Question 1

What's the difference between the three calculation modes?

Accepted Answer

Sensible Load to CFM answers 'how much air do I need to move this many BTU/hr with this temperature split?' — the standard heat equation. Equipment Tons to CFM answers 'roughly what airflow should this size of equipment move?' — a bounded planning rule of thumb. Room-by-Room Allocation answers 'given a known system airflow, how should it split across rooms?' — proportional allocation by load share. They solve different problems and use entirely different inputs; picking the wrong mode for your actual question gives a technically-correct but irrelevant answer.

Question 2

Why can't I use total cooling load in the Sensible mode equation?

Accepted Answer

The equation CFM = Sensible Load / (1.08 × ΔT) is derived specifically from sensible heat transfer — it has no term for latent heat (moisture removal). Total cooling load includes both sensible and latent components. Using total load instead of sensible load overstates the calculated CFM, since you're asking the equation to account for moisture removal it was never built to represent.

Question 3

Why does 400 CFM/ton show up everywhere, and is it always correct?

Accepted Answer

400 CFM/ton is ACCA's own cited baseline — it balances sensible cooling capacity and latent (dehumidification) capacity at standard indoor design conditions (80°F/50% RH). It's a reasonable default, not a universal law: humid climates often benefit from less airflow (350 CFM/ton, more dehumidification), while dry climates can use more (450 CFM/ton, more sensible capacity). The final answer always belongs to the matched equipment's own blower performance data.

Question 4

What actually happens below 350 or above 450 CFM/ton?

Accepted Answer

Below roughly 350 CFM/ton, too little air moves across the coil relative to its cooling capacity, and the coil can ice up. Above roughly 450 CFM/ton, air moves across the coil fast enough that condensed moisture can blow off the coil's fins before it drains away, reducing dehumidification and potentially carrying moisture into the duct system. Both are real physical failure modes on standard residential coils, not arbitrary boundaries.

Question 5

Why is 500 CFM/ton treated differently from 350–450?

Accepted Answer

500 CFM/ton sits outside the bounded 350–450 safe band entirely. It shows up in some hot, dry-climate applications where sensible capacity is prioritized and latent removal matters less, but it isn't broadly supported the way the 350–450 band is — this calculator flags it as needing the specific matched equipment's own manufacturer data before use, rather than presenting it as an equally-valid fourth option.

Question 6

How do I choose the right ΔT for Sensible mode?

Accepted Answer

For cooling, ΔT is the supply air temperature subtracted from the room temperature (a colder supply relative to the room means a bigger split, and less CFM needed for the same load). For heating, it's supply air temperature minus room temperature. Typical residential cooling design ΔT runs roughly 18–22°F; heating design ΔT varies more by equipment type. Use your actual system's design condition, not a generic guess, since CFM is inversely proportional to this number.

Question 7

Does the room-by-room allocation account for actual duct losses?

Accepted Answer

No — it's a proportional planning split based purely on each room's share of the total load, assuming the system airflow you entered is what actually reaches the branch takeoffs. Real ducts have friction losses, fitting losses, and leakage that this calculator doesn't model; the allocated CFM is a design target for balancing, not a guarantee of delivered airflow without a proper duct design and installation.

Question 8

What if my entered rooms don't add up to the full system airflow?

Accepted Answer

That's a valid partial scenario — the calculator reports whatever cooling and heating CFM is left unallocated. This is useful if you're checking only part of a system, but a complete supply-side design needs every room accounted for: add the remaining rooms until the allocation is complete, then verify the resulting per-room targets against your duct design. Return-air path and any bypass duct are separate parts of the overall air-distribution design (see the DOE's air-distribution design guidance) — they aren't a destination for leftover supply airflow that the room allocation itself hasn't assigned yet.

Question 9

Can I mix a detailed Manual J room load with a rule-of-thumb system CFM (like 400 CFM/ton)?

Accepted Answer

Technically the calculator will compute an answer, but it's not a sound combination — a rule-of-thumb system CFM wasn't derived from the same load calculation as your room-level Manual J figures, so the resulting proportional split doesn't represent a real engineering relationship. For a meaningful room allocation, use an actual system design airflow (from the equipment's blower table or a completed Manual D) alongside room loads from the same Manual J run.

Question 10

Does this calculator replace a Manual D duct design?

Accepted Answer

No. This calculator produces CFM targets — what each room or branch should receive. Manual D (or an equivalent duct design method) takes those targets and determines actual duct diameters, routing, and fitting selection to deliver them within acceptable friction and velocity limits. CFM sizing and duct sizing are two separate steps in the same overall design process.

Question 11

What does 'governing season' mean for a room, and why does it matter?

Accepted Answer

Each room in Room Allocation mode gets two CFM figures — one from its cooling load share, one from its heating load share — and the branch duct/register needs to be sized for whichever is larger, since it has to handle both seasons. The 'governing season' just identifies which of the two is larger for that specific room; a room that runs hotter in summer than it runs cold in winter (or vice versa) can have cooling govern in one case and heating in another, even within the same house.

Input	Value
Sensible load	25,920 BTU/hr
Temperature difference	20°F

Step	Calculation	Result
Denominator (1.08 × ΔT)	1.08 × 20	21.6
Design CFM (exact)	25,920 ÷ 21.6	1,200 CFM
Design CFM (rounded up)	ceil(1,200)	1,200 CFM

Cooling Airflow	Typical Application	Important Check
350 CFM/ton	Humid climate, more dehumidification	Confirm coil and blower support this target
400 CFM/ton	ACCA baseline, common starting point	Verify against the blower table at design static pressure
450 CFM/ton	Dry climate, higher sensible emphasis	Check moisture removal is still adequate
500 CFM/ton	Some hot-dry applications only	Needs the matched equipment's own manufacturer data

Application	Typical ΔT Range
Residential cooling (supply vs. room)	18-22°F
Residential heating, gas/electric furnace	35-55°F
Residential heating, heat pump	20-35°F

HVAC Airflow / Room CFM Calculator (Load, Equipment, and Room Airflow)

Inputs

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Knowledge Guides — 105 Articles

What Is an HVAC Airflow / Room CFM Calculator?

Three modes, three different starting points:

HVAC Airflow Formulas

Sensible Load to CFM

Equipment Capacity to CFM

Room-by-Room Allocation

Worked Example

Step-by-Step Calculation

HVAC Airflow Reference Tables

Typical Design Temperature Differences (ΔT)

When to Use This Calculator

Airflow Design Tips

Common Mistakes

Limitations

FAQ