Why reducing exhaust can help (in some cases)
Think of the kitchen as an air-balance equation:
Exhaust (out) – Make-Up Air (in) = Pressure deficit (ΔP).
- When exhaust ≫ MUA, the kitchen goes too negative. Air rushes in through cracks/doors and, more critically, fast cross-drafts form at the cookline.
- Those drafts can starve burners of primary/secondary air right where they need it most, giving lazy/yellow flames, unstable pilots, and sometimes back-drafting from flues.
- If you can’t increase MUA any further (fan at max, blocked intake you can’t clear immediately, damper limits, etc.), then slightly reducing exhaust shrinks the imbalance. That makes ΔP less negative and reduces the local drafts pulling at the flames. The result is often bluer, steadier flames and room O₂ readings returning to normal.
In short: you’re not “adding oxygen” by reducing exhaust — you’re removing excessive suction so the oxygen already being supplied can actually reach the burners without being whisked away to the hood.
A simple number picture
- Before: Exhaust 8,000 CFM; MUA 5,000 CFM → deficit 3,000 CFM → very negative (e.g., ≤ −20 Pa), yellow “lazy” flames.
- After: Reduce exhaust to 6,500 CFM (while holding MUA at 5,000 CFM) → deficit 1,500 CFM → mild negative (≈ −10 Pa), flames regain primary air → blue.
(Exact pressures depend on building leakage and door gaps, but the direction of change is consistent.)
Why this is “last resort”
Exhaust hoods don’t just move air; they capture grease, heat, and smoke. If you reduce exhaust too far:
- Capture/containment drops → haze/grease escape into the kitchen or dining area.
- You could drift out of code or manufacturer ventilation guidelines.
That’s why we only reduce exhaust after:
- Maximizing MUA (fan speed up, dampers open, filters clean),
- Clearing intake blockages, and
- Verifying the hood and space are otherwise set up correctly.
How to do it safely (field playbook)
- Max out supply first
- Confirm MUA fan ON, dampers fully open, filters clean, intake not blocked.
- If you can raise MUA setpoint, do that before touching exhaust.
- Trim exhaust in small steps
- Use the hood fan VFD or balance damper; decrease 5% at a time.
- After each step, measure:
- ΔP across a door (target roughly −5 to −10 Pa in normal operation),
- O₂ at the breathing zone (≥ 20.5% under load),
- Flame quality (sharp blue cones, no lifting), and
- Capture/containment (quick smoke-pen/tissue check at hood edges).
- Stop at the first pass point
- If ΔP and O₂ are back in range and capture looks good, lock settings and log readings.
- If capture suffers (smoke spills), roll back one step and escalate to proper air-balance work (increase MUA capacity, fix duct restrictions, add transfer air, etc.).
- Re-verify under full cooking
- Repeat checks with appliances fully firing and the hood on its normal high setting.
- Log O₂, ΔP, any CO if you have a meter, and acceptance.
Quick cues for the team
- Too negative (doors hard to open, flames lean/yellow near hood) → first choice: more MUA; last resort: slightly less exhaust.
- Not negative enough / positive (odors escape) → increase exhaust or reduce MUA (but keep burner stability and O₂ OK).