The Complete Path of a Commercial Kitchen Exhaust System
A commercial kitchen exhaust system is a continuous, enclosed path that starts at the hood collar and ends where the air leaves the building. In between, the grease-laden air passes through welded grease duct, possibly a fire rated shaft, and an exhaust fan rated for restaurant service.
| Segment | What happens there | Design concern |
|---|---|---|
| Hood and filters | Plume is captured; filters remove part of the grease | Filter pressure drop, collar size |
| Duct collar and riser | Air accelerates into the duct | Velocity and transition losses |
| Horizontal runs | Air travels to a shaft or exterior wall | Slope back toward the hood, cleanout access |
| Shaft or enclosure | Duct passes through floors and rated assemblies | Fire rating, clearances, inspection |
| Exhaust fan | Moves the air against total system resistance | CFM, static pressure, UL 762 listing |
| Discharge | Air leaves the building | Distance from intakes, openings, roof surface |
Each segment affects the others. A long horizontal run raises static pressure, which changes the fan selection, which can change the roof curb and the electrical service. That is why the exhaust system should be laid out before the fan is ordered.
Design Velocity in the Exhaust Duct
Duct velocity is the speed of air inside the grease duct, and it decides both how well grease stays airborne and how much resistance the fan sees. The figure commonly referenced as a minimum in grease duct is 500 fpm, and many designers work in a range around 1,500 to 1,800 fpm. Confirm the values in the edition adopted locally.
Why both limits matter
Too slow, and grease droplets fall out of the airstream and coat the duct walls, which increases fire risk and cleaning frequency. Too fast, and the pressure drop climbs steeply, the fan gets louder and draws more power, and noise can carry into the kitchen.
Velocity also matters when airflow is variable. If a demand-controlled system turns exhaust down at idle, the duct should still stay above the minimum velocity at the lowest airflow. That often limits how far the system can be turned down, or calls for a smaller duct than a fixed-speed design would use.
Duct size is chosen from the design CFM and target velocity. A simple check: divide the airflow in CFM by the duct cross-sectional area in square feet to get velocity in fpm.
Routing the Exhaust: Shafts, Horizontal Runs and Access
The best exhaust route is the shortest one with the fewest fittings that still meets clearance and access requirements. A vertical riser straight from the hood collar to a rooftop fan is the ideal; real buildings rarely allow it.
- Horizontal runs are sloped so grease drains back toward the hood or to a grease reservoir rather than pooling. Long runs get a steeper slope.
- Cleanout openings are placed at changes of direction and at intervals along horizontal runs so every part of the duct can be cleaned.
- Clearance to combustibles is commonly 18 inches for unprotected grease duct, reducible with listed wraps or enclosures.
- Fire rated shafts are typically required where the duct passes through floors or rated ceilings; listed duct wraps may substitute where the AHJ accepts them.
- Separate systems: grease duct does not connect to other exhaust systems, and is kept apart from Type II and general building exhaust.
Construction details of the duct itself (gauges, welds, joints, factory-built options) are covered on our grease duct system page. On this page the concern is the route: how long, how many turns and where it exits.
Static Pressure: Adding Up the Resistance
Static pressure is the total resistance the exhaust fan must overcome to move the design airflow, measured in inches of water column. It is the sum of losses through the filters, hood, collar, duct, fittings and discharge.
| Source of resistance | What increases it |
|---|---|
| Hood and grease filters | Higher airflow per filter, loaded filters between cleanings |
| Straight duct | Longer runs, smaller duct, higher velocity |
| Fittings | Tight elbows, abrupt transitions, offsets |
| Discharge | Wall louvers, extended stacks, rain hoods |
The calculation is done for the longest or most restrictive path in the system. Designers usually allow for loaded filters so the fan still delivers when the filters are due for cleaning, not only on the day they are new. Selecting the fan from CFM alone is one of the most common causes of a kitchen that never quite clears its smoke. See commercial kitchen exhaust fans for how the fan is chosen once the static pressure is known.
Rooftop vs. Wall Discharge
Most commercial kitchen exhaust systems discharge through the roof, because that puts smoke and grease above people, windows and air intakes. Wall discharge is allowed under NFPA 96 and the mechanical code under specific conditions, and it can solve problems in multi-story buildings where a roof route is impractical.
Rooftop discharge
An upblast fan on a curb throws the air vertically. The outlet is commonly required to sit at least 40 inches above the roof surface, and the fan has to be located away from air intakes, property lines and other buildings by the distances in the code. The roof around the fan should be protected from grease, typically with a grease containment system.
Wall discharge
A wall termination uses a fan whose listing covers that installation, commonly a utility set fan discharging horizontally. The code sets distances from property lines, adjacent buildings, operable windows, doors, intakes and grade. Because the discharge is closer to people and neighbors, odor and grease staining on the facade are practical concerns as well as code ones.
Your AHJ will review discharge location carefully, so show distances on the drawings. If the roof route is blocked and the wall route does not meet distances, a pollution control unit or a longer duct run may be the remaining options.
Controls and Demand-Controlled Kitchen Ventilation
Exhaust system controls make sure the fan runs when cooking happens and that the system responds correctly in a fire. At minimum, the exhaust fan is interlocked with the make-up air unit and coordinated with the fire suppression system.
Demand-controlled kitchen ventilation (DCKV)
DCKV varies exhaust and make-up air with cooking activity instead of running at full speed all day. Sensors in the hood, commonly temperature sensors in the collar and sometimes optical sensors that detect smoke or steam across the hood, signal variable frequency drives (VFDs) on the fans.
- At idle, airflow ramps down to a set minimum.
- When cooking starts, airflow ramps up to design rate.
- Make-up air tracks exhaust so the kitchen pressure relationship holds.
The savings come from moving, heating and cooling less air during long idle periods. The payback depends on operating hours, climate and how many hours the line sits hot but idle. Energy codes in some jurisdictions require demand control on larger kitchen exhaust systems, so check your local requirements early in design.
DCKV does not replace good capture. The minimum airflow still has to contain idle appliances, and the duct must stay above minimum velocity at the lowest speed.
Exhaust System Problems We Are Called to Fix
Most failing exhaust systems share a short list of root causes, and nearly all of them trace back to how the path was laid out. Diagnosing the system means checking it from the collar to the discharge, not just looking at the fan.
- Smoke rolling out of the hood: fan underdelivering against high static pressure, or make-up air disrupting capture.
- Grease dripping from the fan: missing or full grease containment, or low duct velocity letting grease settle.
- Odor complaints from neighbors: discharge too close to windows or intakes, or air re-entering the building.
- Loud system: duct velocity too high, fan running at the edge of its curve, or poor fan isolation.
- Fan cycling or tripping: motor overloaded because the system was balanced at the wrong point.
For ongoing cleaning and inspection schedules, read our kitchen exhaust cleaning and inspection guide. For a new system or a redesign, request a quote and include your floor plan and roof plan.
Kitchen Exhaust System Questions
- How do I know if my exhaust system is moving enough air?
- Watch the hood with appliances at cooking temperature. Smoke should rise into the hood and stay there, without rolling out of the front or ends. For a measured answer, a balancing technician can take readings at the hood or in the duct and compare them with the design airflow on the drawings or the hood label.
- Can kitchen exhaust go out a wall instead of the roof?
- Yes, in many cases. NFPA 96 and the mechanical code allow wall terminations when the discharge meets distance requirements from property lines, adjacent buildings, openings, intakes and grade, and the fan is suited to that use. The AHJ reviews wall discharge closely, so show all distances on your drawings.
- Is demand-controlled kitchen ventilation worth it?
- It usually pays off best in kitchens with long operating hours and many idle hours, such as hotels, hospitals and cafeterias, and in climates where make-up air is heated or cooled. In a small kitchen that cooks hard all day, savings are smaller. An energy estimate based on your hours and climate gives a clear answer.
- Why does the exhaust duct have to slope?
- Grease condenses as the exhaust air cools, and some of it will collect on the duct walls. Sloping horizontal runs lets that liquid drain back toward the hood or a reservoir where it can be removed, rather than pooling in a low spot where it becomes a fire hazard and a cleaning problem.
- Can I connect a new hood to my existing exhaust system?
- Only if the existing duct and fan can carry the added airflow at an acceptable velocity and static pressure, and the code allows the hoods to share a system. Adding a hood to a running system without recalculating usually starves one or both hoods. Have the system evaluated before planning on a connection.
- What should be shown on exhaust system drawings for permit?
- Hood locations and exhaust rates, duct size and route, shaft and clearance details, cleanout locations, fan make and model with CFM and static pressure, discharge location with distances to openings and property lines, and the make-up air unit. Your AHJ may ask for more, such as a static pressure calculation.