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Commercial · Kitchen Exhaust & Make-Up Air BalancingKitchen Exhaust & Make-Up Air Balancing
Kitchen exhaust and make-up air balancing is the fix for a restaurant under negative air pressure, where the hood throws more air out than the building brings back in. The working target is a make-up air unit delivering roughly 80–90% of hood exhaust, with the remainder drawn from the dining room so cooking odour stays in the kitchen. If your gas water heater is back-drafting or a carbon monoxide alarm has sounded, that is not a comfort problem: shut the appliance off and call the same day.
A hood removes air whether or not anything replaces it. When the make-up air falls short, the building makes up the difference through every opening it has: the front door becomes hard to pull open and whistles at the weatherstrip, cold drafts cross the dining room toward the kitchen, and smoke rolls out past the front edge of the hood because the fan cannot move its rated air against the vacuum. Pilot lights on ranges and water heaters blow out. The dangerous version is a natural-draft water heater or furnace whose flue reverses and spills combustion gases, carbon monoxide included, into the building; the warning signs are listed in how to tell if a water heater is back-drafting.
The causes we find on site repeat. The make-up air unit was switched off in winter because it blew cold air, which usually means its burner had failed; the supply fan belt is slipping or broken; the intake filters are plugged solid with grease and cottonwood fluff; or the exhaust fan was upsized for a new char-broiler and nobody touched the supply side. Less often the unit runs but its damper is seized shut or the interlock was bypassed. Each has a different price, which is why we measure before quoting, and equipment repair or replacement is detailed on the make-up air unit page.
Three rules frame the repair. NFPA 96, the standard the Ontario Fire Code uses for commercial cooking exhaust, calls for enough replacement air that negative pressure in the cooking area does not exceed about 5 Pa (0.02 inches of water column). CSA B149.1 requires an adequate air supply for every gas appliance and does not allow exhaust equipment to starve it, which in practice means the make-up air unit is interlocked to start with the hood fan. Design practice then sets make-up air at 80–90% of exhaust, leaving the kitchen slightly negative to the dining room and the building close to neutral against outdoors.
A balance visit ends with numbers on paper: exhaust airflow at each hood, make-up airflow at the unit, building pressure at the front door with everything running, and a draft test on each vented gas appliance. That measured airflow report is what a landlord, an insurer or a fire inspector asks to see, and it tells the next technician what normal looked like. A worn fan is handled under commercial exhaust fan repair, and the wider kitchen picture is on restaurant HVAC. The hub is commercial HVAC; to book a measurement, call or message the shop.
Equipment we install and service
The upblast fan on the roof that sets how much air leaves. We read its airflow at the hood, check rotation, belt tension, sheave size and wheel condition, and compare the result with the hood's listed exhaust rate.
The gas-fired or untempered supply unit that puts the air back. Burner, flame safeguard, discharge temperature control, filters, belts and the intake damper are tested, and delivered airflow is measured instead of read off the nameplate.
The wiring that starts the make-up air unit with the hood fan and proves airflow before the burner fires. Bypassed interlocks, failed airflow switches and summer-winter switches left in the wrong position are common finds.
Water heaters, furnaces and unit heaters with natural-draft vents that share the building's air. Each gets a spillage test at the draft hood with the exhaust running, because that is where negative pressure becomes carbon monoxide.
What goes wrong in these buildings
- The front door takes a real pull to open and slams shut behind every customer.
- Smoke hangs under the hood and drifts into the pass when the grill is full.
- The water heater pilot is out again and the dish pit has no hot water.
- Tables near the entrance complain about a cold draft on their ankles all winter.
- Somebody turned the make-up air off in December because it was blowing cold and nobody turned it back on.
- We put in a bigger hood last year and the problems started the same month.
How it works
Hood fans, make-up air, dish exhaust and washroom fans on, doors closed. We record exhaust and supply airflow, building pressure at the door and draft at each gas vent, so the shortfall is a number.
A failed burner, belt, filter bank, damper or interlock is fixed first, since a negative kitchen usually has equipment that already exists and is not working. Added capacity is quoted only if the repaired system still falls short.
Fan speeds and dampers are adjusted to bring make-up air to roughly 80–90% of exhaust, then every reading is taken again. You receive the before and after figures in writing.
Common questions
Is a negative kitchen dangerous or just uncomfortable?
Both are possible. Hard doors and drafts are comfort and energy problems. A back-drafting water heater or furnace is a carbon monoxide hazard and needs attention the same day: shut the appliance off, ventilate, and get out if a CO alarm sounds. If pilots are blowing out, assume the vents are affected too until someone has tested them with the hood running.
Why not just leave the back door open?
It relieves the pressure and creates three new problems: unfiltered, untempered air on the cook line, pests, and a cross-draft that pushes smoke out of the hood's capture zone. In winter it also freezes the kitchen. An open door is a sign the make-up air is short, and the measured shortfall tells us whether a repair or a larger unit is needed.
What does a kitchen air balance and airflow report cost?
It depends on the number of hoods and fans, roof access and whether repairs are needed before the system can be balanced at all. We do not publish a flat rate for it. After a first look you get a written quote that separates the measurement and report from any repair, so you can approve them one at a time.
Should make-up air equal 100% of exhaust?
Not usually. Supplying slightly less through the make-up air unit, roughly 80–90%, lets the last portion transfer in from the dining room, so air moves from the seats toward the kitchen and takes odour and heat with it. The dining-room rooftop units supply that transfer air as part of their outdoor-air intake. The building as a whole should end up near neutral.
Related
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One licensed team for installs, maintenance contracts and 2 AM calls.