 20 ft by 30 ft fellowship room is ventilated only by natural means. What is the minimum openable area to the outdoors under IMC Section 402.2?
2. An interior 400 square foot training room without outdoor openings is ventilated through an adjoining room. What is the minimum unobstructed opening between the rooms under IMC Section 402.3?
3. An outdoor air louver is 6 feet horizontally from the edge of a parking lot and 26 feet above it. Under IMC Section 401.4, is the location acceptable?
4. Under IMC Table 401.5, what opening size is required in screens and louvers protecting outdoor air intakes in a nonresidential occupancy?
5. What people rate (Rp) and area rate (Ra) does IMC Table 403.3.1.1 assign to office spaces?
6. An office suite is 1,500 square feet with 8 people. What is the breathing zone outdoor airflow Vbz using Rp 5 and Ra 0.06?
7. A classroom for ages 9 plus is 900 square feet with a design population of 32. Using Rp 10 and Ra 0.12, what is Vbz?
8. A zone has Vbz of 400 cfm and a heating design with warm air supplied at the ceiling and returned at the ceiling, which has Ez of 0.8 because the footnote f conditions are not met. What is the zone outdoor airflow Voz?
9. A design supplies warm air through floor diffusers with returns in the ceiling. What zone air distribution effectiveness does IMC Table 403.3.1.1.1.2 assign?
10. A 1,000 square foot office uses demand-controlled ventilation and the space is empty during the occupied period. Using Ra of 0.06, what is the minimum outdoor airflow under IMC Section 403.3.1.3?
11. A designer proposes to recirculate return air from a cooking kitchen area, a Table 403.3.1.1 row with note b, into offices next door. What does the IMC say?
12. A public toilet room has 3 water closets and 2 urinals, and its exhaust is designed to run continuously while occupied. Under Table 403.3.1.1 and note e, what is the minimum exhaust?
13. An enclosed parking garage of 20,000 square feet uses carbon monoxide and nitrogen dioxide detectors to cycle its ventilation. What is the minimum full-on airflow under IMC Section 404.1?
14. A four-story Group R-2 apartment building is being designed. Which path do the outdoor air provisions of IMC Section 403.3 send it to?