Venting a Commercial Kitchen Hood

The Room Is the First Component

An exhaust fan does not capture grease. It creates a deficit, and whatever fills that deficit decides whether the plume climbs into the riser or rolls across the pass. Sixteen hundred cfm leaving a kitchen is sixteen hundred cfm entering it somewhere — through a tempered make-up air unit if the design paid for one, through the dining room and the front door if it did not, and through the draft hood of an atmospheric water heater once every easier path is exhausted.

Walk the space before sizing anything. Count every other extract path already bidding for the same replacement air: dish machine hood, restroom and janitor exhaust, a Type II condensate hood over the steamer, walk-in condensers rejecting heat into the room, any appliance flue that draws on its own. Then find the pressure boundary. A kitchen sealed behind doors behaves differently from one open across a six-foot pass, and that opening is a supply path whether or not the drawings call it one.

Depressurisation is measurable on day one. Run a reference tube from a digital manometer through the wall to outdoors, switch on whatever exhaust already exists, and read the room. Numbers gathered before design settle arguments later, particularly on a tenant fit-out where the landlord's rooftop unit is the only outside air on site and its capacity was fixed years ago by somebody else.

Duty, Style and Overhang Set the Number

Exhaust volume follows the appliances, not the room. Mechanical codes classify cooking equipment by duty — light, medium, heavy and extra-heavy or solid fuel — and publish minimum net exhaust rates in cfm per linear foot of hood, differentiated by hood style. Wall-mounted canopy, single-island, double-island, backshelf and eyebrow hoods all draw different numbers for the same appliance line, because each one gives the thermal plume a different amount of help.

Measure the line, not the equipment list. Hood length has to cover the appliances plus the overhang the code of record requires on open ends and the front, commonly six inches, and the hood has to sit low enough that the capture volume actually contains the plume. Every inch of extra mounting height above the cooking surface widens the plume before it reaches the reservoir and pushes the required volume up. Charbroilers and wok ranges are the appliances that punish a short hood; a fryer battery is more forgiving.

Listed hoods complicate the arithmetic in a useful direction. A hood tested to UL 710, Exhaust Hoods for Commercial Cooking Equipment, may carry a manufacturer's rating below the prescriptive code table, and many jurisdictions accept the listing. Take the reduction only where the listing conditions — appliance type, overhang, mounting height, end panels — are reproduced on site. Solid-fuel appliances get their own dedicated exhaust system and their own extinguishing arrangement; do not merge them into the main riser.

Budgeting the Replacement Air

Codes generally require replacement air at a rate approximately equal to the exhaust, less whatever the authority permits to transfer in from adjacent conditioned space. That allowance is the part people abuse. Transfer air is not free air — every cfm pulled from the dining room has to be made up by the rooftop unit serving the dining room, and if that unit's outside-air damper was sized for occupancy ventilation under ASHRAE Standard 62.1, it has no reserve to donate.

Set a pressure target rather than a percentage. A kitchen that sits slightly negative to the dining room keeps odour and grease-laden vapour out of the front of house; a building that sits slightly positive to outdoors keeps infiltration, insects and door problems under control. Designers commonly aim for a few hundredths of an inch of water column negative in the kitchen relative to the space next door, and the make-up air unit is sized to land there — typically somewhere between eighty and ninety-five percent of exhaust delivered mechanically, with the balance transferred deliberately through sized openings.

Two constraints narrow the budget further. Energy codes such as the IECC and ANSI/ASHRAE/IES Standard 90.1 impose demand control kitchen ventilation or make-up air tempering limits above defined exhaust thresholds, which changes what the unit has to do at part load. And heat recovery has no place in the grease stream: the exhaust is loaded with condensable grease and cannot pass through a recovery core. Recovery, if it appears at all, belongs on the non-grease exhaust in the same building.

Delivering Make-Up Air Without Killing Capture

Where the replacement air enters matters as much as how much of it there is. Capture degrades sharply once cross-drafts at the hood face exceed roughly fifty feet per minute, and a standard four-way ceiling diffuser placed near the line will produce far more than that at head height. A make-up air system correctly sized and badly delivered performs worse than an undersized one delivered gently, because it actively strips the plume off the appliance before the hood ever sees it.

Perforated perimeter plenums built into the hood face, low-velocity displacement registers along the back or side walls, and large-face ceiling panels sized for terminal velocities in the tens of feet per minute all work. The intent stays constant: flood the aisle behind and beside the cook, not the reservoir. Aim throw patterns away from the hood, keep supply outlets out of the plane of the hood opening, and treat any diffuser within a few feet of the line as suspect until it is measured.

Short-circuit hoods with internal untempered supply are a category to check against local rules before drawing one, since a number of jurisdictions restrict or prohibit them and their capture margin is thin. End panels and side skirts, by contrast, are cheap physics — they shield the reservoir from room air and let a hood hold capture at a lower exhaust rate.

Comfort has a hard edge here too. Dumping cold outside air on a cook line in January is how the make-up air unit gets switched off at the disconnect, and once that happens the kitchen goes negative and stays negative until somebody with a manometer works out why the back door needs a shoulder.

Tempering, and the Season That Finds You Out

Untempered make-up air is defensible in a narrow band of climates and a narrow band of months. Everywhere else the unit needs heating capacity to lift outdoor design temperature to a discharge setpoint the staff will tolerate — often in the mid-fifties to mid-sixties Fahrenheit — plus freeze protection on any hydronic coil and a low-limit that shuts the unit down before a coil splits.

Direct gas-fired units burn into the airstream and are efficient and compact, but they are listed for specific applications and carry combustion products into the space, so confirm the listing, the required minimum airflow through the burner, and the interlock arrangement. Indirect-fired units avoid that at the cost of efficiency and footprint. Either way, the burner has to modulate; a single-stage burner cycling against a fixed 100 percent outside air stream will swing discharge temperature far enough to be felt at the range.

Summer brings the quieter failure. Warm humid make-up air delivered near a walk-in box or a cold water line condenses on every surface below dewpoint, and the puddles get blamed on the refrigeration contractor. In dry climates, direct or indirect evaporative cooling on the make-up air stream carries real load at low energy cost; in humid ones it does not, and mechanical cooling or simple neutral-air delivery is the honest answer.

The Grease Duct as a Pressure Path

Every fitting between the hood collar and the fan is static pressure the make-up air budget has to overcome. Grease duct is unforgiving about this because its geometry is constrained by fire rules long before it is constrained by aerodynamics: continuous liquid-tight external welds on shop-fabricated duct, no dampers or obstructions in the airstream, clearance to combustibles of eighteen inches unless the assembly is listed for less, and enclosure or listed factory-built duct where the run passes through other spaces, evidenced against UL 1978, Grease Ducts, and UL 2221, Tests of Fire Resistive Grease Duct Enclosure Assemblies.

Velocity carries two competing duties. Fast enough that grease and condensate stay entrained and moving; slow enough that friction loss does not eat the fan's available static. NFPA 96 and the adopted mechanical code set the floor, and the number has moved between editions — older design practice worked around fifteen hundred to eighteen hundred feet per minute, while current text sets a substantially lower minimum. Confirm which edition the jurisdiction has adopted rather than repeating a number from memory.

Slope, access and drainage decide what the system looks like after two years of service. Horizontal runs pitch back toward the hood or an approved reservoir at the code minimum, steeper on long runs; cleanout openings go at every change of direction and at the spacing NFPA 96 requires, positioned where a cleaning crew can actually reach them with the ceiling in place. A cleanout above a fixed ceiling with no access panel is a duct that will not be cleaned.

Support is structural, not incidental. Hanger rod and strap spacing on a grease duct has to account for the duct's own weight plus accumulated grease plus the enclosure, and hangers must not compromise the required clearance or the fire-rated wrap.

Fan, Riser and Discharge

Upblast centrifugal roof fans for this service are listed to UL 762, Power Roof Ventilators for Restaurant Exhaust Appliances, and the listing covers the parts that get overlooked: a hinge kit so the housing swings clear for duct cleaning, a drain to a grease containment device, and a windband that throws the discharge clear of the roof. Utility-set and in-line fans exist for specific geometries but complicate cleaning access and grease drainage.

Discharge location is a code question with neighbours attached. Requirements commonly place the outlet at least forty inches above the roof surface and ten feet horizontally from air intakes, operable openings and property lines, with a vertical offset permitted where that horizontal distance cannot be achieved — verify against the adopted mechanical code and NFPA 96 edition. Discharge velocity high enough to loft the plume above the roof boundary layer is what keeps a complaint call from turning into a relocation.

Select the fan on the calculated system curve, not on the hood tag. Grease duct static climbs as the system loads with residue, so leaving margin — and specifying an adjustable drive or a variable frequency drive — is what allows a balancer to hit the design number instead of arguing about it. Confirm the roof curb is level, insulated, and that the structure below carries the load; a fan mounted on a soft deck telegraphs vibration into the dining room ceiling.

Interlocks and the Balance That Proves It

Exhaust and make-up air must start and stop together. An electrical interlock between the hood control panel and the make-up air unit is the minimum, and the fire suppression system — tested to UL 300, Fire Testing of Fire Extinguishing Systems for Protection of Commercial Cooking Equipment — has to shut the fuel and, on activation, shut down the make-up air supply while the exhaust fan continues per the listing. Where demand control kitchen ventilation is required, the exhaust and supply speeds have to track each other so the pressure relationship survives at every turndown point.

Balance in sequence, and document each step. Nothing about capture can be judged while the supply side is still open-loop.

A commissioning file with traverse data, pressure readings and a dated capture test is what protects the installer when the operator adds a charbroiler in year two and the kitchen goes negative again.

1. Verify the fire suppression interlocks, gas valve shutoff and make-up air shutdown before any airflow work begins.
2. Set exhaust to the design volume first, by pitot traverse in the riser or manufacturer's flow station — not by hood-face readings alone.
3. Bring the make-up air unit up in stages, traversing supply outlets and checking discharge temperature at each stage.
4. Read kitchen-to-dining and building-to-outdoor pressure with a manometer at each stage and adjust transfer openings to land on the target.
5. Run a capture test at full cooking load with a smoke source along the appliance line, watching the front lip and both open ends.
6. Measure cross-draft velocity at the hood face and at the cook's shoulder; retune diffuser throw if it exceeds the capture threshold.
7. Check exterior door opening force and every atmospheric appliance draft hood for spillage with all exhaust running.

Reading the Failure Signatures

Most callbacks on kitchen ventilation are depressurisation wearing a costume. The complaint arrives as smoke, or odour in the dining room, or a pilot that will not stay lit, and the instinct is to speed up the exhaust fan — which makes every one of those symptoms worse. Diagnose the supply side first, every time.

Carbon monoxide risk deserves separate attention. Any atmospherically vented appliance sharing the pressure boundary with the kitchen — water heater, boiler, older make-up air unit — can spill combustion products once the room goes sufficiently negative, and the National Fuel Gas Code governs the venting arrangement that this condition defeats. Test draft with everything running, including the dish machine hood and restroom fans, not with the kitchen at rest.

Seasonal complaints carry information. A system that captures fine in October and fails in January usually has a make-up air unit that staff have disabled, or an operator propping the back door open for relief. A system that fails only during a full service is short on volume at the appliances that matter, not short overall.