7.3 Outside Air, Exhaust Balancing, and Building Pressurization

Outside Air, Exhaust, and Building Pressure

Start with boundaries and modes

Outdoor-air verification is a mass-balance problem tied to a particular operating mode. Identify the air-handling-unit mixing boundary and the building-envelope boundary. At steady conditions, and with all relevant streams included, supply air is the combination of return and outdoor air. At the building boundary, supply, return, exhaust, relief, infiltration, and exfiltration must reconcile. Leakage or an unmeasured relief path makes a simple subtraction incomplete.

Verify occupied mode, fan speed, VAV diversity, economizer command, relief or return-fan operation, doors, and major exhaust systems. Record weather and exterior-reference location because wind and stack effect can dominate a small envelope-pressure reading.

Direct outdoor-air methods

A traverse in a dedicated outdoor-air duct is preferred when a suitable plane and measurable velocity are available. Use the selected traverse procedure, actual net area, density treatment, and a calibrated instrument. A face-velocity sweep at a louver or damper requires a valid manufacturer effective-area factor and a repeatable measurement plane; gross louver dimensions alone generally do not establish flow.

Another useful check is a complete airflow balance. Under stated steady assumptions and with no omitted relief or leakage stream, outdoor airflow at a mixing unit equals supply minus return. Treat this as a diagnostic cross-check, not an automatic substitute for a direct measurement.

Temperature-ratio method

When outdoor and return airstreams mix without intervening sensible heat, the outdoor-air fraction can be estimated from dry-bulb temperatures:

OA fraction = (Tma - Tra) / (Toa - Tra)

The temperatures must represent the mean outdoor, return, and mixed streams. Use a multipoint grid where stratification exists. Coils, humidifiers, fan heat, solar-heated intake surfaces, leakage, and poor mixing can invalidate the energy balance.

The denominator controls uncertainty. As Toa approaches Tra, ordinary sensor error and spatial variation produce a large percentage error. Some field procedures use a screening difference such as 20°F, but the technician must follow the governing method and perform an uncertainty check rather than claiming a universal NEBB minimum. Enthalpy or tracer methods may be more suitable when latent conditions matter, provided the project procedure supports them.

Example: total supply is 20,000 CFM, Toa is 95°F, Tra is 75°F, and a representative Tma is 79°F. The fraction is (79 - 75) / (95 - 75) = 0.20, so the estimate is 4,000 CFM. The result is defensible only after the stated assumptions and mixing quality are verified.

Economizer and minimum ventilation

Follow the approved sequence for minimum-ventilation, economizer, and relief modes. Command only authorized overrides, verify physical damper response and linkage, measure flow, and adjust only the permitted setpoint or mechanism. A command percentage is not an airflow percentage. Recheck outdoor flow after changes in fan speed or duct static because damper pressure drop changes.

There is no universal TAB acceptance band that permits every outdoor-air result from design to 10% high. Apply the project criterion. If a minimum ventilation requirement is involved, report the measured result and limitation without altering data to create a pass.

Building differential pressure

The target and test conditions come from the design, sequence, space use, and governing requirements. An office, isolation room, hazardous compounding room, operating room, stair enclosure, and laboratory can require different pressure relationships and verification methods. Do not apply +0.02 to +0.05 in. w.g. as a universal office target or use pressure alone to predict door-opening force.

Measure across the specified boundary with a suitable low-range differential instrument. Route and shield the outdoor reference as the method requires. Test with the defined doors open or closed and the required exhaust systems operating. Sample multiple facades or conditions when wind is influential. For room relationships, verify airflow direction at the doorway or transfer path using the specified method in addition to pressure where required.

Reconcile the system

Suppose supply is 50,000 CFM, return is 41,000 CFM, and exhaust is 6,500 CFM. If conditions are steady and there are no other intake, relief, transfer, or leakage streams within the chosen boundary, supply minus return implies 9,000 CFM outdoor air, leaving 2,500 CFM for outward envelope flow. In an actual building, use independently measured streams and document uncertainty before assigning the entire residual to exfiltration.

Unexpected pressure calls for a system check: verify fan and exhaust status, damper command versus position, relief path, door and elevator conditions, filter loading, wind, stack effect, and measurement reference. Preserve both the measured value and the conditions under which it occurred.

A valid outdoor-air result records:

• the selected measurement boundary and all included airstreams;
• occupied, economizer, exhaust, return, and relief modes;
• weather, door state, and exterior pressure reference; and
• method limitations such as stratification, leakage, or small temperature difference.