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Room Airflow Distribution Calculator

Split your system’s airflow among rooms in proportion to their heating and cooling loads, and see each room’s design CFM — the larger of the two.

Method reviewed October 26, 2026. Version 1.0.0. How it works

Your inputs

Unit system

The airflow the equipment will deliver at the design blower speed.

Result

Sum of room design airflows
1,063CFM
Heating: CFM per 1,000 BTU/h
41.7
Cooling: CFM per 1,000 BTU/h
55.6

Room 1 gets about 375 CFM; the 4 rooms’ design airflows add up to 1,063 CFM.

  • The room design airflows add up to 1,063 CFM, more than the 1,000 CFM blower, because each room takes the larger of its heating and cooling share. Size each branch for its design CFM; the rooms will not all peak in the same season.

Design airflow by room (larger of heating and cooling)

Room 1
375CFM
Room 2
250CFM
Room 3
188CFM
Room 4
250CFM

Heating airflow by room

Room 1
375CFM
Room 2
250CFM
Room 3
188CFM
Room 4
188CFM

Cooling airflow by room

Room 1
333CFM
Room 2
250CFM
Room 3
167CFM
Room 4
250CFM
Show the working
  1. Heating factor = blower CFM ÷ total heating load
    1,000 ÷ 24,000 = 41.67 CFM per 1,000 BTU/h
  2. Cooling factor = blower CFM ÷ total cooling load
    1,000 ÷ 18,000 = 55.56 CFM per 1,000 BTU/h
  3. Room CFM = room load × factor; design CFM = the larger of the two
    Room 1: heating 375, cooling 333 → 375 CFM

The proportional method

Each room gets the share of the blower airflow that matches its share of the total load:

  • Heating CFM for a room = blower CFM × room heating load ÷ total heating load
  • Cooling CFM for a room = blower CFM × room cooling load ÷ total cooling load
  • Design CFM = the larger of the two

This is the allocation described in ACCA’s Manual D checklist, which asks that every room have a heating and cooling CFM assigned from the room-by-room loads.

Why the room totals can exceed the blower

Taking the larger value for each room means the design airflows usually add up to more than the blower delivers. That is expected: the branches are sized for each room’s worst season, and not every room peaks in the same season. Balancing dampers let you trim between seasons.

Getting the loads

The method is only as good as the room loads. Use a room-by-room load calculation — our room-by-room load calculator is a starting point — rather than floor area, which misses windows, exposure and ceilings.

Worked example

These are the example values the calculator opens with, and the result the engine calculates for them.

Inputs

Blower airflow (design)
1,000 CFM
Number of rooms
4
Room 1: heating load
9,000 BTU/h
Room 1: cooling load (sensible)
6,000 BTU/h
Room 2: heating load
6,000 BTU/h
Room 2: cooling load (sensible)
4,500 BTU/h
Room 3: heating load
4,500 BTU/h
Room 3: cooling load (sensible)
3,000 BTU/h
Room 4: heating load
4,500 BTU/h
Room 4: cooling load (sensible)
4,500 BTU/h

Result

Sum of room design airflows
1,063 CFM
Heating: CFM per 1,000 BTU/h
41.7
Cooling: CFM per 1,000 BTU/h
55.6

Room 1 gets about 375 CFM; the 4 rooms’ design airflows add up to 1,063 CFM.

Assumptions

  • Each room’s heating and cooling airflow is the blower airflow shared in proportion to its share of the total heating or cooling load; the room’s design airflow is the larger of the two (the method shown in ACCA’s Manual D checklist).

Limitations

  • Room loads must come from a room-by-room load calculation; this tool does not calculate them.
  • Rooms that need very different heating and cooling airflow may need dampers or a zoning strategy to balance both seasons.

Frequently asked questions

How do I calculate CFM per room?

Divide the blower airflow by the total load to get CFM per BTU/h, then multiply by each room’s load. Do it for heating and cooling, and use the larger result for the room.

Should I use sensible or total cooling load?

Airflow follows the sensible (temperature) load, so use each room’s sensible cooling load.

Can I just use CFM per square foot?

It is a rough check only. Rooms of the same size can have very different loads because of windows, orientation and exterior walls.

Sources

  1. Manual D calculation checklist (Table 3-1 velocity limits) — ACCA (hosted by AHRI). Friction rate = available static pressure × 100 / total effective length; residential velocity limits (rigid trunk 700 recommended / 900 max; flex 600 / 700).

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