Guide
Smoke not pulling into the hood: troubleshooting before calling a vendor
The fan is humming and the line is still smoky. That is not automatically a dirty duct. Capture is a chain: air enters at the hood lip, passes the filters, travels the duct, leaves at the fan, and an equal volume has to enter the kitchen as makeup air. Any broken link can leave smoke rolling the lip while the motor still spins. Walk the cheap checks before you book the wrong trade.
Short answer
Open an exterior door or window while the line is running. If smoke clears, the exhaust path is moving air and the kitchen is starving for makeup air — call HVAC / TAB, not another canopy wipe. Secondary IMC §508.1 / NFPA 96 §8.3.1 summaries require makeup approximately equal to exhaust and keep cooking-area negative pressure around 0.02 in. w.c. Next: pull a filter (loaded, wrong size, or installed backward); check belts, rotation, and hinge access on the rooftop fan; then book a full-system clean if the plenum, duct, or fan housing is grease-loaded. Filters must stay listed (UL 1046) and in place whenever grease-producing equipment runs (Philadelphia Fire Code §607.3.1 pattern; NFPA 96 grease-removal rules).
Five-minute triage (in order)
| Check | What you look for | If yes — first call |
|---|---|---|
| Door / window test | Smoke clears when you open an exterior door or window with the hood running | HVAC / TAB — makeup air or building pressure. See makeup air balance |
| Filters in place and listed | Missing filters, mesh-only in a hood that needs baffles, or filters not UL/ETL stamped | Staff replace with listed filters; cook only with filters seated. See filters clean vs replace |
| Filter load / orientation | Heavy grease cake; filter put in backward or wrong size after a wash night | Staff wash/replace; if smoke started right after a filter change, re-seat and size-check first |
| Fan running but weak | Belt shredded or loose; wheel won’t free-spin with power locked out; reverse rotation after electrical work | HVAC / sheet metal for belt, motor, rotation. See belt and bearing checks |
| Grease in the path | Plenum behind filters coated; fan housing heavy; long uncleaned duct | Hood cleaner for full-system clean with rooftop photos — not canopy-only. See full-system vs canopy |
| One station only | Smoke escapes at one appliance; rest of the line captures | Geometry / overhang / equipment change — may need hood or appliance move, not just CFM. See menu/equipment changes |
| Drafts at the lip | Supply diffuser blasting across the hood face; pass-through door drafts | HVAC to relocate or throttle supply; high-velocity makeup near the hood can push smoke out before capture |
What “the fan is on” does not prove
- Makeup air: Exhaust without replacement air pulls doors hard, steals dining-room air, and collapses capture at the lip. Secondary IMC language expects makeup approximately equal to exhaust and interlocked to start with the hood.
- Filters and plenum: Loaded baffles raise resistance. A clean filter face with a coated plenum still restricts airflow — nightly filter wash does not replace a professional plenum/duct clean.
- Fan hardware: A motor that hums with a slipping belt or grease-packed blades moves far less air. Hinged upblast access with hold-open is how you inspect the wheel; welded-flat fans hide the problem. See hinge checklist.
- After a “clean”: If smoke got worse after filter change or canopy wipe, check install orientation and whether ducts/fan were never opened. See post-clean fan spin test.
Who to call (split the jobs)
| Symptom cluster | Trade | Not who |
|---|---|---|
| Door slam; smoke clears with door open; MAU dark or damper closed | Commercial HVAC / TAB (measured CFM and pressure) | Not another grease clean alone |
| Filters packed; plenum/duct/fan greasy; overdue interval | Hood / exhaust cleaner acceptable to your AHJ | Not a pressure-washer who only does the canopy |
| Belt, bearings, motor, reverse rotation, curb/hinge | HVAC / sheet metal / listed fan installer | Not the grease-trap pumper |
| Suppression discharged or nozzles disturbed during troubleshooting | Licensed suppression tech before cooking resumes | Not kitchen staff resetting the system. See suppression vs cleaning |
Operator checklist before you pick up the phone
- Note what changed: new equipment, new dish, filter swap, electrical work, shopfront remodel, MAU service
- Do the door test and write the result on the work order
- Photo filters in place, one filter pulled, and the plenum behind them
- Confirm MAU (or transfer air path) is powered and interlocked with exhaust
- Ask the cleaner for full-system scope with inaccessible areas listed if grease is the suspect
- Ask HVAC for measured exhaust vs makeup CFM if the door test points to balance
Who it’s for / not for
- For: GMs and facilities leads with smoke on the line during service; multi-unit ops comparing sites after a “clean” that didn’t fix capture
- Not for: Designing a new hood from scratch; grease interceptor backups (different drain, different vendor); DIY fusible-link or suppression work
Sources
- ICC Building Safety Journal — Exhaust systems and makeup air under 2024 IMC (§508.1 framing)
- SEFA KitchenBiz — Make-up air for commercial kitchens (secondary; IMC §508.1 / NFPA 96 §8.3.1 ~0.02 in. w.c.)
- HoodFilters: NFPA 96 secondary excerpts (listed grease filters UL 1046; mesh limits; Table 12.4 inspection cadence)
- Philadelphia Fire Code §607.3.1 (example AHJ text: ventilation at required rate; listed grease filters in place when equipment under the hood is used)
- CED Engineering — Kitchen ventilation improvement (makeup velocity near hoods can break capture)
Next step
Do the door test on the next busy ticket. Write the result on the work order before you call anyone — it steers the job to HVAC or to the cleaner instead of burning a visit on the wrong trade.