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The Art and Science of Designing and Installing Home Ductwork

Ductwork can make or break your home HVAC system. In this article, we discuss best practices for designing and installing proper ductwork in a home.

The Art and Science of Designing and Installing Home Ductwork

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Joshua Rodriguez


May 4th, 2021

We educate our customers on a lot of HVAC topics, but the one where we find the most misconceptions exist is with ductwork.

Why is this? The answer seems obvious: when people think of HVAC service, they think of the main equipment. The air conditioner, heat pump or furnace. They’re not thinking about ductwork.

You need lungs to breathe, but also an entire network in your body to carry oxygen to your body’s cells. Forgetting to consider ductwork is kind of like thinking the lungs are doing all of the work on their own to oxygenate your body.

That comparison may seem dramatic, but I think it’s more appropriate than some people realize. Improper ductwork is responsible for many more comfort issues than people realize. In fact, I’d estimate about 9 out of 10 jobs we complete include some form of ductwork modification, because the existing ductwork is rarely without flaws that affect a home’s airflow and, ultimately, its comfort.

This article is the “why” and “how” behind that stat. Understanding why ductwork matters will help you make the right long-term decisions for your home.

Why You May Need New Ductwork

None of this matters if it doesn’t affect your home. But a lot of times when you are tackling a new HVAC project, it will affect you. What are the reasons for this? It depends on the household, but some common reasons are below:

  1. The previous system was improperly sized. We’ll talk more about sizing a system below, but often the calculation is done incorrectly that determines how much power your system needs to adequately heat and cool your home.
  2. You have added a room or area to the home since the last system was installed. Your HVAC system could be as much as 20-30 years old. Since then, maybe you  (or the previous owner) added a covered patio that receives air, or maybe you installed a new four-seasons room. Some houses have had entire wings added in previous decades! In these cases, the ductwork will not be able to handle the needs of this new configuration.
  3. You lack ductwork (baseboard, window or alternative heating/cooling). This is the largest job of the bunch, but we’ve handled many jobs like this. If you have never had a ducted HVAC system in your home, an entire system of ductwork will need to be designed and installed.
  4. You have issues in specific rooms. Is your upstairs too hot? Is one area consistently colder than the rest of the home? Often, this isn’t an air conditioner or furnace issue. It’s a problem with ductwork.

RELATED: Why Is My Upstairs So Hot? Solutions for Your Second Floor

Sizing Your System

There’s an (unfortunate) norm in the HVAC industry. It goes like this:

  • Check the square footage of a home
  • Decide what size air conditioner or furnace you need.

This is wrong. And it often includes ignoring ductwork.

For reference, when I talk about “size,” what I’m really talking about is how powerful your system is. And when we talk about the power of a system, it’s usually in tonnage (which relates to airflow and pressure). So a 1-ton system is less powerful than a 4-ton system, but will also cost less. A larger home will require more power to heat and cool than a smaller one. But it’s not ONLY house size that matters.

For starters, the number, size and quality of the windows matter. Old, single-pane windows will leak air, for example.

Insulation also matters. So does ceiling size, and how many stories your home has. And many others.

The other thing this affects is ductwork. Let’s say you have a 2-ton system, but what your house really needs is a 2.5-ton system. The ductwork will be sized for a 2-ton system. And if either the system or the ductwork are improperly sized, you won’t get the necessary airflow to efficiently heat and cool your home.

This is the crux of the problem with sizing. It’s the problem I see the most in homes, and the next most common problem isn’t even close. So the lesson is clear: make sure you’re working with someone who is checking ductwork and doing a full load calculation to determine proper sizing.

A Comparison to Roads

Everyone hates a traffic jam. Or if someone likes them, I have yet to meet that person.

Ductwork that is too small is like a traffic jam.

How do you fix traffic jams? One way is to increase the number of lanes on a road. A major city needs gigantic, 7-lane highways. A small, rural town out in the country probably needs a single 4-lane road at the town’s center, but the rest of the roads are single-lane.

Ductwork is the same. And, for example, having a 3-ton air conditioner and furnace pushing air through a 2-ton ductwork system is like closing down the highways in a major city. The cars (and air) need more room to travel smoothly.

And what’s the result if you ignore this? Increased utility bills costs, and your system will run for a lot longer, and you still won’t get great airflow to certain parts of your home.

Installing new ductwork

The Science of Ductwork Design

We don’t get to proper ductwork sizing by accident.

Let me introduce you to SMACNA! This rather funny-sounding acronym stands for the Sheet Metal & Air Conditioning Contractors’ National Association. They create and test standards for ductwork design that help to inform installation processes across the industry.

SMACNA even provides various tools and apps to help with these tasks, since the calculations can become very intense.

This is similar to the load calculation standards created by ACCA (Air Conditioning Contractors of America). This is how the proper size is determined for your HVAC system.

In practice, what kinds of considerations do these standards include? I’ll give you a few examples:

  1. If a room is a particular size, it will need a certain amount of airflow running to it, which then relates back to the air being moved through the whole system and that individual room’s duct line.
  2. The trunk line is the main duct line running off of your furnace. To stick with the tree analogy, the “branches” are the smaller duct lines that run to individual rooms and ventilation grates. A trunk line must be able to house the necessary airflow, but it also has to stay within certain height and width parameters. The maximum acceptable height-to-width ratio for the trunk line is 4:1 (for example, 8 inches tall, 32 inches wide), but usually it’s safer to get closer to a 2:1 or 3:1 ratio. A 24x8 inch trunk line is typical in many homes, for example.
  3. Turns and bends need to be handled with care. You generally can’t run directly into a T-shaped 90-degree turn, for example, or you’ll mess with the internal pressures that the system has to maintain. Depending on the rate of airflow, turns have specific recommended parameters to ensure proper airflow is maintained at all times.

RELATED: HVAC Ductwork: Best Practices for Cleaning, Modifying and Care

The Importance of Return Air

So far we’ve only talked about air going from the blower fan in your furnace and into your home. But what about the air that returns to the system?

This is often forgotten in ductwork design. The key idea is that heating and cooling isn’t just about sending air into the home, but about creating a circulating airflow pattern that removes untreated air in your home and returns it to the central unit to be heated or cooled. If this circulation doesn’t happen, you’ll never be fully comfortable.

If a room lacks a return air duct, for example, it doesn’t matter how much cool air you pump into it during the summer. It’s going to struggle to stay cool, and will often be muggy and uncomfortable.

So a lot of times, the ductwork running from the furnace to the home is fine, but it’s the return air ducts that are severely lacking. The single most common ductwork modification we make is to install a larger return air drop (the main return line connected back to your furnace) so that proper circulation can take place in a home.

The Art of Ductwork Design

Homes aren’t built the same, so why should ductwork all be the same? This is where the science meets art.

Let’s say 10 years ago a room was added to the home, and now you’re replacing the HVAC system. Proper ductwork is rarely installed in such cases, so we’re often having to work around the existing construction to figure out how to get the proper ductwork to the new area.

Those standards and recommended parameters I mentioned earlier still apply, but we’re often having to work in tight, cramped spaces that weren’t constructed with ductwork in mind.

There’s always a solution, but sometimes it’s not obvious. So a slapdash job that doesn’t take the time to carefully design your ductwork system will, in the end, be a lot less efficient and a lot less comfortable.

So when we say we build custom solutions for homes, that’s not just a tagline. HVAC isn’t “one size fits all,” so why should your system be like everyone else’s?

Air vent in a home

A Comparison to Tires

Where do you look to see what pressure your car’s tires should be at?

Many will answer “on the tires themselves,” and yes, a recommended or maximum PSI (pounds per square inch) will be listed

But this is wrong. Or rather, it has the potential to be wrong. Those tires were created for a bunch of different brands and models of cars, which all have different weight and sizing requirements. So the correct place to look is usually on a sticker that’s on the inside of your driver-side door. And it’s correct because it’s specific to your car.

Companies that offer the same solutions for everyone are only looking at the tires to gauge pressure, so to speak.

Things like air pressure, static pressure, the cubic feet per minute (CFM) that are moved through your ductwork...all of this should be different for you than it is for your neighbor.

With your car, the long-term risks include your alignment being thrown out of whack, or your car slowly becoming less reliable because it wobbles slightly or isn’t properly balanced across years and 10s of thousands of miles.

With your HVAC system, it’s pretty much the same. That repair you’ll have in year 10 is probably preventable, but only if you’ve had a carefully calibrated system for the last 10 years.

Ductwork Expertise vs. “Good Enough”

There’s a trick I like to do when I’m in a home. I check out the ductwork to make sure I understand exactly how it’s functioning. Then, I ask questions: Does this room always seem to be humid in the summer? Is this room hotter than the rest of the home in the summer and colder in the winter?

The answer is usually “yes,” and many homeowners marvel at my ability to predict their problems.

But it’s not magic. It’s simply having a good understanding of how airflow works, and what kind of problems it can lead to when the ductwork is insufficient.

We train our sales team to look out for these problems too. This is important, because it means when we inform you of the work that we recommend, it’s taking the entire system into account. We want you to love your home HVAC system. Ductwork is a necessary part of that.

The downside, of course, is that if another contractor skips this step, their quote will be cheaper. So a lot of homeowners look at the price tag and decide that their existing ductwork is fine.

And yes, it’s fine. It’s good enough to function. But if you have issues with your ductwork, your HVAC system will never be great. This means less comfort. Higher energy costs. And a shorter lifespan for your equipment, leading to things like expensive repairs in the first 5-10 years.

The Cost of Home Ductwork

We like providing price ranges in our articles here at Fire & Ice. We believe in transparency. But this is a difficult one, because “ductwork modification” can be very small, very large, or somewhere in between.

At the low end, you have the replacement of your return air drop. Including installation fees, you’re looking at somewhere between $200-$400.

A new or heavily modified trunk line can range between $1,000-$1,500.

And if you are adding brand new ductwork throughout your entire home (either to replace the old ductwork or because you don’t currently have ductwork), in a typical home it can range as much as $10,000-$12,000.

That last range is rare, but if you’re going from nothing to a full system, it’s a reasonable expectation when accounting for parts, design and labor.

ductwork materials

Next Steps

If you need new or modified ductwork, you probably know it. Or at least you suspect that it may be a problem. Maybe it’s because your airflow is noticeably lower in some rooms. Maybe it’s because you’ve added a new room.

Regardless, good ductwork will help.

We’ve trained our entire team to be able to assess your ductwork and make recommendations that will get you the most for your investment. If you’re ready for that assessment, there’s no time like the present. Give us a call or type in your zipcode below to get started, and get your free in-home estimate.

Start working toward optimal comfort today! We’re looking forward to helping you achieve that.

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