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Duct Sizing Basics Chart

A pointer to the ACCA Manual D friction rate method for residential duct design and the inputs it needs. No duct sizes or airflow values are given here.

Ref HV-0051ACCA Manual DSMACNA HVAC Duct Construction Standards

Draft, open for review. Check every value against the governing standard and the manufacturer’s data before use.

What it is

Residential duct systems in the US are usually designed with ACCA Manual D. Manual D sizes each duct so the blower can move the required airflow (CFM) through the system with the static pressure it has available. The common approach is the friction rate method. Duct construction (gauges, seams, reinforcement) for metal ducts is covered by the SMACNA duct construction standards. Many codes require a Manual J load and Manual D design for new systems.

How to use it

Run a Manual J load calculation for each room.
Select equipment with Manual S. Note the blower's airflow and its available external static pressure from the fan table.
Subtract the pressure drops of components outside the duct (coil, filter, grilles, registers, dampers) to get the available static pressure for the ducts.
Find the total effective length (TEL) of the longest supply run plus the longest return run, including equivalent lengths for fittings.
Design friction rate = available static pressure x 100 / TEL. The result is in inches of water column per 100 ft.
Use a duct calculator or friction chart at that friction rate and each room's CFM to pick round or rectangular sizes. Check velocity limits in Manual D.

Inputs chart

Input	Where it comes from
Room CFM	Manual J room loads and the selected equipment airflow
Blower external static pressure	Equipment fan table at the chosen speed
Component pressure drops	Coil, filter, grille and damper data sheets
Total effective length	Measured run lengths plus Manual D fitting equivalent lengths
Friction rate	Calculated from the items above
Duct material	Sheet metal, flex or duct board, each with its own friction data
Construction details	SMACNA standards and the adopted mechanical code

Key points

Flex duct must be pulled tight and supported. Sagging or compressed flex has far more resistance than the chart assumes.
High-MERV filters and wet coils raise pressure drop. Use real component data, not guesses.
Rule-of-thumb sizing (such as a fixed CFM per ton or per round size) is not a substitute for Manual D.
Commercial duct design uses other methods (equal friction, static regain) from ASHRAE and SMACNA.

Common mistakes

Using the blower's full rated static pressure without subtracting the coil and filter.
Leaving out fitting equivalent lengths from the TEL.
Sizing returns smaller than supplies by habit.

Governing standards: ACCA Manual D (residential duct design); ACCA Manual J and Manual S; SMACNA HVAC Duct Construction Standards; the adopted mechanical and energy codes.

Reference only. Verify against the current governing code or standard, the manufacturer's data and the project documents before use. Apps for Distributors disclaims liability arising from reliance on this reference.

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