Commercial Makeup Air Toronto: Why Your Building's Doors Are Hard to Open
- 4 days ago
- 6 min read
Every fall we get the same call. A tenant can't pull the front door open. Papers blow off a reception desk every time someone walks in. A restaurant's pilot light keeps going out. A property manager assumes the door closer needs adjusting, or that the gas appliance is faulty.
Nine times out of ten, none of that is the actual problem. The building is under negative pressure — it's exhausting more air than it's bringing in, and the difference is being pulled through every door, window seal and flue in the envelope.
It's one of the most common issues we find in GTA commercial buildings, and one of the most consistently misdiagnosed. Here's what's happening and what to do about it before the heating season starts.
What negative pressure actually means
A commercial building should sit at a slightly positive pressure relative to the outdoors. That small cushion of pressure keeps outdoor air, dust, humidity and odours from being sucked in through cracks in the envelope.
Every building has air leaving it on purpose: washroom exhaust, kitchen hoods, dryer vents, welding or paint booth exhaust, dust collection, garage fans. That air has to be replaced. In a properly designed system it's replaced deliberately — by outdoor air brought in through the rooftop units, or by a dedicated makeup air unit (MUA) that tempers and delivers it.
That's what commercial makeup air Toronto buildings are designed to deliver: outdoor air, heated and introduced on purpose, in roughly the volume the exhaust removes.
When the exhaust side outpaces the supply side, the building goes negative. Physics doesn't negotiate. The air will come in anyway — just through the gaps, at whatever temperature it happens to be outside.
Seven signs your building is running negative
Doors are hard to pull open, or slam shut on their own. Exterior doors on the leeward side are usually the worst.
Whistling or howling at door frames, window seals, or elevator shafts.
Persistent cold drafts near entrances, even with working perimeter heat.
Pilot lights or ignition failures on gas-fired appliances — water heaters, unit heaters, boilers, MUA burners.
Odour migration — kitchen smells in the retail unit next door, garage exhaust in the lobby, washroom odours in corridors.
Heating costs climbing year over year with no equipment or occupancy change.
Snow or moisture tracking in around door frames and accumulating inside the threshold.
Any one of these on its own can have another explanation. Three or more together is a pressure problem until proven otherwise.
Why it gets dramatically worse in the fall
Two things change when the weather turns.
Stack effect kicks in. Warm indoor air is buoyant. In cold weather it rises and escapes through the top of the building, pulling replacement air in through the bottom. The bigger the indoor–outdoor temperature difference, and the taller the building, the stronger the pull. A building that was borderline in September becomes obviously negative in November.
Cold air is denser. The same volume of infiltrating air carries more mass and a much larger heating load when it's -15°C outside than when it's 15°C. The load on your makeup air heater climbs sharply — right when it's been sitting idle since spring.
That's why a problem that has existed all year suddenly generates complaints starting in October.
Commercial Makeup Air Toronto Property Managers Keep Getting Wrong
A tenant fit-out that nobody balanced. A new restaurant, dry cleaner, cannabis facility or nail salon goes into a unit and adds significant exhaust. The landlord's base building supply doesn't change. This is by far the most common cause we see in multi-tenant plazas and mixed-use buildings.
A makeup air unit that isn't running. MUA units get shut off after a nuisance trip, a failed burner or a high-gas-bill panic — and never get turned back on. The kitchen hood keeps running regardless.
Exhaust fans that were upgraded, supply fans that weren't. New hood, bigger fan, same intake.
Economizer dampers stuck closed. Actuator failure or a seized linkage on the outdoor air damper on an RTU cuts your fresh air supply without any obvious symptom at the unit itself.
Blocked or undersized outdoor air intakes. Louvres full of cottonwood fluff, screens clogged with debris, or intakes that were never sized for the current exhaust load.
Envelope retrofits. Ironically, sealing and weatherproofing a leaky building can turn a mild pressure imbalance into a serious one. The air was getting in — now it isn't.
What the Ontario Building Code actually requires
This is the part most building owners don't realize: makeup air isn't optional, and it isn't just good engineering practice. Part 6 of the Ontario Building Code sets three requirements for ventilating systems that exhaust air outdoors:
Sufficient makeup air must be provided so that the operation of the exhaust system, other exhaust equipment, and combustion equipment is not adversely affected.
Makeup air facilities must be interlocked with the exhaust devices they serve, so that both operate together.
Makeup air introduced directly from outdoors into occupied parts of the building in winter must include a means of tempering it to maintain the indoor design temperature.
Read that second point again. If your kitchen exhaust fan can run while the makeup air unit is switched off, that isn't just a comfort problem — the interlock is a Code requirement. It's also one of the most common deficiencies we find, because interlocks get bypassed during troubleshooting and never restored.
Separately, ventilation for commercial cooking equipment must conform to NFPA 96, which the Code references directly.
The risks that actually matter
This isn't just a comfort complaint.
Back-drafting and combustion safety. A strongly negative building can reverse the flow in a natural-draft flue and pull combustion products — including carbon monoxide — back into the occupied space. It doesn't take much: pressure differences well below what you'd notice at a door are enough to cause spillage from a naturally vented appliance. This is the single most serious consequence, and it's why we treat pressure complaints in buildings with atmospheric gas appliances as urgent rather than routine. Gas appliance installation and venting in Ontario falls under CSA B149.1, administered and enforced by the TSSA under O. Reg. 212/01 (Gaseous Fuels). This is licensed-contractor work, not a maintenance-staff task.
Egress and door operation. The Ontario Building Code requires that an exit door, once the latch is released, opens under a force of not more than 90 N. Most commercial exterior doors swing outward in the direction of exit travel, and negative pressure holds them shut against that direction — meaning the force needed to push out is exactly the force that matters in an evacuation. A building under significant negative pressure can push real-world door operation past what the Code contemplates. That's a life-safety issue, not just an annoyance.
Energy waste. Untempered outdoor air infiltrating through the envelope is heated by your perimeter units at 100% of the load, with none of the efficiency of a properly designed makeup air path. This is a real and recurring line item on the gas bill.
Freeze risk. Cold infiltration concentrated at doors and low points is a contributing factor in frozen sprinkler and domestic water lines in vestibules, stairwells and loading areas.
IAQ and moisture. Negative pressure pulls in whatever is adjacent — parking garage exhaust, loading dock diesel, soil gas, neighbouring tenant odours.
How it gets diagnosed properly
A real diagnosis takes measurement, not guesswork:
Differential pressure readings between the building and outdoors, and between tenant spaces, taken with a manometer under realistic operating conditions.
An exhaust and supply air inventory — every exhaust fan and every source of outdoor air, with measured CFM rather than nameplate values.
A balance calculation to identify the actual deficit.
Verification of MUA and economizer operation — is the unit running, is the burner firing, is the damper actually opening?
Combustion and draft testing on gas appliances if back-drafting is suspected.
The fix depends on what the numbers show. Sometimes it's as simple as restoring a disabled MUA unit or freeing a stuck damper. Sometimes it's restoring the exhaust/makeup air interlock the Code requires. Sometimes it means adding capacity — which is a capital item worth planning for rather than discovering in January.
What to do before heating season
Late August and September are the right window. Specifically:
Have your makeup air units started and tested now, not on the first cold morning. Burner, gas train, damper, controls, and the discharge temperature under load.
Verify economizer and outdoor air dampers on every rooftop unit as part of fall startup.
Document every tenant fit-out from the past 24 months that added exhaust, and check whether makeup air was addressed.
Verify the exhaust/makeup air interlock actually works — turn on the hood and confirm the MUA starts with it.
Take baseline pressure readings now so you have something to compare against in January.
Treat pilot outages and back-drafting complaints as urgent. Do not let them sit on a work order queue.
If tenants in your building are complaining about doors, drafts or cold entrances every winter, and the answer has always been "it's an old building," it's worth measuring before you accept that.
Burban Air Systems Ltd. services the commercial makeup air Toronto and GTA buildings depend on — MUA units, rooftop units and exhaust systems, from Scarborough across the Greater Toronto Area. If your building shows any of the signs above, we can perform a pressure assessment and a full fall heating startup before the weather turns.
