It's July in Cary. The thermostat reads 82°F. Your AC has been running for hours. The air coming from the vents feels lukewarm at best. You check the outdoor unit — it's running. You check the thermostat — it's set to 72°F. Everything appears to be working, but your house is not cooling down.
This is one of the most common service calls we see across the Triangle during summer, and North Carolina's climate creates conditions that make certain AC failures more likely than in other parts of the country. Here are five causes that are particularly relevant to our region.
1. Frozen Evaporator Coil from Humidity Overload
The symptom: Air from the vents feels weak and barely cool. You might notice ice on the refrigerant lines going to the outdoor unit.
Why NC makes it worse: North Carolina summers combine high temperatures with extreme humidity — we regularly see dewpoints in the 70s from June through September. Your AC system has to remove moisture from the air before it can meaningfully lower the temperature. When the humidity load is heavy enough, the evaporator coil's temperature drops below freezing and ice begins to form.
Once ice builds on the coil, airflow drops. Less airflow means the coil gets even colder. More ice forms. It's a self-reinforcing cycle that can completely freeze the coil solid within a few hours.
Homes in the Preston neighborhood and other Cary subdivisions with larger square footage are particularly susceptible because the systems handle higher volumes of humid air.
Pro Tip: If you suspect a frozen coil, turn the AC off but leave the fan set to ON. This circulates room-temperature air over the coil to melt the ice. It may take 2–4 hours to fully thaw. Once clear, turn the AC back on and monitor it. If it freezes again within 24 hours, you have an underlying issue — low refrigerant, a failing blower motor, or a dirty coil — that needs professional diagnosis.
Contributing factors: Dirty air filters (the number one cause), low refrigerant charge, or a failing blower motor that isn't moving enough air across the coil.
2. Dirty Condenser Coils from Pollen Season
The symptom: The AC runs but the air isn't as cold as it should be. The outdoor unit may be louder than usual or cycling on and off more frequently.
Why NC makes it worse: North Carolina's pollen season is legendary. We consistently rank among the worst states for pollen counts, and it's not just spring — tree pollen runs February through May, grass pollen May through July, and ragweed August through October. That means your outdoor condenser coils spend most of the year being coated with a fine layer of biological insulation.
The condenser is responsible for releasing heat from the refrigerant into the outdoor air. When the coils are coated with pollen, dirt, grass clippings, or cottonwood fluff, they can't dissipate heat efficiently. The system works harder, runs longer, and delivers less cooling.
When to Call a Pro: You can gently rinse your condenser coils with a garden hose (from the inside out, never use a pressure washer). But if cleaning the coils doesn't improve performance, the issue may be deeper — bent fins, a failing condenser fan motor, or restricted refrigerant flow all produce similar symptoms and require professional AC repair.
Simple prevention: Rinse the outdoor unit with a garden hose every month during pollen season. Keep landscaping trimmed at least 2 feet away from the unit on all sides.
3. Low Refrigerant from Aging Systems
The symptom: The AC runs continuously but can't reach the set temperature. The air from vents is cool but not cold. The refrigerant lines may be frosted.
Why NC makes it worse: Many homes in Cary — particularly in established neighborhoods like Kildaire Farms — have AC systems installed in the early 2000s or even the 1990s. These systems use R-22 refrigerant (Freon), which was phased out of production in 2020. As systems age, the solder joints, valve stems, and service connections develop small leaks that slowly reduce the refrigerant charge.
A system that's lost 10–15% of its refrigerant charge will still run, but it won't cool effectively — especially when outdoor temperatures climb above 95°F, which Cary sees regularly from June through August.
The hard truth about R-22: Because R-22 is no longer manufactured, existing supplies are limited and expensive. Recharging an older system with R-22 can cost $150–$300 per pound (most systems hold 6–12 pounds). At that price point, a recharge often isn't economical, and the leak will only lose the new refrigerant over time anyway.
If your system uses R-22 and has a confirmed leak, the practical options are repairing the leak (if accessible and cost-effective) or replacing the system with one that uses R-410A or the newer R-454B refrigerant.
4. Oversized Unit Short Cycling
The symptom: The AC turns on, runs for 5–10 minutes, shuts off, then cycles back on shortly after. The house feels clammy and humid even though the temperature readings seem close to the setpoint.
Why NC makes it worse: Humidity again. An oversized AC system cools the air quickly enough to satisfy the thermostat, but it doesn't run long enough to adequately dehumidify. In a dry climate, oversizing is merely inefficient. In North Carolina's humidity, it creates a genuinely uncomfortable indoor environment — cold and clammy rather than cool and comfortable.
Short cycling also causes uneven temperatures throughout the house. Rooms closer to the air handler cool quickly while distant bedrooms barely benefit before the system shuts off.
Why it happens in the first place: Oversizing is typically an installation error. Some contractors use rules of thumb ("one ton per 400 square feet") rather than performing a proper Manual J load calculation that accounts for insulation, window orientation, ductwork, and local climate data. The result is a system with 20–50% more capacity than the home actually needs.
Pro Tip: If your system is less than 5 years old and short cycling, don't assume the equipment is faulty. Ask your HVAC technician to verify the system sizing against a Manual J calculation. If the unit is indeed oversized, some newer variable-speed systems can be adjusted to partially compensate, though a properly sized replacement remains the best long-term solution.
Partial solutions: A whole-house dehumidifier can offset the moisture problem caused by short cycling. It won't fix the cycling itself, but it addresses the comfort issue while you plan a longer-term fix.
5. Ductwork Leaks in Crawl Spaces
The symptom: The AC seems to work fine at the air handler, but certain rooms stay warm. Your energy bills are higher than expected for your home's size.
Why NC makes it worse: A significant percentage of Cary homes — especially those built before 2010 — have ductwork running through vented crawl spaces. During summer, those crawl spaces reach temperatures of 90°F or higher with near-100% relative humidity. Any leak in the ductwork pushes your expensive cooled air into the crawl space while simultaneously drawing hot, humid crawl space air into the system.
Studies by the Department of Energy suggest that the average home loses 20–30% of conditioned air through duct leaks. In a hot, humid crawl space, those losses hit harder because the temperature differential is so extreme.
The signs: Hold your hand near duct connections in the crawl space (at joints, take-offs, and where flex duct connects to rigid duct). If you feel air blowing, you've found a leak. Also check for duct insulation that has sagged or fallen off — uninsulated duct in a hot crawl space loses cooling capacity through the duct walls even without leaks.
The fix: Professional duct sealing using mastic (not tape — standard duct tape fails within a few years in crawl space conditions) can recapture that lost cooling capacity. In severe cases, replacing deteriorated flex duct with new, properly insulated runs is the better investment.
When the Problem Is Bigger Than a Single Cause
Often, AC performance issues in NC summers involve more than one factor working together. A system that's slightly low on refrigerant, running with a dirty filter, and losing 25% of capacity through duct leaks might cool adequately in May but fail completely in July when outdoor temperatures add the final stressor.
A comprehensive AC diagnostic evaluates all of these factors together rather than chasing a single symptom.
Frequently Asked Questions
What temperature difference should I expect between the supply and return air?
A properly functioning AC system should produce a 15–20°F temperature split between the return air (going in) and the supply air (coming out of vents). You can check this yourself with a simple thermometer. If the split is less than 15°F, the system is underperforming.
Why does my AC work fine in the morning but struggle in the afternoon?
Your AC system is sized to handle a specific heat load. As outdoor temperatures peak in the afternoon (typically 3–5 PM in NC), the heat gain through walls, windows, and the roof increases. If your system is marginally sized or has any of the issues described above, the afternoon peak pushes it past its capacity.
How often should I change my air filter in NC summers?
Every 30 days during peak cooling season (June–September) if using standard 1-inch filters. Our high pollen counts and humidity levels clog filters faster than the manufacturer's 90-day recommendation suggests. A clogged filter is the single most common preventable cause of AC performance issues.
Is it normal for my AC to run all day when it's 100°F outside?
On extreme heat days, yes. Most residential AC systems are designed to maintain a 20°F differential from outdoor temperature. When it's 100°F outside, your system may only be able to maintain 80°F indoors while running continuously. If it can't maintain even that, one of the five issues above is likely at play.
Should I close vents in unused rooms to improve cooling?
No. Closing vents increases static pressure in the ductwork, which can reduce overall system efficiency, increase duct leakage, and potentially damage the blower motor. Keep all vents open and unobstructed for best performance.
