Airflow Distribution Calculator

The Airflow Distribution Calculator splits a system's supply airflow across the rooms it serves. You enter one row per room with its floor area, ceiling height, cooling and heating loads and target air changes, choose how the system airflow is established and how it should be apportioned, and it returns the design airflow for every room along with a check that the parts add back to the whole.

Airflows are in cubic feet per minute, loads in Btu per hour, areas in square feet and temperature differences in degrees Fahrenheit.

Use it to balance supply air across a house once the loads are known. It apportions air; it does not calculate the loads, size the ducts, or select the registers' make and model.

Purpose and model boundary

This model distributes a system supply-air quantity among as many as ten rooms, then selects an indicative register and round branch-duct size for each active room. It supports load-proportional, floor-area-proportional, and air-change methods and lets the design airflow be governed by cooling, heating, or the larger of the two seasonal values.

It is not a complete duct design. It does not size trunks, include fittings or equivalent length, calculate total external static pressure, check noise, establish outdoor-air ventilation, or replace room-by-room HVAC design by a qualified practitioner.

Inputs and units

All calculations use inch-pound inputs. Room rows contain a label, floor area in ft2, ceiling height in ft, cooling and heating loads in Btu/h, and target air changes per hour. A row with zero quantities contributes zero.

System airflow can be entered directly, derived from equipment capacity and airflow per ton, or derived from the total sensible cooling load and cooling supply-temperature difference. The model also accepts the total heating load and heating supply-temperature difference, the target register face velocity in ft/min, and the branch equal-friction rate in in.wg/100 ft.

Governing relationships

For standard air, the workbook uses the sensible-heat relationship

Q_air = q_sensible / (1.08 x deltaT)

where Q_air is airflow in cfm, q_sensible is load in Btu/h, and deltaT is the supply-to-room temperature difference in deg F. The system cooling airflow is either this result, equipment tons x cfm/ton, or the directly entered airflow. Heating airflow is heating load / (1.08 x heating deltaT).

For room i, the selected distribution method is applied as follows:

• load method: cooling share is room cooling load / sum of room cooling loads; heating uses the equivalent heating-load share;
• floor-area method: share is room floor area / total room floor area for both seasons;
• air-change method: room airflow = area x ceiling height x ACH / 60, independently of the system total.

The design-basis selection then returns the cooling value, heating value, or MAX(cooling, heating) for each room.

Required register free area is

free area (in2) = design cfm x 144 / target face velocity.

The workbook rounds up through its illustrative register table; a requirement above the largest entry returns Multiple registers needed. Branch diameter is calculated with the equal-friction power-law relation

d = (0.109136 x cfm^1.9 / friction rate)^(1/5.02)

with d in inches and is rounded up to the next diameter in the workbook's standard-size list. Branch velocity is then cfm / [pi x (d/12)^2 / 4].

Calculation sequence

1. Resolve the three dropdowns to the selected distribution method, design basis, and airflow source.
2. Calculate system cooling airflow and heating airflow.
3. Sum the room cooling loads, heating loads, and floor areas and calculate the differences from the system totals.
4. Calculate each room's cooling, heating, and governing design airflow.
5. Convert each design airflow into register free area, register selection, branch diameter, face velocity, and branch velocity.
6. Sum room design airflows and calculate balance against system airflow and airflow per sensible ton.
7. Evaluate Model_Status in the order shown below.

Outputs and interpretation

The primary results are system supply airflow and the sum of the room design airflows. The room grid and chart show the seasonal and governing airflows by room. The balance ratio is sum of room design airflow / system supply airflow; it need not equal one when the larger seasonal value is chosen room by room. Implied airflow per sensible ton is system cfm / (sensible cooling load / 12,000) and is intentionally on a sensible-load basis.

Register and branch sizes are preliminary selections from workbook lookup lists, not product or construction specifications.

Validation and status logic

The workbook evaluates status in this order; the first matching row is returned.

Assumptions and limitations

The 1.08 air constant assumes standard-air density and specific heat. Load-proportional distribution is only as reliable as the supplied room loads. Floor-area and ACH modes are comparison methods and do not reproduce a load calculation. The register free-area and branch-size tables are illustrative; manufacturer free area, throw, spread, pressure drop, and acoustic data govern a real selection.

The branch equation represents a straight equal-friction sizing relation. It does not add fitting losses, balancing-damper pressure, terminal pressure, leakage, trunk interactions, or fan-system effects. The familiar 400 cfm/ton rule refers to total capacity; this model's diagnostic divides by sensible capacity and therefore can be higher.

Restrictions and non-computing states

This calculator rejects numeric inputs below their declared minimum of zero before the workbook is called. The workbook itself refuses a zero system airflow or a room grid with no cooling load. A room whose numeric design inputs are all zero remains inactive. Zero denominators are protected with workbook IFERROR branches, but a protected zero does not make an otherwise invalid design meaningful; read Model_Status before using the outputs.

Errors and warnings

NOT VALID means the workbook did not receive enough physically meaningful information to distribute airflow. CHECK means calculations were produced but a load reconciliation or customary airflow range needs review. Input checking errors occur before the calculation runs and are separate from workbook status. A network or calculation-service failure is not a model conclusion.

The workbook derives its relations rather than reproducing any table, chart or figure from a manual or standard. The sensible-heat relation and the proportional allocations are computed directly.

• ACCA. Manual D: Residential Duct Systems, the governing residential duct design procedure in United States practice. https://www.acca.org/standards/technical-manuals/manual-d
• ACCA. Physics of heat and airflow, supporting the 1.08 x cfm x temperature difference sensible-heat relation. https://hvac-blog.acca.org/physics-heat-air-flow/
• ASHRAE. Handbook: Fundamentals, Duct Design chapter, for distribution terminology and design context. https://handbook.ashrae.org/Handbooks/F17/SI/f17_ch21/f17_ch21_si.aspx

This tool implements neither Manual J nor Manual D. Room loads are inputs that must come from a proper load calculation, and duct design is separate work this tool does not perform. No trademark or organisation name appearing here implies endorsement by its owner.

Additional source notes migrated from Methodology

The workbook implements the standard sensible-load airflow relationship, the published ASHRAE/ACCA equal-friction power-law relation used by ductulator tables, and ordinary register free-area geometry. It reproduces no proprietary standard table. The workbook's References sheet is the provenance record for the implemented relations and notes that ASHRAE and ACCA do not endorse this model.