Compare measured terminal and system airflows with targets to find signed balancing differences.
Concepts behind this calculation
Compare design and measured terminal flows within explicit balancing groups, identify all lowest-ratio terminals and report proportional references separately from design acceptance. Retain the total supply/return difference check.
Terminal balance compares each measured flow with its design demand. Proportional balancing normalizes those ratios to the lowest ratio within the actual balancing group.
A group can have the correct total while individual outlets are badly distributed. Individual design bands, group-total tolerance and proportional spread therefore answer separate acceptance questions.
How to use this calculator
Choose total supply and return difference for system totals or the measured terminal schedule for grouped terminal measurements.
Document the terminal schedule, group basis and procedure reference. Enter each terminal with its actual group ID, design CFM and compatible measured CFM.
Set the selected design shortfall, excess, group-total tolerance and proportional spread allowances from the procedure, then inspect every terminal and group result.
Calculation method and applicability
rᵢ = measured CFMᵢ / design CFMᵢ; group index = min(rᵢ); proportional referenceᵢ = design CFMᵢ × group index; relative spreadᵢ = 100 × (rᵢ / index − 1). Selected design-band and group-total checks are separate.
rᵢ = measured/design flow ratio for a terminal; index = lowest ratio in its actual group. Both are dimensionless. Design, measured and proportional-reference flows are CFM; spread is percent relative to that index.
Input definitions and units
Airflow balancing method
Use a documented simultaneous terminal schedule with actual independently established balancing groups, or choose the retained total-flow difference. A ratio reference is not an adjusted-flow prediction. Choices: Measured terminal schedule and proportional references; Total supply and return difference.
Total supply airflow (CFM)
Measured/design total with an identified compatible system boundary and operating condition. Applies to Airflow balancing method: Total supply and return difference.
Total return airflow (CFM)
Same operating condition and compatible airflow reference. Outdoor, exhaust or transfer flows may justify a difference. Applies to Airflow balancing method: Total supply and return difference.
Terminal schedule, group basis and procedure reference
Identify actual terminal IDs, source design values, instrument/correction basis, simultaneous operating condition, independently established balancing groups, complete coverage and selected criteria. Supply and return or unrelated branches must not be automatically grouped. A note does not authenticate measurements. Applies to Airflow balancing method: Measured terminal schedule and proportional references.
Measurement, group and acceptance basis
Confirm design and measured flows are already corrected to compatible references within each group, readings belong to one stated condition, actual group membership/coverage is established and the procedure criteria below are selected independently. No automatic density correction or grouping is performed. Choices: Not established; Documented compatible flow references, actual groups and selected criteria established. Applies to Airflow balancing method: Measured terminal schedule and proportional references.
Selected design shortfall allowance (% of design)
From 0 to 100. Lower bound is design flow × (1 − allowance/100). Teaching value only; actual project/TAB criteria and measurement uncertainty are external. Applies to Airflow balancing method: Measured terminal schedule and proportional references.
Selected design excess allowance (% of design)
Nonnegative. Upper bound is design flow × (1 + allowance/100). Applied to terminals and separately to group totals; no universal acceptance rule is selected. Applies to Airflow balancing method: Measured terminal schedule and proportional references.
Selected group-total tolerance (% of group design total)
Symmetric selected range around each actual group total: design total × (1 ± tolerance/100). Independent of terminal bands and proportional spread; no universal acceptance rule is selected. Applies to Airflow balancing method: Measured terminal schedule and proportional references.
Maximum 100 × (terminal ratio / group lowest ratio − 1). This is relative to the group index, not the same percentage as design deviation. A zero index blocks positive proportional references. Applies to Airflow balancing method: Measured terminal schedule and proportional references.
One actual terminal per line; 1–60 rows. IDs are unique integers 1–10000; group IDs are independently established integers 1–10000. Design CFM must be positive, measured CFM nonnegative. Flows must already use compatible group references; decimal point only, no thousands separators. Closed/no-design outlets require another procedure. Applies to Airflow balancing method: Measured terminal schedule and proportional references.
How to interpret the result
Review individual shortfall/excess, group-total error and proportional spread independently. A favorable group total cannot cancel an outlet failure.
Worked examples
Measured terminal schedule and proportional references
Inputs: Constructed snapshot: Group 1 design 200/400/400 CFM, measured 160/400/390; Group 2 design 300/300, measured 330/330. Selected teaching criteria: terminal −10%/+10%, group total ±5%, proportional spread 5%.
Calculation: Group 1 ratios are 0.8/1.0/0.975; index 0.8 yields proportional references 160/320/320 CFM, with 0/80/70 CFM excess. Its group total is 950/1,000 = 95%, meeting its separate selected ±5% band. Group 2 index is 1.1 with both terminals tied.
Result: 4 of 5 terminals meet the selected design band; Group 1 meets its group-total band at the exact lower edge, Group 2 does not. 1 of 2 groups meets the selected proportional spread. Group 1 maximum relative excess is 25%, from (1.0/0.8 − 1) × 100; Group 2 is proportional at 110% of design. References do not predict readings after adjustment.
Total supply and return difference
Inputs: 1,200 CFM supply and 1,150 CFM return.
Calculation: Subtract return from supply; divide the absolute difference by supply.
Result: 50 CFM supply surplus, 4.2% of supply.
Assumptions and limits
Each actual balancing group must be established independently from system topology, control state and the governing procedure. Group IDs do not prove independence; incomplete coverage can hide the actual lowest-ratio terminal. Supply and return are not automatically one group.
Measured/design CFM must already be corrected to compatible flow references within each group and collected under the same stated condition. No density, hood/probe correction or instrument uncertainty calculation is included.
All lowest measured/design ratio ties are retained. An index above one is not capped. A zero-flow index blocks positive proportional references for that group, while design and group-total checks remain available.
Selected asymmetric design-band criteria and relative-to-index proportional spread are separate. Teaching values are not universal TAB or code tolerances. A correct total can conceal terminal errors; proportional agreement can coexist with insufficient or excessive total airflow.
Proportional references describe one snapshot, not actuator commands, a hydraulic proof of the critical terminal or predicted adjusted readings. No automatic damper sequence, fan speed, ventilation minimum, pressure, leakage or full commissioning approval is provided. Adjustments require actual remeasurement.
The retained supply/return difference is not a complete boundary mass balance. Outdoor air, exhaust and transfer may explain a difference; pre-mixed outdoor air must not be added again to supply. It does not predict room pressure.
FAQs
Does the index outlet identify a damper setting?
No. It supplies a proportional reference for the measured group. It does not calculate a damper angle or prescribe field adjustment order.
Does equal supply and return prove room pressure is correct?
No. The simple totals omit boundary leakage and transfer paths. Pressure needs compatible simultaneous measurements and the actual building boundary.
Sources
ASHRAE 2019 Applications: Testing, Adjusting, and BalancingPrimary air-balancing context: actual layout/control conditions, proportional balancing and documented measured/design reporting. This bounded worksheet does not reproduce a complete TAB standard or apply its tolerance guidance as a universal rule.
HEVAC Air Distribution Guide, March 2015Industry association proportional-balancing explanation and initial design/measured coordinates. UK guidance is context, not a US adopted standard. Web extraction was readable; ordinary direct fetch timed out.
Ensign: proportional balancing aidActual peer terminal schedule and ratio/index workflow are observed. Its default tolerances, water-system scope and suggested work order are not adopted as US requirements.
