Balancing Procedures

Air balancing begins with understanding the three pressures that govern airflow in every duct system. Every measurement a technician takes during a balancing procedure traces back to the relationship between total pressure, static pressure, and velocity pressure. Mastering these concepts - and the instruments used to read them - is the foundation of all balancing work.

The Three Pressures in a Duct

Static pressure is the force air exerts equally in all directions against the duct walls. It exists whether or not air is moving and is the pressure that a manometer reads when the sensing tip is perpendicular to the airflow. Static pressure can be positive (on the supply side, pushing outward) or negative (on the return side, pulling inward).

Velocity pressure is the pressure created by air in motion. It acts only in the direction of flow and is always positive. Velocity pressure is what pushes air through registers and grilles and out into the conditioned space.

Total pressure is the sum of static pressure and velocity pressure at any given point in a duct. The critical relationship that every technician must know is:

Total pressure minus static pressure equals velocity pressure.

This can be written as: TP - SP = VP. This is the fundamental equation that makes a Pitot tube work. The Pitot tube has two ports - one facing directly into the airstream (sensing total pressure) and one perpendicular to the airstream (sensing static pressure). When both ports are connected to a manometer, the instrument reads the differential between total and static, which equals velocity pressure.

Note that gauge pressure refers to pressure measured relative to atmospheric pressure (as opposed to absolute pressure), and barometric pressure is the atmospheric pressure itself. When using a manometer for duct measurements, you are reading gauge pressure - the differential across the sensing points. The manometer also reads the differential across a filter or coil when connected on both sides.

The Pitot Tube

The Pitot tube is the standard instrument for measuring velocity pressure inside a duct. It is inserted through a test hole drilled in the duct wall and positioned so the open tip faces directly into the airstream. The Pitot tube is connected to a manometer that displays the velocity pressure in inches of water column (in. w.c.).

Using the Pitot tube is straightforward in moderate to high-velocity duct sections. However, at low velocities, the velocity pressures become so small that the Pitot tube cannot produce accurate readings. As a practical guideline, Pitot tube readings become unreliable below approximately 600 FPM, where velocity pressures are too small for accurate Pitot tube readings on a standard manometer.

The Hot Wire Anemometer

A hot wire anemometer is preferred over a Pitot tube for measuring air velocities below 600 FPM. The hot wire anemometer works by passing an electrical current through a thin heated wire or thermistor element. As air flows across the element, it cools the wire. The instrument measures how much current is required to maintain the wire at a constant temperature - faster airflow requires more current. This principle allows the hot wire anemometer to detect very low velocities that produce velocity pressures too small for accurate Pitot tube readings.

Hot wire anemometers are particularly useful for:

• Measuring airflow at supply registers and diffusers
• Checking velocities in large plenums where air moves slowly
• Verifying low-velocity return air paths
• Measuring velocities in occupied spaces for comfort verification

Calculating Air Velocity from Velocity Pressure

Once velocity pressure is known, air velocity can be calculated using the standard formula:

V = 4005 x sqrt(VP)

Where V is velocity in feet per minute (FPM) and VP is velocity pressure in inches of water column.

For example, if the velocity pressure at a point in a duct is 0.10 inches w.c., the calculation is:

V = 4005 x sqrt(0.10) = 4005 x 0.3162 = 1,266 FPM

This formula is derived from Bernoulli's equation and uses the constant 4005 for standard air density (0.075 lb/ft3 at 70 degrees F). The sqrt (square root) function means that doubling the velocity pressure does not double the velocity - it increases it by a factor of approximately 1.414.

Velocity Pressure (in. w.c.)	Air Velocity (FPM)
0.01	401
0.05	896
0.10	1,266
0.25	2,003
0.50	2,832
1.00	4,005

When using a Pitot tube, total pressure minus static pressure equals velocity pressure. Convert velocity pressure to air velocity using V = 4005 x sqrt(VP). At low velocities below 600 FPM, a hot wire anemometer is preferred over a Pitot tube because the velocity pressures are too small for accurate Pitot tube readings.

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