Why Is My AC Not Cooling My House? (Common Causes + What to Do)

It's 90 degrees outside, your AC is running, and your house still feels like an oven. You check the thermostat — it's set to 72 — but the air coming out of the vents is barely cool. Sound familiar?

This is one of the most common calls we get at BetterTemp Heating & Cooling, especially during peak summer heat in Western PA. The good news: some of these problems you can fix yourself in five minutes. Others need a technician. This guide will help you figure out which one you're dealing with.

Start Here: Quick Checks Before Anything Else

Before assuming the worst, run through these first:

Is the thermostat set correctly? Make sure it's set to COOL, not FAN or HEAT, and that the setpoint is actually lower than the current room temperature. It sounds obvious, but it gets skipped more often than you'd think — especially after a power outage or if kids have been near the thermostat.

Is the outdoor unit running? Go outside and check. If the condenser (the big unit outside) is completely off while the indoor air handler is blowing, you have an electrical issue — a tripped breaker, a blown fuse, or a failed component like a capacitor or contactor.

Check your circuit breaker. HVAC systems often run on two separate breakers — one for the air handler inside and one for the condenser outside. If the outdoor unit's breaker tripped, the indoor unit will still blow air, but it won't be cold. Reset it once. If it trips again immediately, stop — something is wrong and you need a tech.

The Most Common Reasons Your AC Isn't Cooling

1. Dirty or Clogged Air Filter

This is the #1 cause of AC problems we see in residential homes, and it's 100% preventable.

When your air filter gets clogged, airflow across the evaporator coil drops. Without enough airflow, the coil gets too cold and freezes over. A frozen coil can't absorb heat from your home — so even though your system is running, it's not actually cooling anything.

What to do: Turn the system off and let it thaw for 1–2 hours, then replace the filter and restart. If it freezes again quickly, there may be another underlying issue.

How often should you change your filter? Every 1–3 months during cooling season, or monthly if you have pets or allergy sufferers in the home.

2. Frozen Evaporator Coil

A frozen coil is usually a symptom of something else — either a dirty filter (see above), restricted airflow from closed/blocked vents, or low refrigerant. Signs of a frozen coil include:

• Warm air blowing from vents despite the system running

• Ice visible on the refrigerant line coming out of your indoor unit

• Water dripping or pooling around the air handler after the system shuts off

What to do: Turn the system to FAN ONLY (no cooling) to let it thaw. Do not run it in COOL mode while the coil is frozen — you can damage the compressor. Once thawed, replace the filter and try again. If it refreezes, call a technician.

3. Low Refrigerant (Freon / R-410A)

Refrigerant is the substance that actually moves heat out of your home. If your system is low on refrigerant, it can't absorb and reject heat efficiently — and your house won't cool down no matter how long the system runs.

Important: Refrigerant doesn't "get used up" like gas in a car. If your system is low, it means there's a leak somewhere. Simply adding more refrigerant without finding and fixing the leak is a band-aid, not a repair.

Signs of low refrigerant:

• Ice on the refrigerant lines or outdoor unit

• Hissing or bubbling sound near the indoor or outdoor unit

• The system runs constantly but can't reach setpoint

• Higher-than-normal electric bills

This one requires a licensed HVAC technician with the right equipment. Refrigerant handling requires EPA 608 certification — it's not a DIY fix.

4. Dirty Condenser Coil

Your outdoor unit releases the heat it pulls from your home into the outside air. If the condenser coil is caked with dirt, cottonwood, or debris, it can't reject that heat efficiently — and your system's cooling capacity tanks.

A dirty condenser also makes the compressor work much harder, which shortens its lifespan significantly.

What to do: You can gently rinse the outdoor unit with a garden hose (spray from the inside out if possible, never a pressure washer). For heavily fouled coils, a professional cleaning with coil cleaner is worth the investment.

5. Failing Capacitor

The capacitor is a small cylindrical component inside your outdoor unit that gives the compressor and fan motors the jolt they need to start up and keep running. Capacitors are one of the most common failure points on AC systems, especially in summer heat.

A weak or failed capacitor can cause:

• The outdoor fan to spin slowly or not at all

• The compressor to hum but not start

• The system to run but produce very little cooling

This is a relatively inexpensive repair (But other Companies charge $275–$400 typically) but it requires turning off power to the unit and working inside the electrical panel of the condenser — not a DIY job for most homeowners.

6. Refrigerant Line Insulation Damage

The suction line — the larger of the two copper lines running from your outdoor unit into the house — should be wrapped in black foam insulation. If that insulation is cracked, missing, or has been chewed by animals, the line absorbs heat before it even gets back to the compressor, robbing your system of efficiency.

What to do: This is one of the easier fixes. Foam pipe insulation is available at any hardware store. Just wrap the exposed section and secure it with UV-resistant tape.

7. The System Is Too Small for Your Home

If your AC runs constantly and never quite gets the house to setpoint, but everything checks out mechanically, the system may simply be undersized for your home or for the heat load of a particularly hot day.

This is more common in older homes where additions were made without upgrading the HVAC, or in homes where the original system was sized incorrectly.

This isn't a repair — it's a replacement and design conversation. A proper Manual J load calculation determines the right size system for your specific home.

8. Duct Leakage

If your ductwork has gaps, disconnected sections, or deteriorated flex duct, a significant portion of your conditioned air may be leaking into your attic or crawlspace instead of reaching your living space. Studies suggest leaky duct systems can lose 20–30% of airflow before it ever hits a vent.

Signs of duct leakage include uneven cooling between rooms, excessively dusty rooms, and higher-than-expected energy bills.

When to Call a Professional

You should call a licensed HVAC technician if:

• You've replaced the filter and the coil is still freezing

• You suspect a refrigerant leak

• The outdoor unit isn't running and the breaker keeps tripping

• The system runs but can't get within 5+ degrees of setpoint on a normal day

• You hear grinding, squealing, or banging from any part of the system

• The system is 10+ years old and hasn't been serviced

Don't ignore these signs hoping the system will "work itself out." A $100 service call today is a lot better than a $3,000–$5,000 compressor replacement or full system swap next month.

Prevent This From Happening Next Summer

The best time to find out your AC has a problem is before you need it — not on the first 95-degree day in July. An annual tune-up in the spring catches weak capacitors, low refrigerant, dirty coils, and airflow issues before they become breakdowns.

At BetterTemp, our BetterComfort Maintenance Plan is just $20/month and includes seasonal tune-ups, priority scheduling, and repair discounts. It's the easiest way to protect your investment and stay comfortable all summer.

Call BetterTemp — We Serve All of Westmoreland County

If your AC isn't cooling and you've worked through this list without finding the fix, give us a call. BetterTemp Heating & Cooling is locally owned and operated in Jeannette, PA. We serve Greensburg, Irwin, Murrysville, Penn Township, New Stanton, and surrounding communities.

No franchises. No call centers. Just a local tech who knows HVAC and will give you a straight answer.

Call or text us to schedule a diagnostic today.

Better Service. Better Price. Better Temp.