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newSchoolWorks.com › VoTechWorks › Building & Construction › HVAC/R › Ductwork: Design, Fabrication, and Installation › Lesson 11
BLD-HVAC07-05 · Lesson 11 · 22 clock hours · lab

Static Pressure Measurement and Air Balancing

Objective. Students will use a digital manometer and a flow hood (or balometer) to measure total external static pressure and outlet airflow on a live lab duct system, then adjust dampers to bring all outlets within 10% of design CFM.

Why Air Balancing Matters

A duct system that moves air is not necessarily moving the right amount of air to the right places. Rooms that receive too little conditioned air become uncomfortable and may develop moisture or air-quality problems. Rooms that receive too much waste energy and create noise complaints. Air balancing is the process of measuring airflow at every outlet and adjusting the system until each terminal device delivers the volume of air specified on the design drawing. Before you can balance airflow, you must confirm that the air-handling unit (AHU) is operating within its rated pressure range — that task requires measuring total external static pressure.

Total External Static Pressure (TESP)

Static pressure is the outward push that air exerts on duct walls, measured perpendicular to airflow. It is expressed in inches of water column (in. w.c.) or Pascals (Pa). Total external static pressure is the total resistance the fan must overcome outside the AHU cabinet itself — essentially the pressure drop across the supply duct system plus the return duct system. It does not include the pressure drop through the coil, filter, or heat exchanger inside the cabinet (those are internal losses).

Every AHU has a rated TESP range published by the manufacturer, typically found on the equipment nameplate or in the submittal data sheet. If measured TESP exceeds the rated value, the fan is working too hard, airflow is reduced, and the system may be undersized or the ductwork may be too restrictive. If TESP is well below the rated value, the ductwork may be oversized or dampers may be fully open with excess airflow.

Measurement procedure:

1. Locate the static-pressure taps on the supply plenum (downstream of the AHU) and the return plenum (upstream of the AHU). If taps are not pre-installed, drill small access holes per the manometer manufacturer's instructions and seal them afterward.

2. Connect the high-pressure port of the digital manometer to the supply plenum tap and the low-pressure port to the return plenum tap using the supplied hoses and probes.

3. With the system running at full design speed, read and record TESP in in. w.c.

4. Compare to the nameplate rated value. Document both numbers in your lab log.

Using a Flow Hood to Measure CFM

A flow hood (also called a balometer) is a capture device placed over a supply register or return grille. It captures all air leaving or entering that opening and routes it past an integrated sensor array that measures velocity or differential pressure, then converts the reading directly to cubic feet per minute (CFM). Flow hoods are fast and do not require a velocity traverse, making them the standard instrument for balancing work.

Operating procedure:

1. Confirm the hood size matches the register or grille face. Most hoods have interchangeable collars.

2. Hold or seat the hood flush and flat against the ceiling or wall surface so no air bypasses the collar.

3. Wait for the reading to stabilize — typically 5 to 10 seconds — then record the displayed CFM.

4. Record the design CFM from the system drawing next to each measured value on the balance data sheet.

5. Calculate the percentage deviation: % deviation = ((measured CFM − design CFM) / design CFM) × 100.

A rotating-vane anemometer with a traverse kit can substitute for a flow hood by measuring velocity at multiple points across the register face and averaging them, then multiplying by the net free area of the grille. The flow hood method is generally more accurate for balancing work because it accounts for the actual free area automatically.

The Proportional Balance Method

The proportional balance method is the industry-standard approach for adjusting manual dampers. The goal is to bring all outlets to the same ratio of measured CFM to design CFM before fine-tuning individual dampers.

Steps:

1. Record all measured CFM values.

2. Calculate the ratio (measured/design) for each outlet.

3. Identify the outlet with the lowest ratio — this outlet is the most starved and becomes your reference; leave its damper fully open.

4. Working from the outlet farthest from the AHU toward the unit, partially close each other damper until all ratios are approximately equal.

5. If total system airflow is now too low, increase fan speed (if variable) or re-open the reference damper slightly and repeat.

6. Perform a final measurement pass and verify all outlets are within 10% of design CFM.

Always work dampers in small increments and allow airflow to stabilize between adjustments. Slamming a damper shut can distort sheet-metal duct and creates excessive noise.

Safety in the Lab

Key terms8

Total External Static Pressure (TESP)The combined static pressure drop across the supply and return duct systems outside the AHU cabinet, measured in inches of water column (in. w.c.); indicates the total resistance the fan must overcome.
Static PressureThe outward pressure air exerts on duct walls perpendicular to the direction of flow, measured in inches of water column (in. w.c.) or Pascals (Pa).
Flow Hood (Balometer)An instrument placed over a supply or return opening that captures all airflow through that opening and displays a direct CFM reading.
CFM (Cubic Feet per Minute)The standard unit for measuring volumetric airflow rate in HVAC duct systems.
Proportional Balance MethodAn air-balancing procedure in which dampers are adjusted so all outlets achieve the same ratio of measured CFM to design CFM before final fine-tuning, minimizing the number of adjustment passes required.
Manual Volume DamperA hinged or rotating blade inside a duct branch that can be manually set to restrict or open airflow to a specific outlet.
Design CFMThe airflow rate specified on the system design drawing for a particular outlet or zone, established by the HVAC designer to meet load calculations.
Percentage DeviationThe difference between measured and design CFM expressed as a percentage of design CFM, calculated as ((measured − design) / design) × 100; used to judge whether an outlet is within balance tolerance.

Worked examples4

1. The supply plenum tap reads +0.55 in. w.c. and the return plenum tap reads −0.25 in. w.c. on the digital manometer. What is the total external static pressure?
  1. TESP = supply static pressure + |return static pressure|. The supply side reading is positive (air is being pushed) and the return side reading is negative (air is being pulled); both values resist the fan.
  2. TESP = 0.55 in. w.c. + 0.25 in. w.c. = 0.80 in. w.c.
Answer: TESP = 0.80 in. w.c.
2. A supply register has a design CFM of 120. The flow hood reads 138 CFM. What is the percentage deviation, and is this outlet within the 10% balance tolerance?
  1. % deviation = ((measured CFM − design CFM) / design CFM) × 100
  2. % deviation = ((138 − 120) / 120) × 100
  3. % deviation = (18 / 120) × 100 = 15%
Answer: The deviation is +15%, which exceeds the 10% tolerance. The damper for this outlet must be partially closed to reduce airflow.
3. Four supply outlets have the following measured/design CFM ratios after the first measurement pass: Outlet A = 95/100 = 0.95; Outlet B = 110/120 = 0.917; Outlet C = 80/100 = 0.80; Outlet D = 60/80 = 0.75. Using the proportional balance method, which outlet is the reference (leave fully open) and which should be adjusted first?
  1. Identify the outlet with the lowest ratio: Outlet D = 0.75. Outlet D is the most starved; leave its damper fully open — it is the reference.
  2. Outlet farthest from the AHU is adjusted first. Assuming the instructor drawing places Outlet A farthest, begin by partially closing Outlet A's damper until its ratio approaches 0.75.
  3. Move progressively toward the AHU: next adjust Outlet B, then Outlet C, targeting the same ratio (~0.75) at each.
  4. After all ratios are equalized, perform a final measurement pass; if all outlets are within 10% of design CFM, balancing is complete. If total flow is too low, slightly open the reference damper and recheck.
Answer: Outlet D is the reference (fully open). Adjustment sequence: Outlet A first (farthest), then B, then C, closing dampers progressively to match the 0.75 ratio.
4. After balancing, Outlet C reads 94 CFM against a design of 100 CFM. What is the percentage deviation, and does this outlet pass the 10% criterion?
  1. % deviation = ((94 − 100) / 100) × 100
  2. % deviation = (−6 / 100) × 100 = −6%
Answer: The deviation is −6%. Because 6% is less than 10%, Outlet C passes the balance tolerance.

The math in this lesson2

Learners add supply and signed return static pressures to find total external static pressure.
7.NS.17.RP.3
Brush up in the math halls at newSchoolWorks.com:
In class: Connect a digital manometer to static-pressure taps at the supply plenum and return plenum of the lab air-handling unit; record total external static pressure and compare to the unit's rated operating range, documenting findings in the lab log. · Use a flow hood to measure CFM at each of four supply registers and one return grille on the lab system; record readings on a balance sheet and calculate the percentage deviation from the design CFM listed on the system drawing. · Adjust manual dampers in sequence (starting farthest from AHU) using the proportional balance method until all outlets are within 10% of design CFM; perform a final measurement pass and record the balanced readings.
Materials: Digital manometer with static-pressure probes and hoses, flow hood (or rotating-vane anemometer with traverse kit), lab air-handling unit with duct system and manual dampers, balance data sheet, lab-log notebooks, system design drawing (instructor-provided)
Check: All four supply outlets must read within 10% of design CFM on the final recorded measurement pass; total external static pressure must be documented with correct units; lab log entry reviewed and initialed by instructor.

Practice check8

Answer each question, then check it. Nothing is recorded; this is your own practice.

HVAC Excellence – Air Distribution Systems
1. Total external static pressure (TESP) on an AHU is measured by:
NATE – Air Distribution
2. A flow hood reads 108 CFM at an outlet with a design CFM of 120. What is the percentage deviation?
NCCER HVAC – Air Distribution and Balancing
3. In the proportional balance method, which outlet damper is left fully open and used as the reference?
HVAC Excellence – Test, Adjust, and Balance
4. When using a flow hood, which action is most important to obtain an accurate CFM reading?
NCCER HVAC – Safety
5. During this lab, the minimum personal protective equipment (PPE) required includes:
NATE – Air Distribution
6. When adjusting dampers during proportional balancing, you should start at the outlet that is:
HVAC Excellence – Air Distribution Systems
7. An AHU nameplate shows a rated TESP of 0.50 in. w.c. The technician measures 0.82 in. w.c. The most likely cause is:
NCCER HVAC – Safety
8. Which statement about lockout/tagout (LOTO) applies correctly to this lab activity?
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