The facilities supervisor at a Mississauga distribution warehouse called us about a warehouse makeup air ventilation problem — though at first, the three symptoms seemed unrelated. Office staff near the warehouse floor complained of stale air and afternoon headaches. The exterior man-doors slammed shut hard enough to rattle frames. And the dock doors — the big overhead ones — had become noticeably harder to open, as if the building were holding its breath.
The building was, in a sense, holding its breath. All three symptoms pointed to one cause: a warehouse makeup air ventilation failure. The building was running under negative pressure because exhausted air was not being replaced.
Understanding warehouse makeup air ventilation is the key to diagnosing complaints like these.
Why Warehouse Makeup Air Ventilation Goes Wrong
Every cubic metre of air that exhaust fans, dust collectors, paint booths, or process equipment push out of a building has to be replaced by air coming in. That replacement air is called makeup air. If a building exhausts more air than it brings in, pressure inside drops below pressure outside — the building goes negative.
This warehouse had added two new exhaust fans for a packaging line the previous year but never added makeup air capacity to match. Then winter arrived, the overhead doors stayed shut, and the building sealed tight. The result was a building gasping inward:
- Doors slamming and hard to open — outdoor air pressure literally pushing doors closed and fighting anyone trying to open them against the pressure difference.
- Stale air and headaches — with no controlled fresh-air intake, the building pulled air through every crack and gap: around dock seals, through the shipping office, across dusty corners. Ventilation became accidental instead of designed.
- Dust everywhere — unfiltered infiltration air carried in fine dust that settled on racking, product packaging, and equipment.
- Cold drafts in winter — infiltration air in January is −15°C air, and the heating system was working overtime to warm air it was never designed to handle.
There is also a safety dimension that made this urgent: in a building with gas-fired unit heaters, strong negative pressure can interfere with proper venting of combustion equipment. Ontario’s safety regulator, TSSA, expects fuel-burning equipment to be serviced by licensed professionals — check TSSA’s consumer safety resources before booking any combustion work. Balancing a building’s air is never a DIY exercise; it needs a licensed technician’s assessment.
Diagnosing Warehouse Makeup Air Ventilation Problems
The technician started with a manometer — a simple instrument that measures pressure difference — and confirmed the warehouse was running significantly negative relative to outdoors. Then came the walkthrough:
- The existing makeup air unit (MAU) on the roof was running, but barely: belts were glazed and slipping, filters were loaded, and the burner needed service. It was delivering a fraction of its rated airflow.
- Exhaust inventory showed the two new packaging-line fans had no interlock with the makeup air system — they ran whether or not replacement air was available.
- Dock door seals were worn, which explained some of the uncontrolled infiltration, but seals were a symptom-side issue. The pressure imbalance was the disease.
That combination — a struggling MAU running against un-interlocked exhaust fans — is the most common warehouse makeup air ventilation failure we see in GTA industrial buildings.
The Fix: Restoring the Air Balance
- Serviced the makeup air unit: new belts, new filters, burner cleaning and combustion check, and verified airflow back to rated capacity.
- Interlocked the new exhaust fans with the makeup air system so process exhaust cannot run without replacement air being supplied.
- Rebalanced the building to run very slightly positive — the standard target for warehouses — so doors operate normally and infiltration air comes through the filtered, tempered MAU instead of through cracks.
- Flagged the dock seals for the supervisor’s maintenance list as a secondary improvement.
Within a day of restoring proper warehouse makeup air ventilation, the doors behaved normally, the stale-air complaints stopped, and the heating system stopped fighting a losing battle against −15°C infiltration.
Preventing the Next Imbalance
Warehouse makeup air ventilation problems creep back whenever equipment changes or maintenance lapses:
- Quarterly filter and belt checks on the MAU — a slipping belt silently cuts airflow long before anyone notices comfort changes.
- Reassess air balance whenever exhaust equipment is added. A new fan, booth, or process line changes the math. This warehouse’s trouble started the day the second packaging fan went in without a makeup-air review.
- Never let exhaust run without makeup air. Interlocks are not optional extras; they are what keep the building from going negative every shift.
- Install and maintain carbon monoxide detectors in any building with fuel-burning equipment, and schedule an annual combustion safety check with a licensed technician.
- Watch the utility bills. A heating system warming uncontrolled infiltration air shows up as a winter gas bill that keeps climbing — another reason warehouse makeup air ventilation maintenance pays for itself.
Your Building Is Telling You Something
Doors that fight you, stale air, mystery dust, and heating bills that do not make sense — in an industrial building, these are rarely separate problems. They are usually one problem: warehouse makeup air ventilation that has drifted out of balance.
Book an industrial ventilation assessment: Contact The Quality HVAC for warehouse makeup air ventilation service across the GTA, explore our commercial & industrial HVAC services, or see our residential heating and cooling services.