Guide
Makeup air and exhaust balance — when to call HVAC not a cleaner
A greasy duct can starve capture. So can a kitchen that has no makeup air, a dead makeup-air unit (MAU), or a defeated interlock. Hood cleaners remove grease to bare metal along the exhaust path. They do not size, balance, or commission the supply that replaces what the fan pulls out. This page is the split: symptoms that point to exhaust/makeup imbalance, who to call, and what to ask before you pay for another clean that cannot fix the door slam. For grease-path scope see full-system vs canopy-only; for fan hardware see the hinge checklist.
Short answer
Secondary sources republishing IMC §508.1 require makeup air whenever commercial kitchen exhaust runs, approximately equal to total exhaust, with mechanical makeup starting and operating with the exhaust. Secondary NFPA 96 §8.3.1 (SEFA / trade republish) caps negative pressure in cooking areas at about 0.02 in. w.c. (4.9 Pa). Hard-to-open or slamming doors, smoke rolling the hood lip, dining-room odor, cold drafts at the line, or a MAU that does not run with the fan are balance/HVAC problems — call a commercial HVAC or TAB tech for measured CFM and pressure, not another canopy wipe. Your AHJ’s adopted IMC/NFPA edition wins; confirm section numbers locally.
Symptoms → who to call
| What you see / feel | Likely cause (field pattern) | Who to call |
|---|---|---|
| Exterior / kitchen doors hard to open or slam shut | Exhaust pulling more than makeup returns — room goes hard negative | HVAC / TAB — measured balance; check MAU on, filters, dampers, interlock |
| Smoke / grease vapor rolls out the hood face with fans “on” | Capture starved by negative pressure, wrong makeup throw into the capture zone, or fouled duct raising static | First: confirm duct/fan were cleaned to bare metal. If clean and still spilling → HVAC/TAB |
| Cooking odor in dining / adjacent spaces | Kitchen pushed positive (too much makeup) or transfer paths wrong | HVAC/TAB — pressure kitchen-to-dining, not a cleaner |
| Line freezing in winter; staff prop the back door | Untempered or mis-set MAU discharge; door prop then wrecks the balance | HVAC — MAU heat/discharge setpoint (IMC secondary ~10°F differential) |
| MAU silent while exhaust fan runs | Defeated or failed interlock; separate switch; relay fault | HVAC / controls — prove fans start/stop together |
| Heavy grease in duct / fan / grease cup; canopy looks dirty | Cleaning cadence or incomplete scope | Hood cleaner — full system, not HVAC first |
| Fan won’t spin, belts shredded, hinge seized | Mechanical exhaust hardware | Cleaner (reinstall) + HVAC/electrician as needed; see hinge / post-clean checks |
Secondary code notes (verify edition)
| Source (secondary) | What it says in plain terms | Operator takeaway |
|---|---|---|
| IMC §508.1 (ICC Building Safety Journal / SEFA republish) | Makeup air required while commercial kitchen exhaust operates; quantity from all sources approximately equal to exhaust; mechanical makeup starts and runs with exhaust; makeup must not kill hood capture | Ask HVAC to prove interlock and measured makeup ≈ exhaust — not a verbal “it should be fine” |
| IMC §508.1.1 (ICC / SEFA) | Temperature difference between makeup and conditioned space ≤ about 10°F unless HVAC can carry the load (compensating-hood exception exists) | Cold blast at the line is a tempering/setpoint problem, not a cleaning ticket |
| NFPA 96 §8.3.1 (SEFA secondary excerpt) | Replacement air adequate so negative pressure in cooking areas does not exceed 0.02 in. w.c. (4.9 Pa) | Door slam + smoke spill often sit past that limit — need a manometer, not a mop |
| UMC §511.3 (SEFA secondary) | Same 0.02 in. w.c. negative-pressure cap framing where UMC is adopted | Confirm whether your city uses IMC, UMC, or a hybrid |
Section numbers move by edition and local amendment. Treat the cites above as pointers from secondary republishers, then verify with the AHJ and the engineer of record.
Hood cleaner scope (grease path)
- Clean hood, filters, plenum, ducts, fan housing to bare metal
- Report inaccessible sections; restore panels; service label
- Reinstall fan, check spin/belts as part of job closeout (see post-clean guide)
- Flag MAU-off / door-slam symptoms on the report — do not “fix” balance
HVAC / TAB scope (air balance)
- Measure exhaust CFM, makeup CFM, kitchen-to-dining and kitchen-to-outdoors pressure
- Prove MAU–exhaust interlock both directions
- MAU filters, belts, dampers, burner/discharge temperature
- Correct throw locations that blast the hood capture zone
- Written balancing report after fan or MAU replacement
Operator checklist before you call
- Symptom log — Door slam? Smoke spill? Dining odor? Cold line? When (rush only / always)?
- Fan + MAU status — With cooking load on, are both running? Kill MAU power once (safely, with HVAC present if unsure) — does exhaust drop or alarm?
- Last full-system clean date — Fouled duct raises static and can look like a “weak fan.” If overdue, schedule cleaning first; if freshly cleaned and still imbalanced, call HVAC.
- Recent equipment changes — New hood, higher-duty broiler, fan swap without MAU rebalance (common failure per field guides).
- Ask HVAC for — Measured exhaust CFM, makeup CFM, makeup % of exhaust, pressures in in. w.c., discharge temperature, interlock test, door test under full load.
- Do not ask the cleaner for — “Just turn the fan faster,” fabricated CFM numbers, or MAU burner work outside their license.
Field note — why “bigger fan” often makes it worse
- Trade write-ups (SEFA, Anvilfield, Lucas Air) note that raising exhaust without matching makeup deepens the negative, fights doors harder, and can cut effective capture further.
- Secondary industry notes often cite exhaust performance drops on the order of up to ~30% under hard negative — treat that as a secondary illustration, not a measurement of your kitchen.
- A hinged rooftop fan that opens for cleaning still does not prove the MAU is sized or interlocked.
- After any fan or MAU change, require a written balance report — same discipline as commissioning day one.
Who it’s for / not for
- For: kitchen managers fighting door slam or smoke spill after a documented clean; facilities leads after a fan replacement; landlords comparing tenant “ventilation” complaints to grease reports
- Not for: DIY gas-appliance vent work; assuming a canopy polish fixed building pressure; inventing CFM or pressure readings without instruments
Sources
- ICC Building Safety Journal — Exhaust systems and makeup air under 2024 IMC (official ICC article; §508.1 / §508.1.1 framing)
- SEFA KitchenBiz — Make-up air for commercial kitchens (secondary; quotes IMC §508.1 / §508.1.1, UMC §511.3, NFPA 96 §8.3.1 0.02 in. w.c.)
- Anvilfield — Makeup air unit and kitchen ventilation balance (secondary field guide; interlock, tempering, commissioning readings)
- Lucas Air — Commercial kitchen ventilation guide (secondary; imbalance symptoms; do not upsize exhaust alone)
- US Made Supply — NFPA 96 makeup air summary (secondary; door/smoke symptoms + §8.3.1 pressure limit)
- Choice Mechanical — Why commercial MAUs fail (secondary HVAC troubleshooting patterns)
Next step
Write the symptom in one sentence. If grease path is overdue, book a documented full-system clean first. If the path is already bare metal and doors still fight you or smoke still rolls, call commercial HVAC/TAB and ask for measured CFM, pressure, interlock proof, and a written balance report — not another wipe of the canopy.