Why is my AC running but not cooling the house?

Most common causes: dirty air filter restricting airflow, thermostat set incorrectly, refrigerant leak, dirty condenser coils outside, or a frozen evaporator coil. Start with the filter and thermostat — they're free to check and fix 40% of cases.

How do I know if my AC is low on refrigerant?

Signs include: warm air from vents even after running for 30+ minutes, ice forming on the copper lines near the indoor unit, hissing or bubbling sounds from the unit, and significantly higher electricity bills. Refrigerant doesn't deplete with use — low refrigerant means there's a leak.

Why is my AC freezing up?

Frozen AC coils are usually caused by restricted airflow (dirty filter or blocked vents) or low refrigerant. Turn the AC off, switch the fan to 'ON' to defrost for 2–3 hours, then check the filter. If it freezes again, you likely have a refrigerant leak.

How long should it take for AC to cool a house?

A properly sized AC should drop indoor temperature by about 1°F every 15–20 minutes on a hot day. If it takes more than 3 hours to cool 10°F with the unit running continuously, something is wrong.

Why Is My AC Not Cooling? (9 Causes, Ranked by Likelihood)

An AC that runs but doesn't cool is one of the most common summer complaints. Before calling a technician (and paying a diagnostic fee), work through this list — many causes are DIY-fixable in under 30 minutes.

1. Dirty Air Filter (Most Common)

A clogged filter restricts airflow through the system. With insufficient airflow, the evaporator coil can't absorb heat effectively, and the air coming from your vents barely feels cool.

Fix: Replace or clean the filter. Filters should be changed every 1–3 months depending on use. Hold a new filter up to light — if you can't see light through it, it's overdue.

This single fix resolves the problem in roughly 1 in 4 cases.

2. Thermostat Set Wrong

Check three things:

Mode is set to COOL, not FAN or HEAT
Temperature setpoint is actually below the current room temperature
"Auto" fan mode (runs only when cooling) vs "On" (runs continuously) — "On" mode blows uncooled air between cycles, making rooms feel warmer

Also check if batteries are low in a wireless thermostat — they can cause erratic behavior before fully dying.

3. Condenser Unit Is Blocked or Dirty

The outdoor condenser unit expels the heat pulled from your home. If it's surrounded by vegetation, covered in debris, or the fins are caked with dirt, it can't release heat properly.

Check: The unit needs at least 2 feet of clearance on all sides. Clear any plants, leaves, or debris. Gently rinse the fins with a garden hose (from the inside out if possible) to remove dirt buildup.

4. Frozen Evaporator Coil

If the indoor air handler has ice forming on the copper refrigerant lines or on the coil itself, airflow is severely restricted.

What to do:

Turn the AC off at the thermostat
Switch the fan to "ON" (not AUTO) — this circulates warm air to melt the ice
Let it defrost for 2–4 hours
Check and replace the air filter
Turn AC back on

If it freezes again within a day, you likely have a refrigerant issue — call a tech.

5. Refrigerant Leak

AC systems are sealed — refrigerant doesn't get "used up." If your system is low on refrigerant, it has a leak somewhere. Signs: ice on the lines, hissing sounds, warm air even after running 30+ minutes, higher bills.

This requires a licensed HVAC technician to find and repair the leak and recharge the system. Don't just "top off" refrigerant — adding refrigerant to a leaking system is a temporary patch that makes the underlying problem worse.

Typical repair cost: $200–$600 depending on leak location and refrigerant type.

6. Leaky or Uninsulated Ductwork

If your ductwork has disconnected sections or holes (common in older homes), cooled air is escaping into the attic or crawl space before reaching the rooms. You'll notice some rooms cool well while others don't.

Inspect accessible duct runs for visible gaps, tape failing at joints, or sections that have pulled apart. Seal with HVAC foil tape — don't use regular duct tape, which fails with heat cycling.

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7. AC Is Undersized for the Space

A unit that's too small will run continuously but never get ahead of the heat load — especially on very hot days. Signs: the unit runs all day without cycling off, and the house is always 5–10°F above the setpoint on hot days.

This is a system sizing problem and can't be fixed without replacing the unit. As a temporary measure: close blinds on sun-facing windows, use ceiling fans to help distribute cool air, and avoid heat-generating appliances during peak heat hours.

8. Compressor Failing

The compressor is the heart of the AC system — it pumps refrigerant through the cycle. A failing compressor may run but not pump effectively, resulting in weak cooling.

Signs: the outdoor unit hums but the fan doesn't spin, or the unit trips the circuit breaker repeatedly. This is a major repair ($600–$2,000) or full replacement depending on unit age. If the system is over 10–12 years old and the compressor is failing, replacement usually makes more financial sense.

9. Electrical Issues

Capacitors help the compressor and fan motors start and run. A failed start capacitor is a common (and relatively cheap — $150–$300 with labor) repair that causes the outdoor unit's fan to not spin, making the whole system unable to cool.

Check: is the outdoor unit's fan spinning when the system is running? If not, this is the likely culprit.

When to Call a Technician

Call a pro when:

The system freezes repeatedly after filter replacement
You suspect a refrigerant leak
The outdoor fan isn't spinning
The unit trips breakers
Nothing above fixes the problem after a full check

A standard diagnostic visit runs $75–$150. Get quotes from at least two HVAC companies before authorizing major repairs.