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Basic Ductwork Guide: How to Design Your Duct System

Updated: Jun 27, 2023

Residential Duct Starter Guide: Everything You Need to Know


Want to DIY your duct? Have a room that's always too hot or too cold? In this guide we explain how to size a HVAC unit to fit your home, design a matching duct system, and build a material list. This guide includes these rule-of-thumb resources: a Unit Sizing Chart with sqft per ton and CFM per ton, Duct CFM Chart with round duct to square duct conversions, and example system, as well as links to calculators to help with final figures –everything you need to know to correctly size a residential HVAC system.


The Ultimate Residential HVAC Guide

A freshly installed duct system in an attic

Determining the correct tonnage HVAC unit and designing a residential duct system hasn't got to be complicated. In the guide we will break the process down and walk you through the steps necessary to design efficient climate control solutions. For you convenience we have included links to online calculators and provided rule-of-thumb figures based on over a decade of HVAC experience that reduce most calculations to simple arithmetic, and will cover frequent mistakes people make that can be avoided.


HVAC system design made easy.


Table of Contents:

 

Basic Ductwork Guide: How to Design Your Duct System


Ductwork is the network of pipes, commonly called duct, that connects your HVAC unit to the vents in your rooms. It is responsible for distributing the conditioned air evenly and efficiently throughout your home, while also removing stale air and contaminants. A well-designed duct system can improve your HVAC performance, efficiency, comfort, and air quality. In this guide, we will show you how to design your duct system step by step.



What You Will Need


Before you start designing your duct system, you will need some tools and materials. Here is a list of what you will need:


  • A tape measure

  • A pen and paper

  • A duct chart




How to Design Your Duct System


Designing your duct system involves several steps such as sizing your HVAC unit, choosing your duct material and size, calculating your airflow requirements, laying out your duct system, and building a material list. Here are the steps to follow:


Size your HVAC unit


The first step in designing your duct system is to determine the appropriate size and capacity of your HVAC unit. This depends on the square footage of your home and the climate and weather conditions in your area. You can use this online calculator to help you: https://www.calculator.net/btu-calculator.html Alternatively, you can use this simple rule of thumb: 1 ton of air conditioning capacity for each 600 square feet of floor area, rounded up. For example: An 1300sqft home would need a 2.5 ton unit. Take care when sizing your unit as too small a unit will not be able to keep up in extreme weather, whereas too large a unit will suffer from short run times. While short run times may sound ideal, the unit needs at least 15 - 20min average run times to have a meaningful impact on the humidity levels in a home.


Choose your duct material


The next step is to choose the type of material for your duct system. There are different types of duct materials available such as metal or flex; and each has its own advantages and disadvantages in terms of durability, cost, installation ease, maintenance requirements, and energy efficiency. For example metal duct is strong, durable, fire-resistant, and easy to seal. However, they are also more expensive, heavier, noisier, and prone to heat loss or gain. Flex ducts are lightweight, inexpensive, quiet, and insulated. However, they are also more fragile, difficult to clean, and susceptible to mold growth. You can use this online guide to help you choose the right duct material for your home: https://www.lowes.com/n/buying-guide/how-to-choose-ductwork



Choose your duct size


The next step is to choose the size of your ducts based on the airflow requirements of your HVAC unit and the length of your ducting. The size of your ducts affects how much air can flow through them and how fast it can travel. If your ducts are too small, they can restrict airflow, cause pressure drops, reduce efficiency, and create condensation. If your ducts are too large, they can reduce airflow velocity, cause uneven distribution, and increase energy waste and even cause condensation in your attic or crawlspace. To choose the right duct size for your home, you need to consider the capacity of your HVAC unit, the number and location of your vents, and the friction loss caused by bends, fittings, and obstructions in your ducts. You can use this online calculator to help you determine the optimal duct size for your system: https://www.engineeringtoolbox.com/duct-sizing-d_40.html Alternatively, you can use this chart below as a reference:


Unit Sizing Chart:

A HVAC unit sizing chart showing CFM per ton, sqft per ton, and the recommended supply and return sizes for different HVAC capacities and CFM ranges
HVAC Unit Sizing Chart with CFM per ton and starting supply and return size

Notice that the CFM values in the above table are listed as a range. Whether your system flows at the minimum or the maximum of this range is dependent on the system’s friction loss as well as the equipment chosen. Friction loss is the amount of pressure drop that occurs in a duct due to the resistance of the air flow against the duct walls, bends, fittings, and obstructions. In order to minimize friction loss you can: select a duct material with a lower friction coefficient such as metal, use a larger return, avoid sharp or unnecessary bends in the duct, take the most direct route, use metal duct.


Calculate your airflow requirements


The next step is to calculate how much airflow each room in your home needs based on its square footage, ceiling height, insulation, windows, and external walls. This will help you determine how many vents you need in each room and what size they should be. You can use this online calculator to help you calculate the airflow requirements for each room: https://www.engineeringtoolbox.com/air-change-rate-room-d_867.html Alternatively, you can use this simple rule of thumb: 1sqft per 1 CFM. However, this assumes a 8ft ceiling height, normal insulation (R-4 to R-12), just a few windows, and no more than one external wall. Whenever possible vents should be placed near the windows and doors on the exterior walls. This practice is commonly called washing the windows. Conversely, the return should go as close to the center of the system as possible, in an open area not exposed to excessive humidity or grease.



Duct CFM Chart:

A duct sizing chart, also known as a CFM chart, showing the CFM capacity of different sized square and round duct
A duct sizing chart, also known as a CFM chart, showing the CFM capacity of different sized square and round duct

Design your duct layout


The next step is to design your duct layout based on the floor plan of your home, the duct size, and the airflow balance. Your duct layout should be simple, direct, and balanced. This means that you should avoid unnecessary bends, branches, or splits in your ducts that can cause turbulence, noise, and energy loss. You should also ensure that your ducts are properly insulated and sealed to prevent heat loss or gain and air leaks. Additionally, you should make sure that your ducts have enough supply and return vents to provide adequate airflow and maintain pressure balance in your system. You should also place your vents strategically to ensure even distribution and avoid hot or cold spots in your rooms. Placing vent in front of windows is considered good practice and is commonly referred to as washing the windows. You can use this online tool to help you design your duct layout: https://www.ductworkinstallation.com/DuctworkDesign.aspx Alternatively, you can use this simplified duct system pictured below as an example:


Example Duct System:

An example Duct System Diagram
Example 2.5 ton Duct System Diagram

Sizing for a 2.5 Ton unit, we start with a 16" supply at 875 CFM -1125 CFM. We then take a 7" and 5" run (-200 CFM) off of this section bringing the remaining total to 675 CFM - 925 CFM. A drop from 16" to 14" in now required to keep pressure and velocity up. With the 14" supply installed, we take three 6" runs from it (-300 CFM). This brings the total down to 375 CFM - 625 CFM and we are ready to reduce from 14" to 12". With the 12" supply installed we take two 7" runs (-300 CFM) and two 6" runs (-200 CFM) off the remaining trunk for a final total of -125 CFM - 125 CFM. It is ideal to leave between ~100 CFM and ~250 CFM in a duct system as back-pressure on most systems. The last 1' - 2' of the trunk line receives no taps, this helps the system pressurize. To ensure proper airflow, this system is then equipped with a 16" return capable of flowing at 1300 CFM - 1500 CFM.



Determine Your Materials List


The final step is to determine what materials are needed to complete the job based on your planned duct system.


One run requires:


  • R-AT (flanged duct connection with a rounded face to attach to round trunk lines) or,

  • AT (flanged duct connection with a flat face to attach to square trunk lines)

  • R-AT or AT damper (required by building codes in some locations)

  • bag of flex per 25' of run length or,

  • adequate length of metal pipe (sold in 5' lengths) with insulation

  • floor boot or ceiling boot (the register air comes out of)

  • grill or diffuser


Your supply will require:


  • adequate length of metal and insulation

  • any necessary reducers

  • square to round unit adapter

  • end cap


Your return will require:


  • bag of flex per 25' of return length or,

  • adequate length of metal pipe with insulation

  • sheet of galvanized metal

  • AT the same size as your return

  • square to round unit adapter

  • return grill


Other Materials:


  • duct tape

  • aluminum foil tape

  • flex or metal hanging strap

  • 1/4" sheet metal screws

  • 2" sheet metal screws

  • insulation


Notes:


Metal pipe with diameters of less 14" or less are sold in lengths of 5', while diameters of 16" or larger are sold in 3' lengths. Reducers are generally .5' - 1' long depending on brand. While insulation values of R-4, R-6, or R-8 are generally selected you should consult with your local building codes to determine what is required in your area. If you plan on using flex for sections of your supply be sure to include those in your figures.



Final Thoughts


Ductwork is an essential part of your HVAC system that affects its performance, efficiency, comfort, and air quality. By following these steps, you can design your duct system to meet your needs and preferences. Always check your local codes before starting any construction project.


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Glossary of Terms


HVAC - stands for Heating, Ventilation & Air Conditioning


Unit - the heating and air conditioning system


Ductwork - the network of pipes, commonly called duct, that connects your HVAC unit to the vents in your rooms


Supply - the primary pressure or output duct of a duct system, also called the trunk-line


Return - the suction or input duct of a duct system


Run - a secondary pressure or output duct of a duct system


Boot - the floor or ceiling register that air exits from


AT - flanged duct connection with a flat face to attach to square trunk lines, also called taps


R-AT - flanged duct connection with a rounded face to attach to round trunk lines, also called taps


Flex - flexible vinyl lined fiberglass insulated duct with aluminized heat shield


Flex Hanging Strap - cloth, generally nylon, hanging strap for flex duct


Metal Hanging Strap - used when hanging metal duct, commonly called plumbers strap


CFM - cubic feet per minute


Ton - one ton is equal to the heat absorbed in melting 2,000 pounds (1 ton) of ice in 24 hours or 12,000 BTU/hour


Washing the windows - the practice of placing air vent above or below windows on the outer wall


Sheet metal screw - used to connect pieces of sheet metal, also called zip screws

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