AEO Answer · Parking Garages & Structures

How Do Engineers Design CO Exhaust Systems for Parking Garages?

By Jeremy Mills, CEO & Founder, Apex Grid Engineering — USAF Veteran. · Updated 2026-09-15

CO exhaust systems for parking garages are dedicated mechanical exhaust sized to the code minimum, controlled by breathing-zone CO sensors that ramp fans in stages as concentrations rise, with fail-safe operation and commissioning that proves the sequence works.

I'm Jeremy Mills, CEO & Founder of Apex Grid Engineering and a U.S. Air Force veteran. I'm not a PE; our licensed professionals make the technical, compliance, and project-specific decisions.

Illustrative diagram of building MEP systems: ductwork with diffusers, an electrical panel with conduit, and piping runs.

Detection layout and sensor logic

I lay out CO detection from the traffic pattern, not just from a grid. Cars queue at pay stations, idle at gates, and climb ramps under load — each of those is a CO hotspot that gets sensor coverage. Sensors sit in the breathing zone, roughly five feet above the floor, away from direct fresh-air discharge that would give falsely clean readings. The control logic uses voting or averaging where zones overlap so one drifting sensor cannot throw the whole garage to full speed. In garages with both CO and NO2 detection, I sequence the fans on whichever gas rises first, because diesel traffic loads the NO2 side while gasoline loads CO.

Fan selection and electrical reliability

Garage exhaust fans live in a corrosive, dusty environment, so I specify equipment built for it: direct-drive or belt-drive centrifugal fans with corrosion-resistant coatings, mounted where maintenance can actually reach them. Variable-frequency drives give smooth modulation and soft starts. Electrically, the exhaust system needs to run when the building needs it — I coordinate the feeder and, where the garage is part of a larger building, confirm that the fans are not on a load-shedding circuit that would drop them during a power event. Status feedback to the building automation system lets the owner see that the fans are running, not just that they were commanded to run.

CO exhaust system design checklist

A parking garage CO exhaust system is ready when it clears this checklist. The system exists to protect people, so every item ties back to reliable operation. • Exhaust sized to the code minimum with 100 percent outside air • CO sensors in the breathing zone at code spacing, weighted toward ramps and queues • Staged or variable-speed controls that ramp fans as CO rises • Fail-safe operation on sensor or control faults with supervised fault reporting • Commissioning with a controlled CO test proving the sensor-to-fan sequence

Which related engineering resources can help?

When should I hire a structural engineer?What does a PE stamp mean?What is MEP engineering?How Does Demand-Controlled Ventilation Design Save Energy?How Is Ventilation Designed for Vehicle Repair Facilities?How Do You Design Reliable Emergency Ventilation Systems?Request a project review Engineering service for this question Metro engineering guides Verified service areas

What else do project teams ask?

What CO level triggers garage exhaust fans?

Setpoints follow the mechanical code and the adopted standard: a first stage typically activates increased ventilation when measured CO rises above normal background, and a second stage drives the system to full capacity with an alarm at a higher concentration. I set the first stage low enough that the system responds before occupants notice anything and the alarm stage well below concentrations that pose acute risk. Exact setpoints depend on the code edition the project is permitted under, so I confirm them against the adopted code rather than copying a number from another project.

How many CO sensors does a parking garage need?

Sensor quantity follows the mechanical code's spacing requirements for the garage's classification, placed in the breathing zone where exhaust actually collects. I supplement the code minimum in the spots that matter: near ramps, at ramp bottoms, in long dead-end bays, and anywhere traffic patterns create slow-moving queues. More sensors mean finer control and faster response, and the marginal cost of a sensor is small compared to the cost of a system that cannot see the problem. Sensor placement is one of the things I verify on the commissioning walkthrough.

Should garage CO fans run continuously or on demand?

On demand, controlled by detection — and the code allows it. Running full exhaust 24/7 in a garage that is empty half the day wastes enormous energy and wears out fans for nothing. A demand-controlled sequence runs a code-compliant minimum when the garage is quiet and ramps to full capacity as CO rises with traffic. The key is that the detection system is always watching and the fans always respond — the system is never off, it is just at low speed. That distinction is what keeps demand control code-compliant.

What happens if a CO sensor fails?

A properly designed system fails safe. Sensor faults, wiring failures, or loss of the controller drive the exhaust fans to a protective operating state — typically full or increased speed — rather than letting the garage run blind. I also design the controls with supervision: the system reports a fault so maintenance knows a sensor needs attention, instead of silently running on its last reading. Redundant sensors in critical zones are worth specifying in large garages where one blind spot covers a lot of cars.

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