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.
Conversely, negative building pressure (more exhaust than supply air) draws unconditioned air in through every crack, undermining thermal efficiency and indoor air quality control. ASHRAE 62.1-2022 explicitly requires that multi-story buildings be designed to account for stack-effect pressure differences in determining door infiltration, and that pressurization be maintained relative to adjacent occupied spaces and outdoors as appropriate.
Target Pressure Relationships by Space Type
Different space types require different pressure relationships based on their function and contamination control needs:
| Space Type | Pressure Relative to Adjacent | Typical Design Delta-P |
|---|---|---|
| General office / retail | Slight positive vs. outdoors | +0.02 to +0.05 in. wg |
| Hospital patient corridor | Slight positive vs. rooms | +0.01 in. wg |
| Hospital isolation (airborne infection) | Negative vs. corridor | -0.01 in. wg min (FGI 2022) |
| Laboratory (chemical/bio) | Negative vs. corridor | -0.05 to -0.10 in. wg |
| Commercial kitchen exhaust zone | Negative vs. dining | -0.02 to -0.05 in. wg |
| Loading dock / trash room | Negative vs. building | -0.03 to -0.10 in. wg |
| Cleanroom ISO Class 7-8 | Positive vs. corridor | +0.03 to +0.05 in. wg |
Pressurization is achieved by carefully balancing supply, return, exhaust, and transfer air quantities. The net air balance equation for a space is: P_space = f(Supply - Return - Exhaust + Transfer in - Transfer out). Maintaining target pressures across a range of operating conditions (all VAV zones full-open, some zones at minimum, exhaust fans at various speeds) requires either constant-volume exhaust/supply or pressure-controlled variable-volume systems with building pressure feedback sensors.
Makeup Air Units (MAUs) — Types and Selection
A makeup air unit (MAU) is an outdoor-air-handling unit that conditions and delivers 100% outdoor air to compensate for exhaust air removed from the building, maintaining pressure balance. MAU types include:
- Indirect-fired gas MAU: A heat exchanger separates combustion gases from supply air. The combustion gases are exhausted separately. Supply air never contacts flame products. Widely used in commercial applications. Efficiency 75–83% thermal.
- Direct-fired gas MAU: Combustion products mix directly with supply air. Very high efficiency (92–95% thermal) because there are no heat-exchanger losses. Fuel cost savings are significant. Requires excess outdoor air (minimum 15:1 dilution ratio per NFPA 86) and is typically prohibited in occupied spaces by local mechanical code — used for industrial ventilation and large warehouse heating where contamination from combustion gases is acceptable given dilution.
- DX cooling MAU with gas heat: Packaged unit with both direct-expansion cooling coil and gas heating coil. Used where space heating and cooling are both required. Common in commercial applications as a combined DOAS and makeup air unit.
- Energy recovery MAU: Incorporates a rotary wheel, plate heat exchanger, or heat pipe between exhaust and supply air streams to pre-condition incoming outdoor air. Most energy-efficient for climates with large temperature differences between indoor and outdoor air. Required by ASHRAE 90.1 Section 6.5.6 when outdoor airflow and fraction thresholds are met.
Commercial Kitchen Exhaust Makeup Air
Commercial kitchen exhaust makeup air design is one of the most demanding MAU applications. NFPA 96 (Standard for Ventilation Control and Fire Protection of Commercial Cooking Operations) and ASHRAE 154 (Ventilation for Commercial Cooking Operations) provide the design framework:
- Exhaust airflow is determined by Type I (grease-laden) and Type II (heat and odor only) hood design using ASHRAE 154 methods or manufacturer data. Type I exhaust rates: approximately 100–300 CFM per linear foot of hood at the exhaust slot.
- Makeup air must replace 80–90% of exhaust volume to avoid excessive negative pressure in the kitchen. The remaining 10–20% comes from adjacent spaces (dining room transfer air), maintaining kitchen at slight negative pressure relative to dining.
- Short-circuit makeup air (supply within the hood canopy or face of the hood) reduces the amount of outdoor air required to be conditioned but must be carefully designed to not disrupt the thermal plume capture efficiency of the hood. ASHRAE 154 provides plenum delivery, backwall, and front-face makeup air designs with performance data.
- The makeup air unit must handle 100% outdoor air at all times and should include heating, cooling, and dehumidification as required by the climate. Energy recovery from kitchen exhaust air is technically challenging due to grease contamination of the exhaust air stream — only certain heat recovery technologies (run-around coil loops, not rotary wheels) are safe in grease-laden exhaust streams.
Stack Effect and Seasonal Pressurization Challenges
Tall buildings experience significant stack effect in winter: warm air rises and exits through the upper building envelope, creating negative pressure at lower floors and positive pressure at upper floors. The neutral pressure plane (where building pressure equals outdoor pressure) shifts with wind and temperature. Stack effect magnitudes: approximately 0.01 in. wg per story of height difference in winter for a typical U.S. climate — a 10-story building can see 0.10 in. wg stack effect at the ground floor.
Mitigation strategies include vestibules at ground-floor entries (two-door air locks), revolving doors, air curtains at loading docks, pressurized elevator shafts, and careful balancing of supply vs. exhaust airflows to maintain a near-neutral building pressure plane at a central floor. Building pressurization control systems using multiple building differential pressure sensors and modulating fan speed or bypass dampers are increasingly specified on tall and high-performance buildings.
Smoke Control Integration
NFPA 92 (Standard for Smoke Control Systems) and IBC Section 909 govern smoke control system design for large or high-rise buildings. Smoke control pressurization systems use dedicated fans to pressurize stairwells (minimum 0.05–0.10 in. wg relative to adjacent corridor) and elevator shafts to prevent smoke infiltration during fire events. The HVAC engineer must coordinate the smoke control system with the general HVAC system — in many cases, the HVAC air handling units are repurposed as exhaust fans during smoke control mode, and the building's general exhaust and supply systems must be shut down or modulated to avoid interfering with smoke control pressure differentials.