AC Not Cooling with Low Airflow: Duct Leaks and Blockages

Air conditioning that hums along but never quite cools the house usually points to airflow. If the refrigerant circuit is the heart of a cooling system, the ductwork is the bloodstream. Leaks, crushed runs, and blockages slow that flow to a trickle. The system keeps trying, yet heat stays trapped in rooms, humidity creeps up, and the utility bill swells. I have walked into countless attics and crawlspaces where the core problem was not the condenser outside, but the sheet metal and flex duct carrying air to and from the living spaces. Once airflow is restored, the same equipment that felt undersized suddenly handles the load.

Cooling problems rarely come from a single cause. Airflow losses interact with refrigerant pressures, coil temperatures, and blower settings. With poor airflow, evaporator coils run cold, sometimes below freezing, which can produce ice. That ice reduces airflow even further. Meanwhile, duct leaks pull attic air into returns, or push cooled air into the attic. The homeowner sees “ac not cooling,” yet the equipment may be fine. Understanding how air moves through supply and return ducts, where it escapes, and how blockages form is the shortest path to a lasting fix and a longer hvac system lifespan.

How Low Airflow Undercuts Cooling

An AC system relies on a designed volume of air moving across the evaporator coil, commonly in the range of 350 to 450 cubic feet per minute per ton of cooling. When airflow drops below that, two things happen. The coil surface gets colder than intended, which can lead to frost and eventually an ice blanket. At the same time, the system’s ability to remove both heat and moisture drops. The supply air may feel cold at the vent, but you do not get enough of it to change the room temperature or humidity. The thermostat keeps calling, run time stretches, and your electric meter spins.

If you picture the system as a loop, low airflow can occur on either side. On the return side, leaks or blockages starve the blower of air. On the supply side, leaks or restrictions prevent delivery to rooms. Either path lowers the total system airflow. A common pattern is a clean-looking furnace cabinet with a densely packed filter and a return trunk that leaks at every seam. You can have a high-end variable-speed blower strangled by a shrunken return. The motor ramps up, becomes noisy, and still fails to move enough air. Excess static pressure punishes motors and shortens their lives.

Engineers design duct systems with a target static pressure budget, often around 0.5 inches of water column total external static for many residential air handlers, although equipment ratings vary. Real installs drift well above that due to long flex runs, tight turns, pinched ducts near trusses, and undersized returns. Every added elbow and kink steals pressure the blower could use to move air. By the time that air reaches the far bedroom, there is not much left.

The Hidden Cost of Duct Leaks

Leakage is the silent killer. A 20 percent leak rate is not unusual in older homes, and I have tested systems that bleed 30 to 40 percent of their airflow into the attic. That means a three-ton system designed to move roughly 1,200 cfm may deliver 700 to 900 cfm to the rooms, often unevenly. In cooling season, supply leaks dump conditioned air where it does no good, while return leaks pull in hot, dusty attic or crawlspace air. That unfiltered air coats coils, clogs filters, and lowers indoor air quality.

The effect shows up as rooms that never quite reach setpoint, strong temperature differences between floors, and a constant feeling that the AC is undersized. On calls where a homeowner asks about a larger system, I measure total external static, then walk the ducts. Aeroseal or hand-sealed ducts have predictable pressure and deliver solid airflow. Tape-only joints, sloppy panned returns, and boot connections that never got mastic do not. I once found an entire return boot that had fallen away from the return drop in a crawlspace, effectively pulling the entire return from the crawlspace itself. The home cooled fine on mild days, then failed when temperatures hit the mid-90s. Reattaching and sealing that joint solved the problem overnight.

Blockages: From Filter to Grille

Blockages are more straightforward than leaks, but they can be deceptive. Everybody knows a plugged filter chokes airflow. Less obvious are filters that fit poorly, get sucked into the rack, or have MERV ratings too high for the duct design. A 1-inch pleated filter with a high MERV rating can double the external static if the return is undersized. Move up to a deeper media filter with more surface area, and static drops.

Farther downstream, blockages come from collapsed flex ducts, crushed sections under storage boards in the attic, and long runs where the inner liner sags. I have seen kinks so tight they acted like valves. Another regular culprit is a closed or broken balancing damper hidden in a wall cavity or trunk. Renovations often leave dampers offset, or they get knocked partly shut during other work. At the grille, aftermarket magnetic covers and heavy furniture can reduce throw and disturb air patterns. Even an inch of dust on a return grille can add measurable restriction.

Supply trunks and branches also suffer from construction debris, insulation, and in older homes, toys and papers that slipped through returns. If the duct system includes a bypass humidifier or a zoning bypass, poorly set bypass dampers can cause crossflows that rob key branches. On systems with zoning, a single stuck zone damper can make two rooms unlivable while the rest of the house is comfortable.

Symptoms That Point to Duct Issues

When I ask homeowners to describe what they feel, I listen for patterns. If someone tells me the bedroom above the garage is always hot, I suspect a long, exposed run, possible leaks, and insufficient insulation. If the complaint is housewide, with long run times and high humidity, I look at return side restriction and overall leakage.

Other consistent tells: whistling at returns, fluttering filters, or a dusty smell during cooling. Those point to return leaks. If a room has strong airflow but remains warm, that often points to supply temperature problems or envelope issues, not ducts. If a room has weak airflow and is warm, the duct is suspect. Quick checks with an anemometer at grilles give hard numbers. Compare the total measured supply flow to the blower’s expected output, and you will see how much is missing.

How Low Airflow Amplifies Other Failures

Duct problems rarely travel alone. Low airflow makes every cooling issue worse. Dirty evaporator coils collect faster when return leaks drag unfiltered air past the filter. The coil’s pressure drop grows, which lowers airflow further. Low airflow can also run the suction temperature down, increase superheat instability, and trigger icing. An iced coil presents as “ac not cooling,” then suddenly cooling again after the unit has been off long enough to thaw. Homeowners report puddles below the air handler and a musty smell. They change the thermostat batteries and hope for the best. The root cause is the airflow.

On the heating side, the same ducts serve the furnace or air handler. If you see “furnace not heating” or “heater not working” in a home with obvious duct restrictions, do not ignore the ducts. High static pressure can limit airflow across a heat exchanger, forcing the furnace to shut down on high limit. Restart cycles repeat until the homeowner gives up. The fix might be as simple as adding a second return or opening a closed damper.

Measurement Beats Guesswork

Throwing parts at airflow problems wastes money. A few objective tests guide the work. Total external static pressure across the air handler tells you whether the system is stressed. Use manufacturer blower tables to translate static into expected airflow, then cross check with temperature rise in heat or temperature split in cooling. A normal cooling split under load runs somewhere around 16 to 22 degrees Fahrenheit for many systems, but do not chase a number without context. High humidity, longer runtimes, and certain coil designs change the split. If you see a split that is oddly high, that often signals low airflow. A low split can mean either low charge or high airflow, or high latent load. Combine readings to avoid wrong turns.

Pressure diagnostics at supply and return plenums reveal imbalance. A return pressure higher than supply suggests return restrictions. Duct leakage testing, using a duct blaster, quantifies leakage and finds weak spots. Even a hand-held smoke puffer or a thermal camera around boots and seams will expose big holes. If the system uses flex duct, a visual inspection may be all you need. Follow the flex with your hand, feel for kinks, look for sections crushed under planks. Pull back insulation at joints to check for mastic.

Airflow at grilles can be spot measured with a flow hood or a balancing vane to estimate cfm. People often dismiss these numbers as “just for commercial work.” They make residential work faster and more accurate. If a room is designed for 80 cfm and you measure 25, you know where to focus.

Sealing and Repair, Done Right

Sealing ducts is labor more than material. The difference between a quick patch and a lasting fix is prep and product. Use mastic or UL 181 rated foil tape, not standard cloth duct tape. Clean the joint, apply a thick layer of mastic, and reinforce with mesh on larger gaps. At boots, seal both to the duct and to the drywall or subfloor, then install a proper trim ring to prevent gaps. At the air handler, seal the cabinet seams, especially around the filter rack and coil access. Many filter racks leak more than the rest of the system.

For returns made from panned joists, sealing every crack is tedious but worthwhile. Better yet, convert to a sealed metal return where space allows. If you find a return pulling from a garage, stop and rework it immediately. That is both a safety and a performance problem.

When blockages come from poor routing, reroute rather than patch. Move long flex runs off the floor of the attic and hang them with wide straps. Keep bends gentle, with large radius sweeps, and avoid sharp turns. Flex duct should be taut on the inner liner, not stretched to the point of noise, and the outer insulation should be continuous. Undersized branches to distant rooms can be upsized a step or two. A balancing damper at the takeoff makes later tuning possible. In tight attics, metal trunk with short flex drops often outperforms a web of long flex runs.

Return Air: The Starved Side

Most airflow problems trace to returns. It is natural to focus on supplies because that is where you feel air, but the blower can only push what it can pull. If the return filter is small or the return path is narrow, you will fight noise and low cfm no matter how large the supply is. An easy field test: with the system running, crack a door between a conditioned room and a hallway with a central return. If the door slams shut or stands pulled tight by airflow, that room lacks an adequate return path. Jump ducts, transfer grilles, or undercut doors help, but nothing beats dedicated returns sized for the load.

In older homes with a single central return, adding one or two returns in the far bedrooms transforms comfort. Beyond comfort, better return paths reduce static and extend hvac system lifespan by allowing the blower to work within its design envelope.

Insulation and Location Matter

Ducts do not just move air, they also lose or gain heat along the way. A leaky run wrapped in thin insulation that snakes through a 130 degree attic will deliver lukewarm air in cooling season and cool air in heating season. Even with perfect sealing, undersized insulation undermines performance. In colder climates, R-8 insulation on ducts in unconditioned spaces is a practical minimum. In very hot attics, bump it higher if possible, or better yet, bring ducts into conditioned space by sealing and insulating the attic or moving the air handler into a closet inside the envelope.

Ducts that run along the roof deck or lie on attic insulation tend to get stepped on during storage or service, creating flat spots. Raise them on saddles, keep clear pathways, and label them so future trades treat them kindly.

When To Suspect The Equipment Instead

Not every “ac not cooling” complaint is ducts. If airflow checks out and duct leakage is controlled, look to the refrigerant side, the coil, and the outdoor unit. A clogged evaporator coil can mimic duct restriction. So can a failed blower motor or a misprogrammed variable-speed profile. You will also see problems when a heat pump in cooling mode has a stuck reversing valve or when the condenser coil outside is clogged with cottonwood. Balance your troubleshooting: static pressure and airflow first, then the rest.

House envelope issues can overwhelm a good system. West-facing rooms with big windows and no shading, or a bonus room over a garage with bare minimum insulation, will fight you. The duct may be fine, yet the load is too high. Adding supply cfm helps, but sometimes the answer is insulation, air sealing, and shading.

DIY Checks a Homeowner Can Do Safely

A handful of checks are safe and useful before calling for service:

Replace or clean the air filter, and consider a deeper media filter if the system allows. Verify the filter faces the right direction and seals the rack without gaps. Walk the accessible ducts in the attic or crawlspace. Look for obvious disconnections, crushed sections, and open seams at boots. Open all supply registers and returns. Remove heavy rugs or furniture that block airflow. Clean return grilles with a vacuum. Listen for whistling and feel around the air handler for strong air movement that suggests cabinet leaks. Check the thermostat settings and fan mode. If the fan runs continuously, humidity can rise and make rooms feel warmer.

If those simple steps restore airflow and the system cools well, schedule a follow-up to address the root causes permanently. If not, a pro with the right instruments will shorten the path to a fix.

What Good Looks Like After Repairs

When ducts are tight and sized well, the system runs quieter and steadier. Temperature differences from room to room narrow to a few degrees. The system reaches setpoint without marathon cycles. Blower noise softens because static pressure falls. Filters last longer because the return side no longer pulls dusty attic air. On service calls after sealing and rebalancing, I often see a 10 to 25 percent drop in runtime on similar weather days. Energy savings vary, but comfort gains are immediate.

Beyond comfort and bills, the equipment breathes easier. Motors last longer when they are not fighting high static. Coils stay cleaner. Drains see less algae because the coil does not ice and thaw. That is how you stretch hvac system lifespan without replacing major components.

Case Notes From The Field

A two-story colonial with a single upstairs return struggled every July. The upstairs thermostat sat at 76, but two bedrooms stayed at 80 to 82 on hot afternoons. The flex trunk was intact, but every takeoff to the far bedrooms had kinks, and the return static was high. We added two dedicated returns, upsized the worst branch from 6 to 8 inches, hung the flex to remove sags, and sealed all joints with mastic. External static dropped from 0.9 to 0.55 inches of water column. Airflow at those bedroom grilles rose from about 40 cfm to 85 to 95 cfm. The homeowners stopped running box fans in the doorways, and the setpoint held.

Another home had a “heater not working” complaint in January. The furnace fired, ran for two minutes, then shut down on limit and retried. The return filter was new, but the return trunk had a 90-degree entry into a too-small furnace cabinet. We swapped the return drop to a larger size, added a turning vane, and sealed leaks at the filter rack. The furnace ran steadily with a proper temperature rise. In summer, the same fix improved cooling because the blower finally moved the designed air volume.

When To Update Versus Repair

Ducts wear differently than equipment. Well-installed metal ducts can serve for decades with modest sealing and insulation upgrades. Flimsy flex duct that has been abused may be cheaper to replace than to patch. If you have more than 25 to 30 percent measured leakage or widespread crushing, consider a partial rebuild. Pairing a new high-efficiency system with leaky ducts is like putting racing tires on a car with a hole in the gas tank. Plan duct improvements as part of any equipment replacement.

Sizing matters too. Too many homes got oversized systems to compensate for duct losses. After sealing and balancing, you may find that your next system can be right-sized. Correct sizing improves dehumidification and comfort, particularly in humid climates.

Practical Expectations, Not Magic

Sealing and balancing ducts is not a magic wand. If a home has poor insulation or big solar gains, duct fixes will help but not cure everything. If the evaporator coil is matted with dust, airflow repairs must include cleaning the coil. If the system uses a high-MERV filter on a 1-inch rack, expect to rework the filter rack. If rooms lack return paths, accept some carpentry. Honest conversations about constraints produce better outcomes. I have had owners who preferred a small grille in a hallway over a transfer grille in a bedroom door. That choice affects performance, and we plan around it.

Budget also shapes the approach. Hand sealing with mastic is cost-effective for accessible sections. Aeroseal services shine for systems hidden behind finished ceilings, though they cost more. Either way, testing before and after confirms progress.

The Role of Maintenance

Even perfect ducts need upkeep. Replace filters on a schedule that matches your home’s dust load, pets, and fan runtime. Many homes do well with three-month changes for 1-inch filters and six to twelve months for deeper media filters, but look rather than guess. Keep returns clean. Clear debris around outdoor units so the condenser can breathe. Have a technician check static pressure and coil condition every year or two. A half-hour of measurement can prevent a season of frustration.

On the heating side, listen for short cycling, rising noise, or rooms that get stuffy. The same duct restrictions that hobble cooling can trip safeties in heating, leading to “furnace not heating” calls when the real fix is airflow. Treat heating and cooling as a single air delivery problem that changes temperature source by season.

Why This All Adds Up

Airflow does quiet work. You cannot see leaks in the attic during dinner, and you cannot feel a crushed flex in the crawlspace while reading. Yet every minute the system runs with low airflow, you pay. Electricity, comfort, and hvac system lifespan suffer. The good news is hvac richmond ky that airflow problems are solvable with methodical testing and straightforward repairs. When the ducts are tight and the air moves freely, a house feels different. Rooms settle into even temperatures, humidity drops to a comfortable band, and equipment finally operates in the conditions it was built for.

If your AC struggles and the outside unit seems fine, look inward at the ducts. The fix is often closer to home than you think.

AirPro Heating & Cooling
Address: 102 Park Central Ct, Nicholasville, KY 40356
Phone: (859) 549-7341