Restaurant Hood Losing Capture With a Makeup Air Shortfall

Exhaust volume alone does not capture; capture depends on the relationship between the thermal plume, the hood's effective capture volume, and the air motion around it.

Quantity: 4,200 cfm exhaust minus 3,000 cfm makeup leaves 1,200 cfm that must enter through the envelope - doors, windows, and every crack. That is why the doors are hard to open (the 20 × 20 in. transfer grille passes 1,200 cfm at roughly 430 fpm, which also explains the whistle) and why the building sits at −0.12 in. w.c.

Mechanism: makeup air is delivered from diffusers directly in front of and above the hood, so it enters the hood's capture zone at high velocity and disrupts the plume before it can be captured. Instead of a stable rising plume, the hood sees a cross-draft. The untempered supply also creates a cold, dense curtain in winter that pushes the plume sideways. The manufacturer's minimum capture velocity is measured at the hood's lower edge, and it is being defeated by air motion the hood was never designed to overcome.

So the rated exhaust is being satisfied at the fan but not at the hood face: part of the exhaust is short-circuited makeup air, and part is the infiltration path that also carries dining-room air and odors back toward the kitchen.

The building at −0.12 in. w.c. with natural-draft appliances sharing a flue is a carbon monoxide risk, and this takes priority over comfort.

Immediate actions:

1. Test draft at each appliance's draft hood with a smoke pencil or draft gauge while the exhaust fans and any other exhaust devices are running, including bathroom and dryer exhaust. Any spillage or downdraft is a fail.
2. If spillage is present, shut off the affected appliances, ventilate the space, notify the owner in writing, and do not restore operation until combustion air is corrected. Confirm CO alarms are present on every level and in the sleeping areas if the building has any.
3. Do not solve this by adding exhaust or a larger makeup unit. The fix is a dedicated combustion air path: two permanent openings sized per code or direct ducted combustion air to each appliance, terminating where it cannot be blocked by snow or drawn from the negative-pressure zone.
4. Measure CO in the flue gas and in the mechanical room after correction, and record the readings.

Separately, verify that the exhaust and makeup air are interlocked so that the makeup air cannot run without the exhaust and vice versa; a fan failure must not silently depressurize the space.

Targets: makeup air at 85–90% of exhaust, building at about −0.02 in. w.c. (slightly negative, never positive into the kitchen), and transfer airflow sized for low velocity.

1. Makeup air volume. Increase the unit to roughly 3,600–3,800 cfm, or reduce the hood to the minimum listed rate (for example 300 cfm/ft × 12 ft = 3,600 cfm) so exhaust and makeup are matched. Confirm the hood listing for the cooking duty; heavy-duty appliances may require more.
2. Makeup air location. Move the supply diffusers out of the hood's capture zone, delivering air at low velocity (well under 75 fpm in the occupied zone and not aimed at the hood face) and preferably at the perimeter or at the ceiling away from the hood.
3. Tempering. Add heat to the makeup unit so winter supply is at least in the low 60s; untempered air is both uncomfortable and destabilizing to the plume.
4. Transfer path. Provide at least 1,200 cfm of low-velocity transfer between dining and kitchen - two or three grilles rather than one - sized for about 400–500 fpm or less.
5. Combustion air. Add the dedicated path described above before any balancing.
6. Balancing. Set exhaust first, then makeup, then verify building pressure with a manometer at a stable reference. Confirm hood capture with a smoke test at the hood edge and at the cooking surface, and re-check capture with the doors open and closed.
7. Document capture, pressures, and airflow, and add an interlock and a schedule so future fan changes do not recreate the imbalance.