Commercial kitchens are designed to operate at slight negative pressure — air flows from the dining room into the kitchen, keeping cooking smells and grease out of the front of house. But there's a difference between slight negative pressure (by design) and severe negative pressure (a problem).
What Causes Severe Negative Pressure?
The exhaust hood pulls air out. The makeup air unit pushes tempered air in. The difference is the infiltration — outside air that enters through doors, gaps, and openings. This is normal.
The problem happens when the infiltration becomes excessive:
- MAU not working — all replacement air comes from infiltration
- MAU undersized — was designed for an older, smaller hood
- MAU running at reduced capacity — dirty filters, worn belt, failed burner
- Exhaust increased — new hood added or existing fan speed increased without matching supply
- Building sealed too tight — renovation closed off natural infiltration paths
The Real Cost of Excessive Negative Pressure
A kitchen with a 2,750 CFM exhaust hood and a dead MAU is pulling 2,200 CFM of raw outside air through every crack in the building. Here's what that costs in Oklahoma City, OK:
| Season | Outside Air Temp | Building HVAC Cost |
|---|---|---|
| Winter (7 months) | 35°F avg | $4,200/year in gas |
| Summer (4 months) | 90°F avg | $1,060/year in electric |
| **Annual waste** | **$5,260/year** |
That's just energy. Add the operational costs:
- Staff discomfort — cooks in a freezing kitchen in winter, sweltering in summer
- Equipment stress — pilot lights blowing out, gas equipment cycling erratically
- Customer impact — cold drafts from the front door, uncomfortable dining room
- Door damage — the constant suction warps door frames and wears out closers
How to Test for Negative Pressure
The door test: If your front door is noticeably hard to open (pulls toward you when you try to push it open from outside), you have negative pressure. The harder it is, the worse the imbalance.
The smoke test: Light a smoke pencil near the door threshold. If the smoke gets sucked in forcefully, the kitchen is under significant negative pressure.
The professional test: A manometer reading of the kitchen vs. the dining room quantifies the pressure differential. NFPA 96 says negative pressure should not exceed 0.02 inches water gauge.
How to Fix It
1. Check the MAU first — is it running? Is it delivering air? Change the filters.
2. Measure the actual CFM — use a VelGrid or anemometer to verify exhaust and supply volumes.
3. Balance the system — adjust dampers, fan speeds, and MAU settings to achieve 80-90% supply-to-exhaust ratio.
4. If the MAU is undersized — it may need to be replaced with a properly sized unit.
A one-time air balance service typically resolves the problem permanently — and it pays for itself in energy savings within a few months. Contact us for a quote.
True Commercial Service diagnoses and fixes kitchen negative pressure problems for restaurants and commercial kitchens across Oklahoma City, OK. One visit, real measurements, permanent fix.
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