- Every cubic foot of air a kitchen exhaust fan removes must be replaced. When make-up air is short, the building pulls it through doors, cracks, and flues, and comfort, smells, and equipment performance all suffer.
- Most kitchen complaints that look like HVAC failures (hot line, cold dining room, slamming doors, smoky smell) are airflow-balance problems.
- Make-up air units that heat or cool incoming air are a major energy load. Tempering strategy, controls, and filter maintenance matter as much as the unit itself.
- Hood cleaning and grease duct cleaning are a separate, specialized trade covered by fire-safety standards such as NFPA 96. The mechanical side keeps the fans, belts, and air balance healthy, and works alongside that trade.
- A measured airflow and pressure survey beats guessing. It often shows that a simple adjustment or belt change resolves a problem that looked like an equipment failure.
The one rule that explains most kitchen problems
A commercial kitchen hood is a big air pump. A typical exhaust fan moves thousands of cubic feet of air per minute out of the building, taking heat, grease-laden vapor, and combustion byproducts with it. That air does not vanish from the building's inventory. It has to be replaced by air coming in from somewhere, and the only question is whether the building gets that air through a designed path or through whatever openings it can find.
When replacement air is short, the kitchen runs at negative pressure relative to the dining room and outdoors. Doors become hard to open or slam shut. Air is drawn from the dining room into the kitchen, carrying cooled air with it. Gas appliances may spill flue gases or burn poorly. The hood may fail to capture smoke at the edges because the airflow pattern at the hood face is disturbed by drafts. None of this is caused by a broken fan; the fan is working very well.
What makes up the system
- Exhaust hood and plenum: Captures and contains heat and effluent. Type I hoods handle grease-producing appliances; Type II hoods handle heat and steam only.
- Exhaust fan (upblast or utility set): Usually on the roof, often belt-driven. Belt wear, bearing failure, and grease buildup on the wheel all reduce airflow quietly.
- Grease duct: The path from hood to fan. Its cleaning and access requirements are governed by fire-safety codes and standards; this is handled by hood-cleaning specialists, not HVAC technicians.
- Make-up air unit (MAU): Brings outdoor air into the building to replace what the hood removes. May be direct-fired gas, indirect-fired, electric, or an unheated fan, and some include cooling.
- Short-circuit or compensating hood supply: Delivers make-up air directly at the hood, so conditioned dining air is not drawn into the exhaust stream.
- Interlocks and controls: Tie the make-up air to the exhaust so one cannot run without the other. Variable-speed demand control systems modulate both based on cooking activity.
- Dining room and kitchen HVAC: Rooftop or split systems that must handle whatever loads the ventilation system leaves behind.
Reading the symptoms
| What staff notice | Likely airflow cause | What a technician checks |
|---|---|---|
| Cook line is unbearably hot | Hood not capturing; make-up air arriving too warm or at the wrong location | Hood capture at the face, exhaust cfm vs design, discharge location of supply air |
| Dining room is cold or drafty near the kitchen | Dining air being pulled into the kitchen as make-up | Pressure difference across the pass-through, MAU output, door behavior |
| Back door slams or is hard to open | Negative building pressure | Static pressure across the door, MAU operation, exhaust fan speed |
| Smoke or odors drift into dining | Hood not capturing, or building pressure pulling air toward the dining room | Hood fan speed, belts, grease filters, MAU run status |
| Gas appliances light poorly or soot | Combustion air starved by negative pressure | Pressure at appliances, flue draft, burner condition |
| MAU trips or lockouts | Dirty filter, failed burner components, sensor faults, low gas pressure | Filter condition, safeties, ignition sequence, controls |
| Exhaust fan noisy or vibrating | Belt or bearing wear, wheel buildup, loose mounting | Vibration, belt tension, wheel condition (with fire-safety cleaning scope handled separately) |
Pressure balance is a design target, not an accident
The usual design goal is a kitchen that sits slightly negative relative to the dining room, so that cooking odors stay in the kitchen, but not so negative that doors and appliances misbehave. Building codes and design standards describe how much make-up air is expected and how it may be supplied; the exact figures depend on code edition, jurisdiction, and the appliances under the hood, so use your design documents and confirm current requirements with the local authority.
Balance drifts over time. Menus change, a fryer line is added under a hood, a fan belt stretches, a MAU filter loads up, a tenant adjacent to the restaurant modifies its own system. The result is a space that worked at opening and no longer does. A measured survey is the only reliable way to tell: exhaust airflow at each hood, supply airflow at each MAU and diffuser, and static pressure readings between the kitchen, dining room, and outdoors.
If the kitchen feels wrong, ask for airflow and pressure readings before authorizing a new rooftop unit or make-up air unit. Many problems resolve with a belt and pulley change, a cleaned coil, a re-set damper, or a corrected interlock.
Make-up air options and what they cost you in comfort and energy
Outdoor air in Southern California is mild for much of the year, but a kitchen can move so much of it that even modest temperature differences add up. Tempering the air before it enters the building affects how comfortable the line is and how hard the other systems work.
| Approach | Strength | Watch for |
|---|---|---|
| Untempered make-up air | Lowest equipment complexity | Cold or hot air hitting the cook line; dining-room HVAC compensating |
| Direct-fired gas heating | Efficient for heating a large airflow | Burner maintenance, combustion safeties, filter condition, local air-quality rules |
| Indirect-fired or electric heat | Cleaner air path, more control | Higher operating cost at large airflows depending on utility rates |
| Evaporative or DX-cooled make-up | Improves cook-line comfort in warm months | Water treatment, coil maintenance, added electrical load |
| Demand-controlled kitchen ventilation | Reduces airflow when cooking is light | Sensor calibration, programming, and bypass behavior when it faults |
Demand-controlled ventilation, which slows exhaust and make-up fans when sensors detect low cooking activity, is one of the more effective ways to cut energy in a high-volume kitchen. It also adds sensors and drives that must be maintained. Current energy-code provisions address kitchen exhaust in some situations, so check which apply when you plan a hood or fan replacement. Our Title 24 and permits guide covers the general permit process.
Where HVAC work ends and hood-cleaning work begins
It helps to be exact about trades here. Cleaning hoods, filters, and grease ducts to meet fire-safety standards such as NFPA 96, and maintaining the related records, is specialized work. Your insurer, fire marshal, or local jurisdiction may have specific expectations about frequency and documentation, so confirm those requirements with them. Davinci Mechanical does not perform hood or duct cleaning and does not substitute for a certified hood-cleaning vendor.
What the mechanical side does contribute: keeping the exhaust fan, belts, motor, and make-up air units healthy; verifying that interlocks work; confirming airflow and pressure; and flagging conditions the cleaning vendor should know about, such as access panels that are missing or a fan with no hinge kit. Coordination between the two vendors prevents the awkward finding that a fan was serviced but could not be reached for duct cleaning, or the reverse. See the kitchen ventilation and make-up air service page for scope.
Habits that keep the system balanced
- Check make-up air filters on a schedule that matches how greasy and dusty the kitchen is. A loaded filter cuts airflow and quietly unbalances the room.
- Inspect exhaust fan belts and bearings regularly. Replace belts in sets and keep the tension correct.
- Verify the exhaust-MAU interlock any time either unit is serviced or replaced.
- Re-check balance after any change: new appliance, menu expansion, layout change, or adjacent tenant work.
- Record airflow and pressure baselines so that drift can be spotted over time rather than by feel.
- Keep the dining room HVAC maintained. A struggling rooftop in the dining room can look like a kitchen balance problem, and the reverse is also true.
If your restaurant also runs walk-in refrigeration and ice machines, put the whole building on one calendar. Our restaurant annual checklist shows how those pieces fit together across the year.
Frequently asked questions
Why does my restaurant's back door slam shut?
Usually because the exhaust hood is removing more air than the building is receiving, creating negative pressure. A make-up air unit that is off, undersized, or starved by a dirty filter is a common cause. A technician can confirm with a pressure reading.
Does Davinci Mechanical clean hoods and grease ducts?
No. Hood and grease-duct cleaning is a separate specialized trade tied to fire-safety standards. We service the exhaust fans, make-up air units, and HVAC that work with it, and we coordinate with your cleaning vendor.
Can make-up air be cooled?
Yes, in several ways, including evaporative and refrigerant-based cooling. It improves cook-line comfort in warm weather but adds equipment to maintain and energy to pay for. It is worth evaluating against the actual conditions in your kitchen.
How do I know if my kitchen is out of balance?
Typical signs are hard-to-open doors, drafts at the pass, smoke or odors reaching the dining room, and hot cook lines. An airflow and static pressure survey confirms it and shows which side of the balance is off.
Will a new hood or appliance change the requirements?
It can. Adding or changing equipment under a hood changes the required exhaust and make-up air and may require permits and plan review. Confirm requirements with your designer and the local building department before ordering.
Have a site this applies to?
Davinci Mechanical is the commercial and union division of Scottish Tom's Heating & Air. Send us the equipment list or the problem and we'll tell you what we'd check first.