AC Running But Not Cooling? Eight Causes a Birmingham Tech Checks First

1. First, Measure the Temperature Split

Before guessing, get one number. Take any meat thermometer or kitchen probe thermometer. Hold it inside a supply register (the vents blowing air OUT) for 90 seconds. Write down that number. Then hold it inside a return air grille (the big vent your filter sits behind) for 90 seconds. Write down that number too.

Subtract supply from return. That is your delta-T, or temperature split. A healthy residential AC produces 16-22 degrees of split. Anything less than 14 means the system is not effectively cooling the air passing through it. Anything over 22 means restricted airflow.

This single measurement narrows the problem in 30 seconds. Low split with normal airflow points to refrigerant or compressor issues. High split with weak airflow points to a frozen coil or dirty filter. Now you have direction.

2. Cause #1: Clogged Air Filter (Free)

Always check this first because it is the most common cause and costs $5 to fix. A clogged filter starves the evaporator coil of airflow. With less air moving across the coil, less heat is removed from the building. The system runs continuously trying to satisfy the thermostat — but it cannot, because it is moving too little air.

In Birmingham, filters load faster than national averages. Pollen counts in spring exceed 5,000 grains per cubic meter. Red clay dust kicked up by lawn equipment. Pet dander. New construction dust from neighborhood remodels. The "90-day filter" the manufacturer printed on the box is closer to 30-45 days here in cooling season.

Fix

Pull the filter. Hold it up to a bright light. If you can barely see through it, replace. Match dimensions exactly (16x25x1, 20x20x1, etc.). Stay between MERV 8 and 11 for residential — higher MERV ratings restrict airflow on systems not designed for them. Per the Department of Energy maintenance guidance, monthly checks during cooling season are the minimum.

3. Cause #2: Frozen Evaporator Coil ($0 to diagnose)

If your delta-T was huge (28+ degrees) but the air flow felt weak, the coil is probably frozen. Open the air handler access panel. If you see ice on the copper coil or the suction line, that is your answer.

Frozen coil = restricted airflow OR low refrigerant. The dirty filter you just replaced may have caused it. Or refrigerant is low from a leak. Either way, the ice now blocks airflow completely and no cooling reaches the house.

Fix

Set thermostat to OFF. Set fan to ON. Let the blower move room-temperature air across the coil for 2-4 hours until it fully thaws. Replace the filter. Restart system and re-measure delta-T after 15 minutes. If split is back to 18-20 and stable, you fixed it. If the coil refreezes within an hour, the refrigerant charge is low and an EPA 608-licensed tech needs to find the leak. We cover this deeper in why your AC freezes up in Alabama summer.

4. Cause #3: Dirty Outdoor Condenser Coil (Free)

The outdoor unit's job is to dump heat into the outside air. The coil — those aluminum fins wrapping the outside of the condenser — is where that heat exchange happens. When the fins are wrapped in pollen, grass clippings, cottonwood fluff, dog hair, or dryer lint, heat cannot escape. The system loses cooling capacity even with a perfectly fine indoor unit.

Birmingham specifically: oak pollen in March, pine pollen in April, cottonwood seed fluff in May, mowing season debris all summer. Homes near power lines, freshly mowed lawns, or trees with significant seed shed end up with condensers half-blocked.

DIY Fix

1. Shut off power at the disconnect box (the gray metal box on the wall next to the outdoor unit).
2. Remove any visible debris by hand — leaves, twigs, accumulated grass.
3. Use a garden hose with a normal spray nozzle (NOT a pressure washer — pressure bends fins) to rinse from the inside of the unit outward. Spray the fins from inside the unit if you can access them. Otherwise rinse from outside-in.
4. Let dry 30 minutes before restoring power.

Birmingham condensers benefit from this rinse two to three times per cooling season. Homes with mature trees or near pastures, more often.

5. Cause #4: Weak Run Capacitor (Tech-Required)

A run capacitor is a small cylindrical or oval component that gives the compressor and outdoor fan motor the kick they need to start and run efficiently. They degrade over time, and Birmingham humidity accelerates the degradation. A capacitor that has lost some of its rating can let the compressor run weakly — drawing power but not pumping refrigerant at full capacity. The system runs, the fan spins, the homeowner gets confused, and the house keeps warming up.

Symptoms: humming or laboring sound from the outdoor unit when it tries to start. Compressor that takes a long time to spin up. Sometimes hot-to-the-touch capacitor (do not touch — they hold charge).

Why This Is Not DIY

Capacitors hold lethal voltage even with the unit unplugged. They must be discharged with an insulated tool before being handled. They are also a $20 part with a $200 service call surrounding them — finding the bad cap takes a meter, replacing it takes 10 minutes if you know what you are doing. We replace dozens of caps per cooling season across Hoover, Pelham, and Helena. It is one of the most common single-call fixes.

6. Cause #5: Refrigerant Leak (Tech + EPA 608 Required)

If the indoor coil is clean, the filter is fresh, the outdoor coil is rinsed, the capacitor reads correctly, and the system still cannot produce a 16+ degree split, the next stop is refrigerant charge. Per EPA Section 608 regulations, only certified technicians can legally handle refrigerant. Anyone selling you "refrigerant top-off" without proper certification is breaking federal law.

A refrigerant leak is also not a "fix" by itself — finding and repairing the leak matters. The EPA does not allow techs to simply add refrigerant to a known-leaking system above certain thresholds. Common leak points: evaporator coil, refrigerant line set joints, Schrader valves, condenser coil. Each requires different repair and verification.

Symptoms beyond the temperature split include: oil residue or stains on outdoor copper line connections, hissing sound from the indoor or outdoor unit, ice buildup that returns within hours of thaw. If you suspect a leak, do not run the system continuously — running a low-charge AC damages the compressor, which is the most expensive component to replace.

7. Cause #6: Thermostat Reading Wrong

The thermostat thinks the house is the temperature it senses. Hang an old digital thermostat next to the wall thermostat and compare. If they disagree by more than 2 degrees, the wall thermostat is reading wrong — and the system is running according to bad data.

Causes: thermostat installed on an exterior wall (south- or west-facing walls in Birmingham can run 5-10 degrees hotter than interior walls), direct sun hitting the unit at certain hours, a lamp or TV mounted nearby radiating heat, low batteries on a Wi-Fi thermostat causing erratic readings.

Smart thermostats can also "learn" wrong. A Nest or Ecobee that has been "training" with bad data for weeks may be holding back cooling cycles thinking the house is cooler than it is. Reset to factory and let it relearn. We covered Wi-Fi thermostat troubleshooting in our Nest and Ecobee emergency override guide.

8. Cause #7: Leaky Ductwork

The Department of Energy estimates that typical residential ductwork loses 20-30% of conditioned air through leaks, gaps, and disconnections. In Birmingham attics, where summer temps reach 130-140°F, that lost cold air is being delivered to the attic instead of your living space.

Common in: 1980s-2000s Birmingham construction with flex duct in unconditioned attics, especially Hoover, Helena, and Trussville subdivisions where flex duct connections at supply boots loosen over decades. Gravity, expansion-contraction cycles, and rodent damage all contribute.

Symptoms: certain rooms much warmer than others, especially rooms farthest from the air handler. Air handler running constantly. High summer power bills. Dust accumulating around supply registers (a sign that the room is being depressurized by leaks elsewhere).

A duct blaster test isolates leak locations. Sealing involves mastic, foil tape (NOT cloth duct tape — it fails in attic heat), and sometimes replacement of damaged sections. See our duct services page.

9. Cause #8: System Undersized for Heat Load

If the system is in good repair and the temperature split is healthy but the house still cannot keep up on Alabama's hottest days, the system may simply be too small for the heat load it is fighting. This is not a failure — it is a design problem that has always been there but only shows up in the worst weather.

Common contributors: home additions built without HVAC capacity upgrades, replaced windows that did not match original specifications, attic insulation that has settled or been removed, heat-generating appliances (server room, multiple monitors, gaming setup) added in finished basements or attic offices.

An ACCA Manual J load calculation reveals the real cooling demand based on square footage, insulation, window glazing, orientation, infiltration, and internal heat sources. Birmingham's design temperature for sizing is 94°F per ACCA Manual J. Older systems may have been sized for 90°F. The four-degree gap matters when 95+ days hit.

If a tech confirms the system is running at full capacity but the home is still not cooling, the conversation moves to system sizing. See our AC installation page for what a Manual J replacement looks like.

FAQ: AC Running But Not Cooling