Your exhaust fan is running. The hood looks fine. But smoke is escaping — billowing out from under the hood, drifting across the cooking line, or migrating into the dining room. Here's what's actually wrong.
The 4 Most Common Causes
1. Exhaust CFM Is Too Low The most common cause. The hood was designed for a specific CFM — say 2,750 for a 10-foot hood. But the actual exhaust has dropped due to: - **Dirty fan wheel** — grease buildup reduces efficiency by 20-40% - **Slipping belt** — fan turns slower than it should - **Worn bearings** — increased drag slows the fan - **Damper partially closed** — someone closed it and forgot - **Dirty filters** — restricted airflow (less common with baffles)
Test: Measure actual exhaust CFM with a VelGrid or anemometer. Compare to the hood's design CFM (usually on the UL nameplate inside the hood). If measured is more than 10% below design, the exhaust system needs service.
Fix: Fan service (belt, wheel cleaning, bearing) or damper adjustment. Contact us for pricing.
2. Makeup Air Disrupting Capture The supply air from the makeup air unit is hitting the cooking surface or the hood face at too high a velocity. This scatters the thermal plume — the column of hot air and smoke rising from the cooking — and pushes it out from under the hood instead of into it.
Test: Run a smoke pencil test with the MAU on vs. off. If capture improves dramatically when the MAU is off, the supply air direction or velocity is the problem.
Fix: Redirect supply air diffusers, reduce MAU fan speed, or adjust dampers. May require a full air balance. Contact us for a quote.
3. Insufficient Overhang The hood doesn't extend far enough past the cooking equipment. Smoke rises at an angle — not straight up — especially from equipment at the front of the cooking line. If the hood doesn't overhang the equipment by at least 6 inches on exposed sides, smoke escapes before reaching the capture zone.
Test: Look at where the smoke escapes. If it's always from the front edge of the hood, overhang is likely insufficient.
Fix: This is a design issue. Options include adding an air curtain, extending the hood with a front extension panel, or repositioning the cooking equipment further under the hood.
4. Cross-Drafts Air movement from HVAC supply registers, open doors, pass-through windows, or even foot traffic creates cross-drafts that push cooking effluent out from under the hood. Walk-in doors near the cooking line are a common culprit.
Test: Observe when capture fails. Is it when the back door opens? When the HVAC kicks on? When servers pass by? The pattern reveals the source.
Fix: Redirect HVAC registers away from the hood area, install strip curtains on walk-in doors near the line, or add side panels to the hood.
The Smoke Pencil Test
This is the definitive field test. Hold a smoke pencil at the front edge of the hood at cooking height. The smoke should be captured immediately and pulled into the hood. If it drifts, billows outward, or stalls — the hood is not capturing.
Every air balance we do includes a smoke capture test as the final verification. Numbers on a report mean nothing if the smoke test fails.
What It Costs to Fix
Most smoke capture problems are solved with fan service and/or an air balance visit. Contact us for a quote — we price per hood system, not hourly. Physical modifications like air curtains or hood extensions are quoted separately.
Most smoke capture problems are cause #1 or #2 — and both are fixed with a single air balance visit.
True Commercial Service diagnoses and fixes hood capture problems for commercial kitchens across Union County, NJ. We bring the instruments, take the readings, run the smoke test, and fix it. One visit.
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