Building Pressure Management
Commercial building pressurization principles, measuring building pressure relative to outdoors, balancing supply and exhaust air to maintain neutral to slightly positive pressure, and solving infiltration and exfiltration problems.
- Explain the causes and consequences of positive and negative building pressure in commercial facilities
- Measure building pressure relative to outdoors using a digital manometer
- Calculate the supply-exhaust air balance to predict building pressure conditions
- Recommend corrective actions for buildings with excessive negative or positive pressure
Leçon 1
Building Pressure Fundamentals
Why Building Pressure Matters
Every commercial building exists in a state of pressure relative to the outdoors. If more air is supplied to the building than is exhausted, the building is positively pressurized and air leaks outward through the envelope. If more air is exhausted than supplied, the building is negatively pressurized and outdoor air infiltrates through every crack, gap, and opening.
Building pressure has a direct impact on energy consumption, indoor air quality, moisture management, occupant comfort, and the operation of combustion appliances. Uncontrolled building pressure is one of the most overlooked factors in commercial HVAC performance.
Slight Positive Pressure (Ideal)
Target: +0.02 to +0.05" w.c. (+5 to +12 Pa)
Benefits: Prevents uncontrolled infiltration, keeps outdoor pollutants out
IAQ effect: Building controls what air enters
Moisture: Reduces moisture infiltration in cooling climates
Negative Pressure (Problems)
Reading: Below -0.02" w.c.
Problems: Uncontrolled infiltration, doors hard to open
IAQ effect: Outdoor contaminants pulled in through envelope
Moisture: Hot humid air pulled in during cooling season
Consequences of Negative Building Pressure
Entry door problems: The most visible symptom. Negatively pressurized buildings have entry doors that are difficult to open because the pressure differential holds them closed. In severe cases (-0.10 inches w.c. or more), glass entry doors require excessive force to open and can be a safety hazard and ADA compliance issue.
Uncontrolled infiltration: Outdoor air enters the building through every unsealed gap in the envelope - around windows, through wall penetrations, at roof curbs, and through loading dock openings. This unfiltered, unconditioned air brings humidity, dust, pollen, and pollutants directly into the occupied space.
Moisture problems: In humid climates, negative building pressure pulls warm, moist outdoor air into wall cavities and ceiling spaces. When this air contacts cool interior surfaces (near the AC supply ducts or behind cooled walls), the moisture condenses, causing mold growth, rotting drywall, and ceiling tile staining. This is one of the most expensive building damage mechanisms in the southeastern United States.
Increased energy consumption: Every cubic foot of unconditioned air that infiltrates must be cooled or heated by the HVAC system. A building running at -0.10 inches w.c. can infiltrate 20-30% more outdoor air than its HVAC system was designed to handle, dramatically increasing energy costs.
Combustion appliance backdrafting: As covered in the CO/Combustion Safety certification, negative building pressure can reverse the flow in natural draft flues, pulling combustion products into the building.
Consequences of Excessive Positive Pressure
While slight positive pressure is desirable, excessive positive pressure creates its own problems:
Energy waste: Over-pressurizing forces conditioned air out through the building envelope. In winter, this warm, humidified air enters wall cavities and can cause condensation inside the walls (in cold climates).
Door control issues: Doors may not close properly or may swing open unexpectedly if the pressure differential is too high.
Moisture problems in cold climates: Warm, humid indoor air forced into wall and roof cavities by positive pressure can condense on cold surfaces, causing moisture damage from the inside out.
What Creates Building Pressure
Building pressure is determined by the balance between air supplied to the building and air exhausted from it:
Supply sources:
- HVAC system outdoor air intake (economizer or minimum outdoor air damper)
- Makeup air units (MAU)
- Pressurization fans
- Transfer air from adjacent pressurized spaces
- Wind pressure on the windward side of the building
Exhaust sources:
- General exhaust fans (bathroom, storage)
- Kitchen exhaust hoods (500-5,000+ CFM each)
- Laboratory fume hoods (300-1,500 CFM each)
- Process exhaust (manufacturing, printing)
- Restroom exhaust (75-150 CFM per fixture)
- Return air duct leakage in unconditioned spaces
- Stack effect (warm air rising out through upper-floor openings)
- Wind suction on the leeward side of the building
The Kitchen Exhaust Factor
Commercial kitchen hoods are the single largest source of building depressurization. A single commercial kitchen hood can exhaust 2,000-5,000 CFM, which is more than many small commercial buildings' total HVAC supply air. Without dedicated makeup air for the kitchen, the entire building becomes severely depressurized when the kitchen hood operates.
Commercial buildings should maintain slight positive pressure (+0.02 to +0.05 inches w.c.) to prevent uncontrolled infiltration, moisture problems, and door issues. Negative pressure is caused by exhausting more air than is supplied, with kitchen hoods being the most common culprit in commercial facilities.