6.3 Recirculation, Transfer Air, Makeup Air & Energy Recovery

Once Table 403.3.1.1 tells you how much outdoor air a room needs, Chapter 4 and Chapter 5 decide where that air is allowed to come from and where the exhaust is allowed to go. Four paths get tested: recirculation, transfer air, makeup air, and energy recovery. Mixing the names is how a legal DOAS becomes an illegal toilet-to-office return.

Recirculated air (Chapter 2) is air removed from a conditioned space and intended for reuse as supply air. Transfer air is air moved from one indoor space to another. Makeup air is any combination of outdoor and transfer air intended to replace exhaust air and exfiltration. Energy recovery uses an air-to-air heat exchanger to preheat, precool, humidify, or dehumidify outdoor ventilation air. ERV airstreams are not recirculated air when cross-leakage is limited to less than 10 percent of total design airflow (513.4).

Recirculation limits — Notes b and g

403.2.1. The outdoor air required by 403.3 shall not be recirculated. Air in excess of that required outdoor air may be recirculated as a component of supply air, with four limits:

1. Ventilation air shall not be recirculated from one dwelling to another or to dissimilar occupancies.
2. Supply air to a swimming pool and deck shall not be recirculated unless it is dehumidified to 60 percent RH or less. Air from that area shall not be recirculated to other spaces where more than 10 percent of the resulting supply is pool air. Dehumidification shall comply with ANSI/ACCA 10 Manual SPS.
3. Where Table 403.3.1.1 Note b requires mechanical exhaust, recirculation of air from those spaces is prohibited. Recirculation of air contained completely within the space is not prohibited. Where recirculation is prohibited, all air supplied to the space shall be exhausted, including air in excess of the table rate.
4. Where Note g requires mechanical exhaust, recirculation from those spaces is prohibited where more than 10 percent of the resulting supply airstream is from those spaces. In-space recirculation is still allowed. Note g also caps wheel-type ERV leakage from the exhaust airstream into the outdoor airstream at less than 10 percent of the outdoor-air volume — except science laboratories, which do not get that wheel leak path.

Note b occupancies you will see on M2: smoking lounges, kitchens (cooking), common garages, private dwelling kitchens (exhaust), automotive motor-fuel-dispensing stations, beauty salons, embalming rooms, nail salons, pet shops (animal areas), repair garages, and enclosed parking garages. Note g occupancies include cells with plumbing fixtures, art classrooms, locker/dressing rooms, science laboratories, sports locker rooms, wood/metal shops, private bathrooms/toilets, public toilet rooms, and shower rooms. A public toilet that dumps into the office return plenum is a Note g / 403.2.1 Item 4 failure if that air is more than 10 percent of the office supply — and a field failure even sooner if the inspector treats the toilet as a dedicated exhaust room that should never have been tied to the return.

Transfer air — when it is allowed

403.2.2. Except where recirculation from such spaces is prohibited by Table 403.3.1.1, air transferred from occupiable spaces is not prohibited from serving as makeup air for required exhaust in kitchens, baths, toilet rooms, elevators, and smoking lounges. Transfer air and exhaust air shall be sufficient to provide the 403.3.1.1 flow rates. The required outdoor airflow in the table shall be introduced directly into those spaces, or into the occupied spaces from which air is transferred, or a combination of both.

Read that with Note b. A cooking kitchen is Note b: you may not recirculate kitchen air to the dining room. You may transfer dining-room air into the kitchen as makeup, provided the dining room (or the kitchen, or both) still receives its Table 403.3.1.1 outdoor air. Smoking lounges are listed as a permitted transfer-air destination, but they are also Note b — lounge air does not return to the casino floor.

Field arithmetic. Dining room Vbz = 1,057.5 cfm (Section 6.2). Cooking kitchen exhaust = 280 cfm plus whatever the Type I hood adds under Chapter 5. Transfer 280 cfm of dining air into the kitchen, keep the dining room’s outdoor-air intake at 1,057.5 cfm (or split that outdoor air between dining and kitchen), and exhaust the kitchen. Do not take 280 cfm of kitchen air back to the rooftop mixing box.

Makeup air and pressure

501.4 Pressure equalization. Size mechanical exhaust to remove the Chapter 5 quantity and operate it when exhaust is required. Where mechanical exhaust is required in a room other than Group R-3 and R-2 dwelling units, that space shall be maintained at a neutral or negative pressure. If supply exceeds exhaust, provide a path for the excess. If only exhaust is installed, or exhaust exceeds supply, provide makeup air to satisfy the deficiency.

Chapter 2 already defined makeup air as outdoor air plus transfer air. 501.4 is the pressure sentence that fails a restroom with a 50 cfm fan and a tight door, no undercut, and no transfer grille: the fan cannot move 50 cfm if the room goes deeply negative, and the corridor does not get to donate air through a rated corridor door that was never designed as a transfer opening.

Commercial kitchen makeup is Section 508, taught with Type I hoods. The pointers you need from a Chapter 4 inspection:

• 508.1: supply makeup during commercial kitchen exhaust operation; building makeup from all sources shall be approximately equal to building exhaust; makeup shall not reduce hood effectiveness; gravity, mechanical, or both; mechanical makeup shall start and run simultaneously with the exhaust; intakes still obey 401.4.
• 508.1.1: kitchen HVAC must handle the extra latent and sensible load, or dedicated makeup shall be within 10°F of the kitchen setpoint. Compensating-hood makeup is excepted from that temperature rule.
• 508.1.3: plans shall include an outdoor-air balance showing all exhaust, all replacement air, and net exfiltration. Total replacement air equals total exhaust plus net exfiltration.

Energy recovery — 2024 Section 513 (was 514)

The 2024 IMC moved energy-recovery ventilation from 514 (2018/2021) to Section 513. Tab 513, not a memory of 514. 513.1: install ERVs per this section; where the IECC requires recovery, follow the IECC; ducted heat-recovery ventilators listed to UL 1812; nonducted to UL 1815.

513.2 Prohibited applications. ERV shall not be used in:

1. Hazardous exhaust systems (2024 Section 509 — the hazardous-exhaust chapter).
2. Dust, stock, and refuse systems that convey explosive or flammable vapors, fumes, or dust.
3. Smoke-control systems (Section 512).
4. Commercial kitchen exhaust serving Type I hoods.
5. Clothes-dryer exhaust (Section 504).

Exception: ERV that recovers sensible heat only using coil-type heat exchangers is not limited by 513.2. A run-around coil that never mixes airstreams can sit on a stream a wheel cannot. A total-energy (enthalpy) wheel on a grease duct is still a fail.

513.3 requires access to the exchanger for service. 513.4: if cross-leakage is 10 percent or more, treat the ERV path as recirculation and apply 403.2.1. That is how a “we added an ERV so the toilet can share the office AHU” argument dies: either leakage is under 10 percent and the application is not on the 513.2 list, or you have built a recirculating toilet.

Exhaust to outdoors — not to the attic

501.3. Every mechanical exhaust system shall discharge outdoors at a point that will not cause a public nuisance and at the 501.3.1 distances. Discharge to a location from which the air cannot be readily drawn back into a ventilating system. Air shall not be exhausted into an attic or crawl space, or be directed onto walkways. Exceptions are narrow: whole-house ventilation-type attic fans into a dwelling unit’s private attic; listed commercial cooking recirculating systems; and listed domestic ductless range hoods where Chapter 4 ventilation is otherwise provided. A commercial toilet fan that terminates in a roof truss bay, a dryer that “vents into the soffit,” or a warehouse exhaust that blows across a sidewalk fails 501.3.

501.3.1 distances worth pairing with 401.4: environmental-air exhaust 3 feet from property lines and operable openings (1 foot above a gravity intake is the operable-opening relief), 10 feet from mechanical intakes. Product-conveying and explosive/flammable outlets use the much larger Item 1 and Item 2 setbacks (30 feet / 10 feet / 6 feet — see the hazardous-exhaust chapter).

501.2 keeps environmental-air exhaust independent of other exhaust. Dryer, domestic kitchen, and hazardous exhaust shall be independent of all other systems. Type I exhaust is independent except as 506.3.5 allows. Do not manifold a toilet, a dryer, and a grease duct into one ERV.

Field: office DOAS versus mixed-air VAV

Two legal office systems show up on the same floor plate.

DOAS (100 percent outdoor air). One air handler supplies only outdoor air to one or more zones. Equation 4-4: Vot = Σ Voz. Each zone still gets Vbz from Table 403.3.1.1 and Voz = Vbz / Ez. Parallel fan-coil or VRF units recirculate within the zone for temperature. Toilet exhaust is a separate Note g stream to outdoors; it may use transfer air from the office for makeup, but it does not return through the DOAS. An ERV on the office exhaust/intake pair is legal if it is not serving a 513.2-prohibited stream and leakage stays under 10 percent.

Mixed-air VAV. One air handler mixes outdoor air with recirculated return and serves many zones. 403.3.1.1.2.3: compute each zone’s primary outdoor-air fraction Zp = Voz / Vpz (use the lowest expected occupied primary airflow for VAV), take Ev from Table 403.3.1.1.2.3.2, then Vot = Vou / Ev. 403.3.1.4 still requires outdoor-air controls that hold the 403.3 rate over the entire supply range. A return-mounted toilet or locker grille is a 403.2.1 problem, not an “energy-saving mixing” feature. Parking-garage exhaust at 0.75 cfm/ft² is Note b — it never enters that mixed-air unit.

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