2018 IMC Reference
Ventilation and Makeup Air Basics
A practical guide to outdoor air, exhaust, makeup air, building pressure, rooftop units, tenant spaces, restaurants, and the difference between ventilation and normal AC airflow.
2018 International Mechanical Code
Mechanical code references used on this topic
The 2018 International Mechanical Code covers ventilation in Chapter 4. Section 401 covers general ventilation requirements and intake openings, and Section 403 covers mechanical ventilation. Commercial exhaust and makeup air are addressed separately in Chapter 5.
Model code reference
References are based on the 2018 IMC, the mechanical code book used for Arizona HVAC contractor licensing study. Local adoption decides the enforceable version.
Manufacturer instructions
2018 IMC Section 304.1 ties equipment installation to approved equipment, listing, manufacturer instructions, and the code.
Local inspection
2018 IMC Chapter 1 covers administration, permits, inspections, and the code official role. Permitted work follows the local jurisdiction.
Ventilation Basics
Ventilation and cooling airflow are different
Cooling airflow is the air the HVAC system recirculates through the blower, coil, ductwork, and rooms. Ventilation brings outdoor air into the building or exhausts stale air out. Makeup air replaces air that exhaust fans remove.
Those differences matter in restaurants, offices, tenant spaces, salons, shops, kitchens, bathrooms, storage areas, and commercial buildings. A space can have an AC unit that runs, but still have odor, pressure, comfort, or exhaust problems because the ventilation side is not right.
- Outdoor air is not the same thing as recirculated cooling air.
- Exhaust fans can create negative pressure if makeup air is missing or poorly balanced.
- Commercial tenant spaces may need ventilation review when use changes.
- Restaurants and kitchens can involve hood exhaust, makeup air, and rooftop equipment coordination.
- Comfort complaints can involve pressure balance, return air, exhaust, outside air, and thermostat settings.
Outdoor Air
Outdoor air intake and control
Some systems bring in outdoor air through dampers, rooftop units, economizer sections, ventilation equipment, or dedicated outside-air paths. That air has to be filtered, controlled, and conditioned enough for the space.
If outdoor air is missing, stuck closed, stuck open, poorly controlled, or not matched to the building use, the complaint may show up as stuffy air, comfort problems, high humidity, hot rooms, odors, or an AC that seems to work too hard.
Exhaust And Makeup Air
Exhaust air and makeup air
When a building exhausts air, replacement air has to come from somewhere. If the makeup air path is not planned, the building may pull air through doors, gaps, ceiling spaces, water heater rooms, kitchens, or other paths that create comfort and pressure problems.
This is one reason commercial kitchens, restroom exhaust, tenant spaces, and high-exhaust businesses need ventilation thought instead of treating every issue like an AC repair.
Pressure Balance
Building pressure and HVAC operation
Negative pressure can make doors hard to open, pull hot outdoor air into the space, move odors, affect flame performance on some combustion appliances, or make the HVAC system seem weak. Positive pressure can push conditioned air out and create its own comfort issues.
A pressure problem is not solved by equipment size alone. Exhaust, makeup air, outside air, return air, ductwork, and controls may need to be checked together.
Tenant Spaces
Tenant space changes
An office, retail suite, restaurant, salon, storage space, and small shop do not all use air the same way. A tenant improvement or business change can create new exhaust, outside air, occupancy, odor, heat load, or comfort concerns.
For commercial HVAC, CTS looks at the equipment, roof access, controls, ductwork, filters, tenant complaint, and whether the issue sounds like repair, maintenance, ventilation, or a broader design conversation.
Residential Context
Residential ventilation and return-air issues
Most residential calls are not commercial ventilation design jobs, but homes can still have pressure and airflow issues. Bath fans, kitchen exhaust, tight rooms, poor return air, duct leakage, dirty filters, and closed interior doors can change how air moves.
A home that feels stuffy, dusty, uneven, or hard to cool may need an airflow and ductwork check before assuming the outdoor unit is the only problem.
Related CTS Pages
Related service pages
Related service pages connect the reference topic to diagnostics, repair planning, and replacement decisions.
Commercial HVAC
Rooftop units, package units, tenant spaces, restaurants, controls, maintenance, ventilation-adjacent issues, and replacement planning.
IAQ and ductwork
Comfort, dust, airflow, returns, ducts, filtration, hot rooms, and indoor air quality service conversations.
Kitchen exhaust
Commercial kitchen exhaust and makeup air can affect restaurants, rooftop units, comfort, doors, odors, and cooling performance.
HVAC ducts
Ductwork, return air, supply air, transitions, filters, and airflow can make or break the comfort side of the system.
Ventilation and makeup air FAQs
Answers about repair, replacement, maintenance, and service.
Is ventilation the same as AC airflow?
No. AC airflow is usually recirculated indoor air moving through the cooling system. Ventilation brings outdoor air in or exhausts indoor air out.
What is makeup air?
Makeup air is replacement air brought into a building to replace air removed by exhaust fans or hood systems.
Can missing makeup air affect comfort?
Yes. Missing or poorly balanced makeup air can create pressure problems, hot-air infiltration, odors, door pressure, and comfort complaints.
Do restaurants have special ventilation concerns?
Yes. Restaurants may involve hood exhaust, kitchen heat, makeup air, rooftop units, grease duct coordination, fire-safety systems, and business downtime.
Can CTS check ventilation-related comfort problems?
CTS can check the HVAC side, airflow, rooftop equipment, filters, controls, and ductwork. Some specialty ventilation or hood work may require additional specialists.
Call CTS Air Conditioning
CTS handles AC repair, HVAC service, replacement, maintenance, water heaters, and other plumbing across the Phoenix area.
480-696-5033