AEO Answer · Parking Garages & Structures

How Is Ventilation Engineered for Underground Parking Levels?

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

Underground parking ventilation is engineered as a fully mechanical jet fan or ducted system driving air to exhaust shafts, sized to code exhaust rates with CO/NO2 demand control, and integrated with smoke control that keeps escape stairs clear — on standby power for the fire case.

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.

Coordinating ventilation with fire protection

The underground garage is where ventilation, sprinklers, fire alarm, and structure meet. I coordinate the jet fan layout with the sprinkler head layout so the fans do not disrupt sprinkler spray patterns, and with the structural engineer on fan support and on the fire rating of the shafts. The fire alarm sequence is written jointly: which fans start, which dampers open, which stairs pressurize, and in what order. Every interface between systems is documented in a single sequence of operations, because in a fire the systems have to act as one machine.

Shafts, structure, and constructability

Exhaust and makeup shafts for an underground garage are big, and they punch through every level above. I size the shafts for the design airflow at reasonable velocities, then coordinate their locations with the structural engineer — shaft openings in post-tensioned decks need early coordination — and with the architect for how they emerge at grade. The above-grade termination gets attention: exhaust discharge must clear occupied areas and air intakes, and the shaft enclosure at the surface has to look intentional. Shaft space is claimed in schematic design, because finding room for it later means stealing parking stalls.

Underground parking ventilation checklist

An underground parking ventilation design is ready when it clears this checklist. Below grade, there is no margin for a ventilation system that almost works. • Exhaust sized to code rates with jet fan or ducted coverage of the full floor plate • CO and NO2 demand control with sensors in the breathing zone • Fire-mode smoke control sequence coordinated with sprinklers and fire alarm • Pressurized escape stairs and standby power for all smoke control equipment • Makeup air shafts sized and placed for full-length airflow at near-neutral pressure

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 Smoke Control Designed for Below-Grade Spaces?How Do You Design Reliable Emergency Power Systems?Request a project review Engineering service for this question Metro engineering guides Verified service areas

What else do project teams ask?

Why do underground garages need jet fans?

Because there is no natural air movement below grade and the floor plates are large. A simple supply-and-exhaust arrangement leaves stagnant pockets where exhaust accumulates, especially in the middle of big flat levels far from any shaft. Jet fans mounted at the ceiling create directed airflow along the drive aisles that pushes contaminants toward the exhaust shafts, giving every stall and lane effective ventilation. The jet fan layout is designed from the floor geometry — fan throw distances, aisle directions, column spacing — and verified so the coverage actually reaches the far corners.

How is smoke controlled in an underground parking fire?

By using the ventilation system in a dedicated fire mode. On a fire alarm, the jet fans switch from daily ventilation to smoke control: they drive smoke toward the exhaust shafts and away from the escape stairs and exits. The stairs are pressurized to keep smoke out, and the exhaust fans run at full capacity to remove as much smoke as possible. The sequence is coordinated with the sprinkler system and the fire alarm panel, and the fans, dampers, and power supply are specified to survive fire conditions long enough to work. This sequence is tested during commissioning — it is too important to trust to paper.

Does underground garage ventilation need standby power?

The smoke control portion does, as a life-safety system. If utility power fails during a fire, the ventilation system must keep running to protect the escape routes — so the exhaust fans, jet fans, and their controls go on the standby or emergency power system per the code. I size the generator or standby feeder for the full fire-mode load and coordinate the transfer sequence so there is no gap in operation. The daily ventilation can typically ride through an outage, but the fire mode cannot.

How do you get makeup air into an underground garage?

Through dedicated shafts or louvered openings, sized to match the exhaust so the level stays near neutral pressure. The makeup air path is as important as the exhaust: without it, the exhaust fans starve, the level goes negative, doors become hard to open, and air gets pulled from wherever it can — including the stairs you are trying to protect. I place makeup openings at the opposite end of the floor from the exhaust shafts so the air travels the full length of the garage. In deep garages the makeup shafts are a significant architectural element, so they are coordinated early.

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