AC Running But Not Cooling? Most Common Causes in the Bay Area

It’s a warm Bay Area afternoon, your AC is clearly running, you can hear the fan… and yet your house still feels like it’s stuck in “mildly annoying” mode.

You walk to a vent:

Air’s coming out.

But it’s not really cold.

So what’s going on?

In this guide, Galaxy Heating & Air Conditioning breaks down the most common reasons an AC runs but doesn’t cool in Bay Area homes — and when it’s a quick DIY fix vs when you really should call a pro.

📍 Serving Contra Costa, Alameda & San Francisco counties

📞 AC not keeping up? Call or text (925) 578-3293.

Quick Checklist: AC Running But Not Cooling

Before we dive deeper, here’s the short version of what to check first:

Is the thermostat set to “Cool” and fan on “Auto”?
Is the air filter dirty or clogged?
Are supply vents and return grilles open and not blocked?
Is the outdoor unit running and not buried in leaves/debris?
Is there ice on the refrigerant lines or indoor coil?
Do you feel any temperature difference between return air and supply air?

If everything looks okay and you still have warm air, it’s time to look at some of the more common causes.

1. Thermostat Settings & Issues

Let’s start with the simplest things.

1.1 Wrong Mode or Fan Setting

Make sure:

Mode is set to Cool (not “Heat,” “Off,” or just “Fan”)
Fan is set to Auto (not “On”)

If the fan is set to On, it will blow room-temperature air even when the compressor outside isn’t cooling. That can make it feel like the AC is “running but not cooling” when in reality the cooling cycle is off.

1.2 Miscalibrated or Bad Thermostat

Signs your thermostat might be the issue:

Display is blank, glitchy, or unresponsive
Room feels much warmer than the set temperature
System short cycles (turns on and off quickly)

Sometimes a low battery or a wiring issue at the thermostat can stop the cooling signal from getting to the system. Replacing batteries or checking basic settings is fine DIY; digging into wiring is usually pro territory.

2. Dirty or Clogged Air Filter (Super Common)

A clogged filter is one of the most common reasons for poor or no cooling.

When the filter is too dirty:

Airflow drops dramatically
The evaporator coil (the part that gets cold) can freeze up
You may feel weak airflow and not much temperature change

If the coil is frozen:

You might see ice on the refrigerant lines near the indoor unit
The outdoor unit might be running, but hardly any cooling is happening

What to do:

Turn the system off at the thermostat.
Replace the air filter with the proper size and MERV rating.
If there’s visible ice, leave the fan on “On” (no cooling) for a few hours to help melt it, or just let the system sit off until completely thawed.
Once everything is thawed and the filter is new, turn cooling back on and see if performance improves.

If it freezes again, there’s likely another underlying issue (coil cleanliness, refrigerant level, airflow, etc.) that needs a tech.

3. Dirty Coils (Indoor or Outdoor)

Your AC relies on two key coil surfaces:

Indoor evaporator coil (the one that gets cold and absorbs heat from the air)
Outdoor condenser coil (the one that dumps that heat outside)

3.1 Dirty Outdoor Condenser Coil

In the Bay Area, outdoor units can get coated with:

Dust and dirt
Leaves and yard debris
Lint, pollen, and spider webs

When the outdoor coil is dirty:

The system can’t reject heat efficiently
You’ll get warm-ish air inside, long run times, and possible system strain

Homeowner-safe step:

Gently clear leaves and debris away from the unit.
Make sure there’s 12–24 inches of clear space around it.
You can lightly rinse the outside of the coil with a garden hose (power off first!), never using a high-pressure washer and never bending fins.

For deeper cleaning or if the coil is caked, that’s a job for a tech.

3.2 Dirty Indoor Evaporator Coil

If the indoor coil is dirty:

Heat transfer drops
Coils may freeze up
Cooling drops even though the system runs

This coil is usually behind a panel and can be easy to damage if you don’t know what you’re doing, so indoor coil cleaning is typically a professional service.

4. Low Refrigerant / Refrigerant Leaks

Another big one: low refrigerant charge.

Despite what some people say, refrigerant isn’t like oil in your car – you don’t “use it up.” If your system is low, it usually means:

There’s a leak somewhere in the system.

Symptoms of low refrigerant:

AC runs a long time but doesn’t cool well
Ice on indoor coil or refrigerant lines
Hissing or bubbling sounds (sometimes)
Higher energy bills with worse comfort

Just “topping off” refrigerant every year without addressing the leak is like refilling a bucket with a hole in it.

This is not DIY.

You’ll need:

Leak detection
Repair (if feasible)
Proper evacuation and recharge to manufacturer specs

In many older systems, especially if the leak is major or the system uses older refrigerants, replacement may be the better long-term choice.

5. Blower or Airflow Issues

If your indoor blower isn’t moving enough air, the AC can’t do its job.

Common issues:

Blower motor failing or running at the wrong speed
Bad capacitor
Loose or damaged fan belt on older units
Control board issues impacting fan operation

Signs:

Weak or inconsistent airflow from vents
System runs but you barely feel air movement
Strange noises from the indoor unit

Some airflow issues (like a capacitor) are relatively simple for a tech; others may require more involved repairs. Either way, blower problems are usually professional fixes.

6. Ductwork Problems (Leaky, Disconnected, or Undersized)

In Bay Area homes, duct issues are a huge reason for “AC running but not cooling.”

Problems we see all the time:

Leaky ducts blowing cold air into the attic or crawlspace instead of rooms
Disconnected or partially connected ducts
Undersized ducts that can’t carry enough cold air to distant rooms
Crushed or kinked flex duct strangling airflow

Symptoms:

Some rooms okay, others barely cool at all
Unit runs constantly yet the house never quite reaches setpoint
Dusty or uncomfortable areas even when the AC is new

Sometimes you’ll even feel the attic is cold while the house isn’t — that’s your cooling going to the wrong place.

Fixes range from:

Sealing and repairing problem sections
Resizing ducts for proper airflow
In some cases, full duct replacement and redesign

7. System Sizing and Age

If your AC is running but not cooling well, and it’s old, sizing and wear might be part of the problem.

7.1 Undersized System

An undersized system will:

Run nearly non-stop on hot days
Still struggle to bring indoor temps down
Keep you in that “not miserable, but never comfortable” range

You might think, “Isn’t that just because it’s hot?”

Sometimes, yes. But if it never seems to catch up, even when evenings are cool, it may simply not be big enough or properly matched to the house and ducts.

7.2 Oversized System

Oversized systems bring a different problem:

They short cycle – turning on and off a lot
They may cool the air quickly near the thermostat but fail to fully cool the whole house or properly dehumidify
Air doesn’t get a chance to mix evenly through the ducts

You may feel like the AC works “for 5 minutes, then shuts off, then comes back again,” but the house never feels evenly comfortable.

8. Bay Area-Specific Issues: Older Homes & Additions

A lot of Bay Area houses:

Were built before AC was common
Have additions, converted garages, or sunrooms
Have unusual ductwork or DIY expansions

In these homes, your AC might technically be working but still not cooling certain areas because:

Those rooms were never properly ducted or sized
Insulation is poor
Windows and sun exposure overwhelm what the system can handle

Sometimes the best fix isn’t forcing the existing system to do more, but:

Ductless mini splits in problem areas
Zoning and duct modifications
New heat pump systems better matched to the actual load

9. When to Shut It Off and Call a Pro

You should turn off the system and call a pro if you notice:

Ice on refrigerant lines or the indoor coil
Burning smells or signs of electrical issues
Loud, new grinding, squealing, or banging sounds
The outdoor unit isn’t coming on at all, but indoor is running
Repeated breaker trips

Continuing to run the system in those conditions can cause more damage and more expensive repairs.

10. How Galaxy Handles an “AC Running but Not Cooling” Call

When we come to your Bay Area home for this kind of issue, we don’t just guess and throw parts at it. We typically:

Listen to your symptoms:
- When did it start?
- Certain rooms worse than others?
- Any changes (remodel, new thermostat, filter, etc.)?
Check basics first:
- Thermostat, filter, blower operation, vents, outdoor unit
Measure actual performance:
- Temperature difference between return and supply
- Refrigerant pressures/temperatures (for licensed techs only)
- Electrical readings on key components
Inspect ducts and coils:
- Look for obvious duct leaks or disconnections
- Check coil condition (indoor and outdoor)
Explain what we find in plain language and give you options:
- Repair (for specific failed parts or leaks)
- Maintenance/cleaning to restore performance
- Upgrades if your system is at the end of its life or badly sized

Our goal is to solve the root cause, not just reset something and hope it holds.

11. FAQ – AC Running But Not Cooling

Q: How long should it take my AC to cool the house?

A: It depends on outdoor temperature, insulation, sun exposure, and system size. On a typical warm Bay Area day, you should see steady progress toward your setpoint. If it runs for hours with almost no change, something’s off.

Q: Is it safe to run the AC if it’s not cooling well?

A: If you suspect freezing, low refrigerant, or electrical issues, it’s better to shut it off and call a pro. Running it that way can stress the compressor and other components.

Q: Can I just add more refrigerant myself?

A: No. Handling refrigerants requires proper licensing and tools. Adding refrigerant without fixing leaks or checking charge correctly can damage the system and violates regulations.

Q: My upstairs is hot but downstairs is fine. Is that the AC or the house?

A: Usually a mix of both — heat rises, and many older duct systems weren’t designed for perfect upstairs comfort. Duct balancing, zoning, or ductless mini splits upstairs often solve this.

Q: Is it worth fixing my old AC, or should I replace it?

A: If the system is 10–15+ years old, needs a major repair, and doesn’t keep you comfortable, replacement is often smarter. If it’s relatively young and the issue is something simple (capacitor, contactor, minor leak), repair can be a good choice. We’ll walk you through both options.

12. AC Running But Not Cooling in the Bay Area? We Can Help.

If your AC is running but your home still feels like a lukewarm disappointment, you don’t have to guess what’s wrong.

Galaxy Heating & Air Conditioning can:

Diagnose whether it’s a simple fix, airflow problem, refrigerant issue, or ductwork
Perform professional maintenance and repairs to restore cooling
Recommend smart upgrades (heat pumps, ductless, zoning, or new ducts) if your current system is at the end of the line