How much positive building pressure should I target for a commercial office building?

ASHRAE 62.1-2022 does not specify a minimum building pressure, but the standard practice for commercial office buildings in mixed and cold climates is to maintain +0.02 to +0.05 in. wg (0.002 to 0.005 kPa) relative to outdoors. This is enough to prevent infiltration through door gaps and envelope penetrations while not creating uncomfortable drafts when exterior doors open. In hot-humid climates, positive pressure is more critical — maintain +0.05 in. wg minimum to prevent moisture-laden outdoor air from infiltrating through the building envelope and condensing within wall assemblies. Monitor with building pressure sensors and verify with smoke pencil tests at key envelope locations during HVAC commissioning.

When is a direct-fired makeup air unit appropriate vs. indirect-fired?

Direct-fired MAUs (where combustion products mix with supply air) are appropriate for non-occupied industrial spaces such as warehouses, aircraft hangars, auto dealerships, and large retail stores with high ceilings and high ventilation rates (15:1 minimum dilution ratio maintained per NFPA 86). The cost savings from 92–95% thermal efficiency (vs. 75–83% for indirect-fired) are significant in cold climates with high outdoor air quantities. Direct-fired units are generally NOT permitted for occupied commercial, institutional, or healthcare occupancies. Always verify with the local AHJ and check whether the project jurisdiction has adopted NFPA 86 restrictions or local amendments before specifying direct-fired equipment.

What is the minimum exhaust-to-supply ratio for commercial kitchen ventilation?

ASHRAE 154-2016 and NFPA 96 do not specify a fixed percentage, but the common design practice is to supply 80–90% of exhaust volume as makeup air directly to the kitchen, with the remaining 10–20% supplied as transfer air from adjacent dining or corridor spaces. This maintains the kitchen at approximately -0.02 to -0.05 in. wg relative to dining, preventing cooking odors from migrating to the dining area while avoiding excessive negative pressure that would cause hood spillage. The exact ratio depends on hood type, exhaust rate, and space configuration. Always model the kitchen airflow balance and verify pressures across the kitchen-to-dining door gap during commissioning.

How do I calculate stack effect pressure in a tall building?

Stack effect pressure difference between the top and bottom of a building is approximately: ΔP = 0.52 × H × P_atm × (1/T_out - 1/T_in), where H is building height in feet, P_atm is atmospheric pressure in in. Hg (29.92 at sea level), T_out and T_in are outdoor and indoor absolute temperatures in Rankine. For a rough rule of thumb, stack pressure is approximately 0.008–0.010 in. wg per story in winter for typical U.S. conditions (0°F outside, 70°F inside). A 20-story building can experience 0.16–0.20 in. wg stack effect — enough to slam vestibule doors, make lobby doors impossible to open against the pressure, and draw smoke from lower floors to upper floors during a fire event. Consult ASHRAE Handbook of Fundamentals Chapter 24 for detailed stack effect calculations.

Engineering·9 min read·August 5, 2025

🔧 Building Pressurization Design and Makeup Air Units

Engineering guide to building pressurization control — pressure relationships between spaces, makeup air unit selection, smoke control, kitchen exhaust compensation, energy recovery, and ASHRAE 62.1 outdoor air compliance.

📖Belt Conveyor System Design Drawing Set (Full Access)

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Why Building Pressurization Matters

Building pressurization — maintaining the interior at a slightly positive pressure relative to outdoors — is one of the most important but often overlooked aspects of HVAC system design. Positive pressure prevents infiltration of unconditioned outdoor air, moisture, pollutants, and pests through envelope gaps, which is especially critical in hot-humid climates where infiltration of warm moist air into a cooled interior causes condensation in wall assemblies and interstitial mold growth.

Topics covered

building pressurizationmakeup air unitMAUpositive pressure buildingpressure relationshipssmoke controlkitchen exhaust makeup airair curtainstack effectinfiltration controlASHRAE 62.1 outdoor airdirect-fired MAUenergy recovery ventilation