Building Pressurization, Exhaust, and Ventilation Verification
Core concepts
Understand building pressurization as the balance of supply, return, exhaust, and infiltration/exfiltration. Positive pressure reduces infiltration; negative pressure draws in unconditioned air and can back-draft combustion appliances. Know ASHRAE 62.1 ventilation-rate procedure: Vbz = Rp × Pz + Ra × Az, then Voz = Vbz / Ez (zone air distribution effectiveness), and system-level outdoor air with the multiple-spaces equation (critical zone). Verify exhaust flows for kitchens, restrooms, and labs, and confirm makeup air.
Measure building pressure with a reference (outdoor or a stable neutral space) and a sensitive differential gauge; small pressures (0.02–0.05 in. w.c.) matter. Understand stack effect, wind, and door-opening transients as error sources.
Traps
- Confusing zone outdoor airflow (Voz) with breathing-zone airflow (Vbz).
- Forgetting Ez (distribution effectiveness) reduces or increases required outdoor air.
- Using the simple sum of zone outdoor airflows instead of the critical-zone multiple-spaces equation at the system level.
- Measuring building pressure through a door that is opening/closing or in high wind.
- Assuming exhaust equals design without verifying makeup air.
- Ignoring that a negative building can back-draft water heaters and fireplaces.
Worked example
An office zone: 10 people, 2,000 ft². Using Rp = 5 cfm/person and Ra = 0.06 cfm/ft²: Vbz = (5 × 10) + (0.06 × 2,000) = 50 + 120 = 170 cfm. With Ez = 0.8, Voz = 170 / 0.8 = 212.5 cfm. If measured zone outdoor air is 180 cfm, the zone is under-ventilated relative to design.
Rule of thumb
Voz = Vbz / Ez; system outdoor air must satisfy the critical zone, not the average.