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Cost of HVAC Ductwork Modification

Without proper ductwork, your HVAC system won’t have the ability to breath properly and provide your home with adequate heating and cool. Learn the costs in the modification process.

Cost of HVAC Ductwork Modification

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Joshua Rodriguez


January 10th, 2024

When you think about your HVAC system, your thoughts probably go to your air conditioner and furnace. Maybe it includes a whole-home, ducted humidifier, dehumidifier, or other accessories.

Without proper ductwork, though, none of those will function well.

Your ducts are the veins of your HVAC system, and the airflow moving through the ducts is the lifeblood that allows the equipment to properly heat and cool your home.

When problems occur, as they eventually will for any system, no matter how reliable, homeowners tend to look at the A/C or furnace units. Just as often, though, ductwork can be the culprit.

This article intends to do a few things:

  1. Teach you the benefits of good ductwork (and conversely, the perils of bad ductwork)
  2. Talk about what we’re looking for when we inspect the ductwork in your system
  3. Explain why ductwork occasionally needs to be modified along with a new installation of an air conditioner and/or furnace
  4. Walk you through the costs associated with ductwork modification

If you’re thinking about a new furnace or A/C, but your home’s ductwork is damaged or improperly sized, to get the benefit of the new heating and cooling power, you’re also going to need to adjust the ductwork accordingly.

So it’s important to be ready for this, so that you aren’t shocked when there’s more work than anticipated for your next HVAC investment.

Problems Associated With Bad Ductwork

Problems with ductwork can come about because of the age of your ducting, or because it was improperly sized or installed previously.

Duct Leaks

Leaks are probably one of the more self-explanatory problems. They can result in reduced airflow, which is then going to make your system work harder to achieve your desired temperature in any season.

They can also allow contaminants into your airflow, and it may be at a point where they won’t be caught by a filter.

So it amounts to reduced air quality, higher bills, and less comfort. Identifying and fixing these is very important.

Patching leaks can be done in several ways, including with specially-lined tape, heated caulk, or a specific type of paint that seals leaks. These are generally considered among the more minor modifications of your ductwork. If things are bad enough, though, it may necessitate the replacement of a section of ductwork.


It makes intuitive sense that ducts that are too small can be detrimental to proper airflow. But what about if they’re too big?

While this is a rare issue, it’s not unheard of. Older homes with “gravity furnaces,” for example, often have massive ductwork. When replacing these furnaces for newer models, the ductwork generally needs to be gutted to size the ducts for the new equipment.

The downsides to oversized ductwork include the system not running enough to remove enough moisture from the home, uneven temperatures throughout the home, and increased run times for the equipment due to having to move air through a wider path.


It’s easier to understand how ducts that are too small can negatively impact your system. Airflow in an HVAC system is generally quantified in cubic feet per minute (CFM). If a system is unable to achieve the target CFM for the equipment, all kinds of downstream problems can occur related to cost, comfort, and the lifespan of your equipment.

More than anything, though, this chokes your system. If the blood can’t move well through the veins (to borrow the analogy from earlier), the entire system will suffer. And that includes you and your family.

Installation Faults

You’ve probably seen images of octopus-looking ductwork systems that seem to twist and curve in a dozen directions. This is terrible.

This happens for one of two reasons: either the installer didn’t know any better, or they didn’t care.

While that sort of negligence may be an extreme example, something as simple as a turn in a ducted path can be problematic if the ducting is kinked or bent in a way that reduces airflow. This becomes more of an issue in tight spaces that may include crawl spaces or areas inside of floors, ceilings, or walls.

Ductwork in a home.

Necessity of Proper Ductwork

If the section above outlines the problems with bad ductwork, the reverse is true in nearly every case. Benefits of quality ductwork include the following:

  • Even temperatures throughout your home
  • Proper heating and cooling to all floors of the home
  • Thorough moisture control
  • Quiet operation
  • Allows for full efficiency of the HVAC equipment
  • Cleaner air
  • Lower energy bills

Those are the most obvious benefits. Long-term, your entire system, and thus your entire home, will be healthier due to your equipment’s smooth, efficient operation.

When an HVAC representative visits your home, they should be performing a Manual J Load Calculation to ensure that your home is accurately measured for any HVAC equipment.

The ductwork corollary to the Manual J calculation is the Manual D Calculation. This, similarly, takes a variety of measurements to ensure that what you have is exactly what you need. The Manual D takes into account things like the size of your home, potential points of pressure loss within an HVAC system, and formulates the necessary ductwork to support the home.

Another method of calculating ductwork involves the use of a tool called a Ductulator. While that might sound like an instrument of destruction wielded by a malevolent mallard, the truth is less dramatic (but thankfully, more useful). A Ductulator is merely a calculative device that takes into account many of the same parameters of the Manual D to determine duct requirements.

This calculation should be free as part of an in-home estimate with a sales professional, and the results should be discussed with you by that representative at the time of the estimate. By knowing the parameters of your home and ductwork, you are best positioned to assess all of your options.

RELATED: Sizing Your Air Conditioner, Heat Pump and Furnace

Examples of Ductwork Modification

We often learn best by example, so here are a couple of examples of modifications, and why they need to happen in the first place.

Home Addition

You purchased your home, then three years later you added a room addition. This is not uncommon, and often involves turning a porch or patio into an indoor extension of the rest of the home. However, this area, by default, won’t have ductwork being run to it.

Ideally, you’re able to have an individual duct run installed that goes to the new area and terminates at vents that provide air to the room addition.

When this sort of addition happens, it’s important to take stock of the sizing of your whole system. If your airflow is now being split more ways than before, it might not have the proper power to a specific duct run. This can result in hot or cold spots throughout your home.

This will depend on the size of the addition and the size of your existing HVAC system, so it’s worth having a conversation of all your options with a trusted HVAC partner.

Older Home, New HVAC System

Modern HVAC equipment is more efficient than many older systems, but also can require adjustments to older ductwork. An old home with failing or inadequate ductwork may need a complete overhaul.

Another common pitfall is a home that was improperly sized for HVAC years ago. If the entire duct system is too small, for example, it’s going to struggle with modern equipment.

Odds & Ends

Maybe you didn’t add an addition to your home, but since the last time HVAC equipment was installed, several new windows were installed. Something as seemingly innocuous as this can have big implications for the power required to heat and cool your home.

Whatever the particulars, it’s worth having a full assessment to determine your ductwork needs and options.

Ductwork in a utility room.

Cost Factors

A handful of common factors contribute to cost. We run through the major ones below.

Duct Length

If a line needs to be run from your basement to the second-story master bedroom, it can cost several times the amount of a shorter run to a first-floor room. Similarly, this rule can apply to sections of damaged or leaking ductwork.


Metal, hard pipe ductwork is, in our opinion, the most reliable for the majority of homes. Another common style of ductwork is flex duct. Both can be excellent options when installed properly, but flex duct comes with more danger of kinks and twists that can inhibit airflow.

We recommend hard pipe ductwork for one big reason: it is smooth inside, which facilitates airflow. Flex duct has ridges that can (and will) inhibit airflow. Over months and years, this will result in significantly higher energy costs and reduced comfort.

The main benefit of flex duct is that it can be installed more quickly. Flex duct can also occasionally be useful for tight areas, or areas where the ductwork would need to have insulation added. Flex duct comes standard with this insulation, which gives it a leg up in certain situations.

The cost variance between these two isn’t vast, but metal ductwork will tend to run a bit more expensive.


Labor costs should be expected with any HVAC installation. They can be as costly as the equipment sometimes, due to how specialized and complex HVAC work can get.

In the case of ductwork, if a contractor needs to go into or through a floor, ceiling, or wall to maneuver or add ductwork, this is the biggest factor that will contribute to cost beyond the base installation fee.

Cost of Ductwork Modification

Unlike a furnace or air conditioner, ductwork modification isn’t always a larger job. So the range of potential costs will be quite wide. However, below we’ve listed the most common modifications we do, as well as larger ranges for whole-home ductwork installation and a few considerations for determining what your job might cost.

The return air drop is the item we see that most commonly needs to be resized. HVAC systems alter the air pressure in your home, and excess air needs somewhere to go. The return vents in your home facilitate this pressure adjustment, and move air to the return air drop, which is positioned near your furnace or air handler. And as a result of new equipment, return air drops are sometimes too small to accommodate the power of many modern systems.

A return air drop replacement will run around $300-$400, including installation. In rare cases where a second drop needs to be added, the cost can be up to twice that amount.

Sometimes, one or two individual duct runs need to be modified or added, for situations like the room addition we mentioned earlier. Depending on the length and size of the duct, as well as whether or not additional labor is involved to maneuver through floorboards and walls, individual duct runs generally cost anywhere from $300 to $1,200, though this is an average. If the line is very short, or it requires intensive labor through layers of floorboards or walls, this range can widen.

Whole-home ductwork replacement can cost as much as $5,000-$15,000 for larger homes. It’s in these cases that we’ll often look at alternative options with homeowners, including ductless systems.

RELATED: Ductless Mini-Splits 101: What They Do & How You Benefit

Ductwork FAQs

Still have questions? Here are a few common ones…

Can I install ductwork myself?

This is not recommended, as specialized knowledge, tools, and preparation are necessary to execute an installation well.

How long does installation take?

Most jobs are a single day, but larger, whole-home installations can last up to three days.

When should ductwork be replaced?

Other than when it’s improperly sized, ductwork may eventually crack and deteriorate to the point where replacement is advisable. A well-maintained duct system can last well over 20 years, but the exact timing of replacement should be discussed with an in-home consultation with an HVAC professional.

Installing flex duct in a home.

Sized & Ready

What does all of this mean for you? If you’re lucky, maybe nothing. Ideally, your ductwork is properly sized for your home, so that when you’re ready to invest in a new HVAC system, it will integrate seamlessly with the existing ductwork.

However, if this isn’t the case, it’s important to know what the costs and benefits are to matching ductwork to other HVAC equipment.

If you tried to breathe through a straw, it would be very difficult. Similarly, if you took nothing but deep breaths for several minutes, you’d undoubtedly start to feel light-headed. Ductwork is kind of like this with respect to the airflow in your system, and how efficiently it can heat or cool your home.

What can you do to avoid these situations? A few things:

  1. Early in the process of considering a new investment, talk with an HVAC partner to determine if your ductwork is adequate for your needs.
  2. Regular system maintenance is a must for HVAC systems, and this will occasionally include duct cleaning. By removing dirt and grime, you’re freeing up the space for airflow and ensuring the quality of your air.
  3. Understand that without complimentary ductwork, the most sophisticated HVAC equipment in the world won’t function well. The two work hand in hand and must be considered when calculating cost.

With these steps in place, you’ll be prepared for any level of work that may need to be done.

Ready to take one of those next steps? Are you in Columbus, OH (or surrounding areas)? Give us a call. We’d love to hear from you.

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