It's a hot Colorado afternoon and your AC is running — you can hear it, you can feel air coming from the vents — but the house just won't cool down. This is one of the most common HVAC calls we get in the Denver metro every summer. The good news: some causes are free fixes you can handle yourself right now.

1. Dirty or Clogged Air Filter (Free Fix)

This is the #1 cause of AC problems. A clogged filter blocks airflow, which causes the evaporator coil to freeze up — and a frozen coil cannot cool your air. Check your filter: if it's gray or you can't see through it, replace it immediately ($5–$15 at any hardware store). After replacing, turn the AC off for 2 hours to let the coil thaw, then restart.

2. Thermostat Set Incorrectly (Free Fix)

Check that your thermostat is set to COOL (not HEAT or AUTO-HEAT), and that the fan is set to AUTO rather than ON. If the fan is set to ON, it runs constantly — including when the AC isn't actively cooling — which pushes warm air through the vents and makes it feel like the AC isn't working.

3. Frozen Evaporator Coil (May Need Pro)

If you see ice forming on the copper lines near your indoor unit, or if there's water pooling around your air handler, your evaporator coil is frozen. This often happens because of a dirty filter (see #1) or low refrigerant. Turn the AC off and let it thaw for 2–3 hours with just the fan running. If it refreezes after, call a technician — you likely have a refrigerant leak.

4. Low Refrigerant / Refrigerant Leak (Needs Pro)

Refrigerant is the substance that actually transfers heat out of your home. When it's low, your AC loses its ability to cool. Signs include: ice on the coil or copper lines, hissing or bubbling sounds from the unit, and a house that never reaches the set temperature. Low refrigerant always means a leak — refrigerant doesn't "run out" on its own. A technician needs to find and fix the leak and recharge the system ($200–$600).

5. Dirty Condenser Coils (Sometimes DIY)

Your outdoor unit has condenser coils that release heat outside. If they're coated in dirt, grass clippings, or cottonwood fuzz (very common in Colorado summers), heat can't escape efficiently. You can spray them down gently with a garden hose from the inside out. If heavily fouled, a professional coil cleaning ($100–$200) will recover significantly more cooling capacity.

6. Failing Capacitor (Needs Pro)

Capacitors are small cylindrical components that start and run the compressor and fan motors. When they weaken or fail, the compressor may hum but struggle to start — meaning the AC runs but doesn't compress refrigerant efficiently. Capacitor replacement is one of the most common summer repairs we do ($150–$300) and can usually be completed in a single visit.

7. Undersized or Aging System

Colorado's elevation affects cooling performance — systems sized for sea level may underperform at Denver's 5,280 feet. If your home was built before 2010 and the AC was never upgraded, it may simply be undersized for the square footage. Similarly, units over 12 years old lose efficiency each year. If your system is struggling every summer and getting older, it may be time for a replacement quote.

When to Call a Professional

• You replaced the filter and the AC is still not cooling after 2+ hours
• You see ice on any part of the system
• You hear hissing, banging, or grinding noises
• The outdoor unit is not running at all
• Your energy bill has spiked without explanation