This information is aimed at the homeowner who wants to fully understand the mechanical systems in their home – in this case, the ducts, their purpose, proper design and cost.
There’s a glossary of terms you might find useful too. It will prove useful for DIY installation too, though we don’t recommend you do it yourself. It requires more technical knowledge than a guide this length can contain – and a wealth of experience too.
You’ll benefit from this HVAC ductwork guide if:
>You’re building a home or addition and plan to install or replace a ducted system
>You’re comparing ducted split systems to your options that don’t require ductwork:
Here is a quick navigation of this article:
The Purpose of Ductwork
The ducts in your home are the channels through which heated or cool air is dispersed to rooms and zones of your home and then returned to the air handler or furnace to be treated again.
This requires two sets of ducts in most homes.
Supply ducts lead from the furnace or air handler to the grilles where they disperse conditioned air pushed there by the blower. This is why traditional split systems are called “forced air” systems.
The affect of air being pushed through the supply ducts is non-conditioned air being pulled into the system through the return grates and ducts.
Some small homes, especially mobile/HUD homes, do not have return ducts. Air is pulled directly into the furnace or air handler from nearby rooms.
Ductwork Glossary
If the names for components of a ductwork system aren’t familiar, this glossary explains them. Knowing the terms will assist you in discussing your ductwork with an HVAC contractor.
Terms are listed in logical order rather than alphabetical.
Conditioned air: Air that has been heated or air conditioned and dehumidified. Conditioned air is also called treated air and supply air.
Air handler: The unit that pushes air through the system of ducts. Furnaces are used in systems with or without central air conditioning. Air handlers are used in heat pump systems. For our purposes in this article, the term air handler can refer to either.
Blower fan: Furnaces and air handlers have a powerful fan known as the blower. Its purpose is to circulate air through the system, pulling in non-conditioned air and pushing out conditioned air.
Ducts: There are several types, but two are most common: Galvanized steel sheet metal ducts and flex ducts made from wire and plastic. Ducts carry air to and from the furnace or air handler. Metal ducts are rectangular, round and oval. Spiral ducts are a type of round metal duct. They are aesthetically better looking than plain metal ducts, so are used in open areas where the ductwork is visible. Flex duct is round and often covered with R6 or R8 insulation.
Supply and return ducts: Supply ducts carry conditioned air to living spaces in your home. Return ducts carry non-conditioned (spent) air back to the system where it can be heated or cooled/dehumidified.
CFM: Cubic feet per minute. Every blower fan is rated for how many cubic feet per minute of air it can push. Ductwork is also rated for CFM. The larger the duct, the more air it can carry. To have adequate airflow and even temperatures throughout your home, the ductwork must be properly sized – the trunk, branches or all radial arms – for the amount of air that will flow through it.
Dampers: Manual dampers allow you to reduce or close off air flow to a room or zone. In zoned systems, the dampers are controlled electronically.
Joist panning / panned joists: These are sections of sheet metal attached to the bottom of two or three joists in the basement to produce channels through which return air can flow to the HVAC system. They use less material and take less labor to build than enclosed ducts, so they cost less.
Warning: Panned joists were once standard and are still used frequently. However, they are infamous air leakers when not properly installed and sealed. The money saved in using theminstead of enclosed ductwork could be lost in one to two years of wasted energy.
Discuss this issue with your HVAC contractor before allowing the use of joist panning. This article from the International Association of Certified Home Inspectors (InterNACHI) is loaded with useful information on the topic.
Supply and return grates/grilles/openings: Supply grates are the endpoints for supply ducts. Return grates provide the openings through which untreated air is pulled back into the ductwork and system. Both are commonly installed on floors, walls and ceilings depending on the design of the HVAC system and ductwork. Optimal design calls for the same number of supply and return grates.
Trunk and branches: The trunk is the largest duct carrying conditioned air from the furnace or air handler. It typically runs most of the length of a home. Like a tree trunk, branches run off the trunk to individual rooms and zones. The trunk is also called the trunk line. Endcaps are installed on each end of the trunk.
In climates that use more heat than AC, the trunk and possibly the entire HVAC system is located in the basement or crawlspace, since heat rises. In warm climates where AC is used most, the trunk/system is usually located in the attic, since cool air naturally sinks.
Plenum: Made from sheet metal, the plenum is the connection between the air handler and the trunk or where arms are attached in a radial system (explained in the next section).
Return air drop/connection: This is the return air equivalent of the plenum. Built from sheet metal, this component connects the return ductwork to the air handler. In basement/crawlspace installation, the return air drop connects near the bottom of the air handler. Return air is conditioned and forced by the blower through the plenum and into the trunk.
Takeoff and boot: For each branch, a hole is cut into the trunk, and a fitting is installed called a takeoff. It’s branch/trunk connection. A boot is a sheet metal connection between the branch and the air grate.
Duct reducer: Near the end of the trunk line, the width of the trunk might need to be shrunk to ensure pressurized air flow as the air gets further from the blower fan.
Wall stacks: These are ducts installed between studs in walls. They are used to carry air to and from upper floors.
Insulated ducts: When ductwork travels through space such as an attic that isn’t heated and cooled, it should be insulated to prevent energy loss.
Total Effective Length (TEL): This is the length of all the duct runs in a home plus the equivalent for all fittings. TEL is used when determining duct airflow requirements.
Ductwork Design: Things to Consider
Here’s an overview of planning ductwork. As we’ve said, having basic knowledge of the process ensures the best outcome for the homeowner.
Manual D: The first step to properly designing a duct system is to use Manual D. This manual from the Air Conditioning Contractors of America is the tool contractors use to calculate correct ductwork sizing for your home and HVAC system. The goal is to distribute air evenly, so indoor temperatures and humidity are balanced.
Manual D factors the location of the air handler, distances required for air to flow and the cubic feet per minute (CFM) of treated air needed in each room or zone.
Some HVAC contractors wing it without doing a Manual D calculation. If they’re very experienced, they might get it right. When they get it wrong, the home has hot/cold rooms because not enough or too much conditioned air is getting to them, enormous amounts of energy are wasted – along with money on energy bills higher than they should be.
Good ductwork design:
1). Delivers conditioned air to living spaces using the shortest routes possible to minimize heat loss from heated air and the heating of air-conditioned air. In short, compact design is best, with the HVAC system air handler centrally located.
2). Has the same number of return air grates/grilles as supply grates/grilles. This ensures proper airflow balance. Where this isn’t practical, then transfer grilles and/or jumper ducts should be installed in walls between rooms to allow for airflow between them when the HVAC system is running – pulling in non-treated air.
3). Uses expansion collars. Sheet metal expands and contracts as it heats and cools during HVAC cycles. If the trunk is too tightly fitted to the plenum, the expanding and contracting makes a “boom” noise. This is eliminated by installing expansion collars at the connections. Don’t let your HVAC installer cut this corner to save a few bucks.
4). Gives ducts room. Ducts installed between joists, in wall cavities and near pipes should have an inch on all sides to allow for expansion during heating. This avoids contact with surrounding features that can cause noise and damage to the ducts. A damaged duct is a leaky duct.
5). Includes sealed ducts. Metal ducts should be secured with sheet metal screws. All seams should be sealed with mastic and metal tape. Poorly sealed ducts waste up to 20% of conditioned air, according to the US Department of Energy.
Think of 20% of air-conditioned air escaping the ductwork in the basement, where it’s already cool, or into the attic where it does no good. The remaining 80% won’t be enough to adequately cool your home, causing your AC to run too much, leading to premature mechanical failure.
The image above explains how to test your current home to determine if your ducts are leaking.
6). Considers the radial duct alternative. Radial ducts are like spokes of a wheel. There is no trunk in a radial design. Instead, individual ducts extend from the plenum to carry conditioned air to each room or zone. They offer excellent airflow balance for even temperatures. There are also fewer duct seams, so less potential leakage. The plenum is often larger or extended on a radial system to accommodate the ducts.
Radial systems increase the efficiency of zoned HVAC systems when the damper door is placed near the plenum. Little conditioned air is wasted.
Ask your HVAC contractor if a radial system design would be better for your home. Factors to consider are home design and the HVAC system’s design and performance. Most radial systems use insulated flex duct. Keep in mind that flex duct is not considered as durable as sheet metal ductwork.
7). Insulates ducts in unconditioned space. Attics are a major cause of wasted heat in winter and air-conditioned air in summer if the ducts aren’t insulated. Air-conditioned air is about 58 degrees Fahrenheit. It will rapidly heat up in an attic that’s 115F, if the ducts aren’t insulated. The same is true for air from the furnace at 125F if ductwork runs through an attic that’s 20F.
The code in most places calls for R6 or R8 insulation for ducts in attics. We recommend you talk to your ductwork installer about doubling that to R12 to R16. The cost will be minimal, and you’ll recoup it in a single year or two through lower energy costs.
Note: Ductwork shouldn’t be run in unconditioned space unless absolutely necessary. If your contractor plans ductwork in an attic or unheated crawlspace, ask if there are alternative options.
8). Might not be complete without duct inspection and testing. The integrity of the ductwork should be verified. Looking and listening can find obvious leaks. Air pressure can be tested with a blower door test, which can also help locate small leaks. A blower door is placed in a main entry door of your home. One or more powerful fans in the door push air out of your home. When the indoor pressure drops, air is drawn in wherever there are gaps in your home’s envelope. Air will also be pulled out of ducts, identifying where they are leaking.
Ask your contractor if a blower door test is recommended.
The blower door test can also help identify places where air is leaking in and out of your home through drafty doors and windows, poorly caulked door/window frames, around outlets and other locations. A blower door test followed by necessary repairs and upgrades is a great way to maximize the energy efficiency of your home and lower your energy costs.
HVAC Ductwork Cost
This table shows you where your money will be spent.
HVAC Ductwork Cost By Types
| Duct Types | Cost Per Foot | Common Sizes | Uses |
|---|---|---|---|
| Duct-Rectangular | $6.25-$11.50 | 8x16 to 10x24 | Trunk, large branches |
| Duct-Round | $2.50-$12.85 | 3" to 18" diameter | Trunk, large branches |
| Spiral | $2.35-$13.50 | 3" to 24" diameter | Trunk, branches, exposed ducts |
| Duct-Oval | $2.15-$4.45 | 3x6 to 4x8 | Branches, tight areas |
| Duct-Flexible | $0.80-$2.65 | 4" to 20" diameter | Branches |
| Duct-Flexible insulated | $0.95-$5.65 | 4" to 20" diameter | Branches in unconditioned areas |
| Wall stacks | $4.80-$6.85 | 2.5x10 to 3.5x12 | Suppy & return in walls |
| Plenums | $48-$160 | 16x20 to 24x48 | |
| Cold-air drop kits | $115-$150 | 20x8 to 25x16 |
HVAC Ductwork Accessories Cost
| Accessories | Cost | Common Sizes | Uses |
|---|---|---|---|
| Takeoffs | $8.00-$12.50 | 4" to 10" | See Glossary |
| Boots | $6.00-$18.75 each | 4" to 14" | |
| Dampers | $5.50-$13.00 | 4" to 10" | |
| Joist pan | $1.50-$2.35/foot | 12" to 24" wide | |
| Various fittings | $3.35-$12 each | 4" to 20" | |
| Tools | $115-$225 | N/A | |
| Duct sealing materials | $85-$150 for most homes | N/A |
Every job requires some large duct for the trunk and branches serving big rooms or zones. And they use smaller duct for branches to small rooms and zones. Some duct systems are built with a blend of sheet metal, flex and panning.
Then, there are the required accessories, which can add up too.
Total ductwork materials cost will be $4.85 to $9.50 per linear foot of supply and return ductwork in your home.
The type and size of ductwork will be the main factor in cost.
The Sample Home Costs section below will help you get an estimate for your house based on its square footage and layout.
HVAC Ductwork Installation Cost
Here’s what you could save by doing the installation yourself:
Ductwork installation cost is $5.35 to $7.75 per linear foot.
These costs are for homes during the framing stage. If the home is finished, and drywall, drop ceilings and other finished components must be removed/replaced to install ducts, the cost will be two to four times higher.
Those are the costs for hiring a professional HVAC contractor.
The potential costs of DIY ductwork are installing the wrong size for your HVAC system and home’s airflow needs, placing ducts and grates in non-optimal locations and not properly sealing the ductwork. Leading duct manufacturer Snappy sums it up: “Ducts that are not well-designed result in discomfort, high energy costs, bad air quality, and increased noise levels.” Those are risks most homeowners don’t want to take.
Related Article:
HVAC Installation Cost – What’s the Fair Price For New HVAC Systems in 2018
New Furnace Replacement Cost and Installation Cost
Sample Home Ductwork Costs
Here are the key numbers used to determine how much your ductwork might cost:
- $10.20 to $17.25 per linear foot: Cost range for materials and labor combined
- 190 and 350 feet of ductwork: Average range of ductwork in homes 1,800 to 3,500 square feet.
- $1,938 to $6,037: Potential total ductwork cost in homes that size.
- $4,200: Average total cost in homes that size.
Remember: These costs apply to most jobs when ductwork is installed during framing of the home or addition. As we noted, retrofitting an existing home with ducts costs as much as three times more in total expenses. In such homes, a mini split system, aka ductless system, is a far less expensive choice.
Now, here are two real-life samples of ductwork design and cost:
Example 1: 2,750 square foot two-story
- HVAC system location: Basement
- Duct system: Trunk, branches and wall stacks
- Duct type: Sheet metal
- Total length of supply and return ductwork: 315 feet
- Materials: $2,598
- Labor: $1,921
- Total cost: $4,519
Example 2: 1,800 square foot ranch/single-story
- HVAC system location: Attic
- Duct system: Radial
- Duct type: Insulated flex
- Total length of supply and return ductwork: 255 feet
- Materials: $1,440
- Labor: $1,632
- Total cost: $3,072
Why DIY Ductwork is a Risk
The main problems is improperly sized ductwork for the HVAC system and home. This results in poor airflow and rooms that are hotter or colder than others. Ducts that are too small are noisy and spread dust and other allergens rapidly through a home. Pressure that is too high causes duct damage and leaks that waste energy and money. Moisture condenses in too-small ducts during AC cycles, and moisture leads to mold.
Ducts that are too large don’t have enough pressure to effectively get treated air to rooms and zones distant from the blower. Your HVAC system will work overtime and have mechanical problems before it should. Air conditioning cycles will do a poor job dehumidifying the air in your home.
Those are just issues with size. Bad design causes airflow restrictions and temperature imbalances too.
Proper duct design is part science and part art combined with experience. Our recommendation is that you entrust your ductwork and HVAC system installation to a professional with a proven track record of doing the job right. If you’d like written estimates from top local installers in your area, our referral service is free of cost and obligation. All installers in the service are prescreened, licensed and experienced.
I have a home 1000 sq ft need new a/c and duct work. What size unit and approximately how much for home with 5 rooms. Thanks… Carol
how much more does it cost per square foot to have the duct placed above the joist as compared to below the joist?