When Retrofitting A/C, Do the Ducts Too?

October 31, 2014
November/December 2014
A version of this article appears in the November/December 2014 issue of Home Energy Magazine.
Click here to read more articles about Indoor Air Quality

When retrofitting A/C systems in hot-humid climates, contractors provide many choices that can help homeowners increase the efficiency, comfort, health, and affordability of their homes. What they recommend about whether to include duct retrofits depends mainly on the situation—whether the system is old, worn out, or malfunctioning and it’s time to replace it; whether there is a specific problem that requires a solution other than upgrading the A/C unit; or whether the upgrade is part of a more-comprehensive energy upgrade, such as a deep energy retrofit or a whole-house weatherization upgrade.

Weighing the options for repairing, reconfiguring, and replacing ducts is complicated. I spoke with three experts to find out what determines the choices a contractor presents to a customer, and to learn some of the merits of those choices.

Duct configurations such as this one in a spider pattern are not efficient and may warrant reconfiguring when retrofitting A/C. (LSU)

A duct leakage test is well worth the money. The test lets you know the level of duct leakage and therefore know how to approach the problem. (DOE Building America Solution Center)

Excessive duct lengths and sharp bends lead to an inefficient air delivery system and should be repaired. (DOE Building America Program)

A compressed flex duct is an example of a poor installation and should be repaired. (LSU AgCenter)

UntitledIn homes that are converting from window A/C units to central air, ducts and duct testing should be part of the plan. (Morguefile.com)

Unplanned A/C Repairs

If the only homeowner complaints are that the energy bills are too high and the equipment is old, the existing A/C system is probably inefficient and needs to be replaced or repaired. Claudette Reichel, professor and specialist in extension housing at the LaHouse at the Louisiana State University (LSU) AgCenter, says that before looking at A/C units that are available for upgrade, you should assess the status and quality of the ductwork, if it’s accessible.

A duct leakage test reveals the tightness of the ducts. Reichel says it is important to see if any ducts are not connected, are not properly sealed, are installed incorrectly, or are leaky for other reasons. However, there is some disagreement among experts as to when, or even whether, duct testing is necessary.

Homeowners may not need to test if they already know that their ducts are leaky, according to Sydney Roberts, director of Applied Building Science at the Southface Energy Institute and principal investigator for the DOE Building America and Advanced Commercial Building Initiative research programs. Roberts is also the director of the Southeast Weatherization and Energy Efficiency Training Center, which trains contractors.

She says that some homeowners may be more likely to pay for duct sealing if they see test results that will show a 20% savings. But some homeowners will pay for duct sealing after seeing photos of their ducts that a contractor has taken in the attic or under a crawl space—places where most homeowners never venture and that they may not even be aware of.

If the renovation will be part of a program to earn rebates, it is important to do duct leakage testing both before and after the A/C system is updated to document the results. But if rebates are not an issue and homeowners can see that their ducts are leaky, it’s only necessary to test afterward, to prove that the repairs were done, she says.

Comfort Problems

Whenever homeowners are considering a new A/C unit, contractors should complete Manual J load calculations in order to rightsize it. In so doing, they should take into account the design of the house, the insulation values, and the type of windows. Contractors should not simply replace the unit with one of the same size, especially when a home has a specific, unsolved problem, such as drafts or overheating in certain rooms.

When homeowners are looking to solve problems related to comfort or inadequate distribution or delivery of conditioned air, Reichel says, they should ask the contractor to perform a duct leakage test before they make any decisions. It’s important to know how leaky the ducts are. But homeowners don’t always ask, of course.

Roberts says that the choices a contractor offers depend on how long homeowners will stay in the house and how much they want to invest. If they plan to stay a long time and are willing to make the investment, a contractor will recommend buying the best A/C box they can afford, since it probably won’t need to be replaced before they sell the house. Most A/C units last 15–20 years, and by the time they do need to be replaced, they are out of date and better equipment is available.

Contractors should also consider the specific problem when presenting choices to homeowners. If the house is too hot or too cold, or is drafty, there is probably an airflow problem. Armin Rudd is principal of ABT Systems and has more than 25 years of experience as a building consultant, engineer, and researcher focusing on conditioning, ventilation, and dehumidification in hot-humid climates. He says one way to solve airflow problems without compromising the speed needed to get adequate mixing is to add ductwork. He explains that not having enough duct-work is like trying to drink a super-thick milkshake out of a straw—it makes an A/C system work harder.

Typically, contractors install oversized A/C units in older houses, and they are sometimes installed even in newer construction. An oversized A/C unit may give homeowners enough airflow to have the option, if there is room, of running additional ducts to bonus rooms that are hot in the summer. Roberts agrees with Rudd and adds that the ductwork in most new construction is undersized.

A/C systems are designed to have a certain static pressure drop across the air handler in order to run quietly and efficiently, and to last a long time. Contractors must always measure the static pressure; this gives them a sense of how much the system is operating like or unlike a thick milkshake. They can also measure the airflow at all of the registers to see exactly where and how much air is going into and out of the house.

Planned Improvements

When weatherizing or making other improvements to a building to make it more efficient—including retrofitting the HVAC system—contractors can make major changes in both the heating-and-cooling load on the system and the dehumidification needs of the house, both of which can affect duct design.

Before installing a new A/C system, Rudd suggests making sure that the building is properly insulated and air sealed. After a building has been properly sealed, the highest air loss is through the windows, so replacing windows should be a serious consideration. Upgrading windows may reduce the A/C load, which may allow for a smaller, more cost-efficient system.

“However, when an A/C system breaks in a hot-humid climate, uncomfortable homeowners won’t entertain replacing the windows,” Rudd says. “If the system breaks when it’s comfortable outside and there is an opportunity, windows should be considered. Ideally, a homeowner will have a proactive plan to replace old systems rather than waiting for them to break. New windows and a smaller cooling system will save money and allow smaller ductwork that may fit better.”

Roberts says that if homeowners plan to make other efficiency upgrades, such as weatherizing or adding insulation, they need to tell the HVAC contractor before the A/C system is retrofitted. When contractors calculate loads, they need to know what they have to add to their model. New efficiency upgrades will probably reduce the size of the system to be installed. This may save homeowners enough money to pay for other energy upgrades, such as new windows.

Roberts suggests looking at the whole house as a system and considering all of its parts at the same time. Again, it’s important for the homeowner to have a plan, so that current decisions don’t limit future possibilities.

The Merits of Repairing or Replacing Ducts

Before the homeowner invests in a new A/C unit, ducts should be inspected and/or tested for leaks. The inspection can pinpoint where problems may lie in a system, possibly showing dust, mold, infestations, clogs, or other types of damage. Leak testing can show where ducts aren’t properly sealed.

Before the homeowner invests in a new A/C unit, ducts should be inspected and/or tested for leaks. The inspection can pinpoint where problems may lie in a system, possibly showing dust, mold, infestations, clogs, or other types of damage. Leak testing can show where ducts aren’t properly sealed.

Repairing ducts is one solution that contractors typically offer. This involves sealing leaks, cleaning ducts, or replacing them. Sealing leaks and reducing heat gain to poorly insulated ducts can make the A/C system run less often and not work as hard. The system will consume less energy and make the building more comfortable. Repairing ducts can reduce utility bills, and it may solve mold problems and improve air quality.

Roberts says that experts at the Southface Energy Institute are huge proponents of sealing ductwork, which is often very leaky. All joints and connections should be sealed
with mastic.

If attic ducts aren’t moldy, Reichel says, it may be possible to do several other things to improve building conditions so that the ducts don’t need to be replaced. Most A/C units are located in the attic in homes built on slab-on-grade foundations in hot-humid climates that don’t have room under the house. Older homes with ducts in a vented attic typically lose about 30% of their heating and cooling ability through leaky, poorly insulated, and poorly designed duct systems.

Correcting all of this might allow the homeowner to downsize the A/C unit because it was probably designed by the rule of thumb long after the home was built. That is, the A/C unit was oversized to compensate for the leaky, poorly insulated ducts in the blazing hot attic. The downsized system will also control indoor humidity better, because it will no longer be pulling humid attic air into the conditioned spaces.

Rudd says that in addition to sealing the ductwork, minimizing holes, penetrations, and gaps between drywall and partitions may make it unnecessary to replace the ducts themselves. Problems can also occur at block walls that have furring strips on the inside, where an air gap at the drywall that isn’t capped off can go all the way up to the attic. These gaps should be blocked to eliminate airflow.

Rudd says that in some states the Weatherization Assistance Program (WAP) will weatherize the homes of low-income families and will consider the ducts as part of the whole house when making retrofit decisions. But many homeowners who don’t qualify for WAP assistance may not know whether to replace the ducts or not.

All three experts interviewed agreed that the time to replace ducts that are reachable is when there is evidence that they’ve taken a beating, or that UV daylight has damaged them. Rudd also adds that in this case, old ducts with R-4.2 insulation that are accessible should be upgraded at the same time, since adding more insulation to sealed ducts will keep the attic cooler and will help the A/C system to cool the house more efficiently. R-8 insulation is required by IECC 2012 codes for supply ducts in unconditioned spaces. For ducts located inside conditioned spaces that a contractor can’t reach, such as those in some two-story homes, R-4.2 insulation is fine.

When It Makes Sense to Clean

Duct inspections can reveal problems that homeowners may not have known existed, and it’s important to eliminate the cause of these problems to prevent them from recurring. For example, the presence of mold in supply ducts signals a need to seal the return. But after the cause of the problem is corrected, the question remains whether to clean the moldy ducts or replace them. Answering that question is more complicated.

Cleaning can save the homeowner the expense of replacing ducts if they’re not old or leaky, and if they are lined with smooth, flat metal. Flex ducts and those with fiberglass duct board linings, scalloped surfaces, or any interior surface that isn’t smooth will trap any dirt and mold inside and can’t be cleaned effectively. If the ducts are insulated and the insulation is moldy, the ducts should be replaced.

There are a few times when it is appropriate to clean the ducts rather than replace them. Ducts should be cleaned if they house rodents, or if they are dirty to the point of being blocked or
releasing visible particles into a home. If ducts cannot be reached to be replaced, it may still be possible to reach them with a
cleaning system.

Another reason that it may make sense to clean ducts is that cleaning the ducts makes it possible to seal them from the inside. A proprietary product called Aeroseal can be sprayed on the inside of the ducts, but it will adhere only to a clean surface. This product seals holes and gaps up to ¼ inch wide and can be used to seal ducts that are between floors, behind walls, or otherwise unreachable. It cannot be used to seal large gaps or places where ducts are disconnected, but it offers a way to seal ducts without tearing the walls apart or threading new ducts down old chases.

EPA provides guidelines on duct cleaning. These guidelines suggest when it is appropriate to clean ducts, what cleaning involves, and how to select a contractor. They include several decision guides to help homeowners evaluate their specific situation. EPA has also published studies showing that duct cleaning had no measurable impact on residents’ health. So while homeowners may feel more comfortable knowing that their ducts are clean, it’s more important to worry about why they are dirty in the first place. “When you fix the cause, you don’t have to clean them,” says Roberts.

Rudd says that it makes sense to clean metal ducts when a more-powerful ECM fan is going in. The new fan may cause dust and dirt to come loose and be distributed throughout the house through the ducts, possibly lowering IAQ. It is a good idea to clean the metal ductwork first to avoid contamination in this instance.

It’s not always clear that cleaning makes things better. Roberts says that the cleaning process involves drilling holes in the ducts and feeding equipment into them, and the leaks that these holes create in the system can do more harm than good if they’re not sealed afterward. Make sure the company repairs all the holes it makes in the ducts when it finishes cleaning them.

Rudd adds that cleaning brushes can be hard on the ductwork, creating perforations, knocking the ducts off their loops, or tearing them up. Fiberglass duct boards, which aren’t cleanable, can also be knocked loose or broken off, creating air quality problems later. Rudd suggests coating the inside of flex duct board with a thin layer of mastic, which makes the duct board cleanable with a damp rag.

Reconfiguring Ducts

There are several situations in which contractors can suggest reconfiguring the ducts to improve airflow. When replacing an A/C unit, homeowners may want to reconfigure the ducts to have a better insulated, more efficient, tighter delivery system.

During inspection a contractor can simply look at the ducts in the attic, crawl space, or basement to see if they look like a big pile of spaghetti with lots of twists and sharp turns. Flex ducts with U-turns and S-curves, crimps, or spider designs that have eight long ducts extending out from a central manifold are all examples of duct installations that were poorly designed and/or inefficiently installed, and that make for an inefficient air delivery pathway.

Duct designs should have a central trunk that goes through the center of the house with flex ducts to create smooth transitions and curves for its branches to reach all the rooms. The ducts should be sized properly by Manual D to provide the right air delivery to each room.

Another time to reconfigure ducts is when they show visible signs of damage. In older homes, the metal ducts and duct board typically used at the supply and return plenums and reduction boxes usually hold up well over time. But flex ducts, typically used in newer homes, are easily kinked or crushed when the duct is run over pipes or around corners and the airflow is restricted. Homeowners can identify these kinds of problems themselves by looking to see what kind of ducts they have and their condition and arrangement.

Reconfiguring may be needed if the ducts run all the way to the exterior walls. According to Rudd, duct designers had to do that in old homes that have single-pane windows and no wall insulation in order to warm the coldest surfaces in the home—the ones exposed to the weather. But if a home has upgraded windows and there is at least some insulation in the walls, Rudd says that the ducts should not be run all the way to the exterior walls. Reconfiguring them with a more efficient duct layout that has shorter branches and a more central supply will minimize the linear footage used and do a better job of moving air across the rooms.

During a retrofit, most homeowners won’t reconfigure their ducts because they don’t want to move the registers. Rudd says shortening duct runs and patching up register holes typically doesn’t improve efficiency that much. When adding on or remodeling, it’s more important that the duct layout be sufficient, with smooth curve transitions from a central trunk line. The largest efficiency reducers are the crimped flex ducts, the U-turns, the S-curves, and the 90° angles, along with leakage, he says.

learn more

ABT Systems

LSU Ag Center LAHouse

Southface Energy Institute

Read fact sheets developed by Southface with support from the Georgia Environmental Finance Authority and the South Carolina Energy Office between 1997 and 2002, including Ductwork Questions and Answers and Air Distribution System Installation and Sealing.

See a YouTube video from the Florida Green team, Why Look at Your Ducts When Replacing AC System.

U.S. Environmental Protection Agency. Should You Have the Air Ducts in Your Home Cleaned? Washington, D.C.: EPA, 1997.

It’s not news that locating ducts in conditioned spaces instead of unconditioned attics and crawl spaces greatly reduces energy loss. Anyone considering reconstruction or remodeling should make sure that, if possible, the ducts and the HVAC unit are inside. Like sealing the crawl space or insulating the walls, this is a great improvement to make.

There Is No Sweet Spot for A/C and Ducts

Rudd says that duct sizing is based on rules for friction loss per 100 feet of duct. He likes to look instead at duct sizing based on velocity. Knowing the linear foot per minute (fpm) and the cubic fpm required, you can calculate the cross-sectional area required. Rudd uses 750 fpm for supply trunks, 500 fpm for run-outs, 500–600 fpm for return air trunks, and 300–350 fpm for return grilles. For the grilles, divide the net free area by 0.7, increasing the size of the grill in the calculation, because part of the area is covered with metal. The calculations are straightforward.

“The design principles are fundamentals, and aren’t something to trade off against cost. Instead, trade off SEER efficiency level against cost. I’ve found a SEER rating of 15 or 16 is probably the upper end of good value,” Rudd says.

Marrying the duct size and the size of the A/C unit should be part of any retrofit package, according to Roberts. If ductwork is not addressed whenever an HVAC system is replaced, the homeowner won’t get the full benefit of the energy efficiency of the new unit.

“Often both homeowners and contractors don’t understand the importance of solving the issues with the duct system. It’s not easy work, sealing or straightening out ductwork, especially if the ducts are in a crawl space. It doesn’t get the attention it deserves,” says Roberts.

Debbie Sniderman is an engineer and CEO of VI Ventures, LLC, an engineering consulting company based in Charleston, South Carolina. She can be reached at info@vivllc.com.

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