It is 94°F outside, the unit is running, and the air coming out of the vents feels like a warm hair dryer. A window AC not blowing cold air usually traces to one of nine causes, and seven of them are fixable in under 30 minutes with no tools more specialized than a screwdriver and a garden hose. The other two need a tech, but you should rule out the easy ones first because a service call runs $90 to $150 just to show up.
- 1. Check the Air Filter
- 2. Look at the Coils
- 3. Verify the Thermostat Setting
- 4. Confirm the Compressor Is Running
- 5. Measure Voltage at the Outlet
- 6. Check Refrigerant Lines for Damage
- 7. Seal Around the Unit
- 8. Right-Size the Unit for the Room
- 9. Inspect the Drain Pan and Slope
- When It Is Time to Replace
- Pro Tip: The 15-Minute Annual Tune-Up
- FAQ
Run through this list in order. The early checks catch about 80% of the problems homeowners face.
1. Check the Air Filter
A clogged filter chokes airflow before any cooling happens. Pull the front grille — most units use a slide-out filter behind it — and hold it up to a window. If you cannot see light through it clearly, vacuum it or rinse with warm soapy water. Let it dry completely before reinstalling. Manufacturers recommend cleaning every 30 days during cooling season; most homeowners go 6 months and wonder why the unit struggles.
2. Look at the Coils
The evaporator coils sit just behind the filter. If they are coated in gray fuzz or, worse, frosted over with ice, the unit cannot transfer heat. Turn the AC off, leave the fan running for 1 to 2 hours to thaw any ice, then vacuum the fins gently with a soft brush attachment. Bent fins can be straightened with a $6 fin comb from any HVAC supply.
The condenser coils on the outdoor side need the same treatment. A garden hose set to gentle spray, directed from inside out, clears pollen, cottonwood fluff, and bug debris.
3. Verify the Thermostat Setting
Sounds obvious, but it catches plenty of people. The unit should be set to “Cool” — not “Fan” or “Energy Saver” with a high target. Drop the setpoint to 65°F as a test. If the compressor kicks on within a minute and you hear the deeper hum behind the lighter fan motor, the thermostat is fine.
4. Confirm the Compressor Is Running
Stand outside or near the back of the unit. A working compressor produces a low rumble distinct from the fan whir. If you only hear the fan, the compressor is not engaging. Causes include:
- A tripped internal thermal overload (let the unit sit unplugged for 30 minutes)
- A failed start capacitor — looks like a small soda can inside the unit, $15 part, requires basic electrical work
- A seized compressor — replacement is rarely worth it on units under $400
5. Measure Voltage at the Outlet
Window units pull a serious load. A standard 5,000 to 8,000 BTU unit needs 110V to 120V; larger 12,000+ BTU models often need a 220V dedicated circuit. A multimeter at the outlet should read between 114V and 126V. Anything below 108V can prevent the compressor from starting even though the fan runs fine.
Long extension cords are the silent killer here. Manufacturers explicitly void warranty when units are run on extension cords, and the voltage drop alone often explains weak cooling.
6. Check Refrigerant Lines for Damage
You cannot legally recharge a sealed window unit at home — refrigerant work requires EPA Section 608 certification — but you can inspect for obvious leaks. Look for oily film around the copper lines, hissing sounds, or frost forming on a single spot of the coil. These point to a refrigerant leak, which on a unit under warranty (typically 1 year parts, 5 years compressor) is worth claiming. Out of warranty, replacement usually beats repair.
7. Seal Around the Unit
If cool air is coming out of the vents but the room is not getting cooler, the gaps around the unit may be undoing the work. Foam side panels compress and shrink over years. Replacement insulating panels run about $15 on Amazon. Add weatherstripping foam tape around the sash where it meets the unit, and stuff any visible gaps with backer rod or rolled foam.
8. Right-Size the Unit for the Room
An undersized AC will run nonstop on a hot day and never feel cold. The rule of thumb:
- 5,000 BTU: up to 150 sq ft
- 6,000 BTU: 150 to 250 sq ft
- 8,000 BTU: 250 to 350 sq ft
- 10,000 BTU: 350 to 450 sq ft
- 12,000 BTU: 450 to 550 sq ft
Add 10% for sun-facing rooms, 30% for kitchens, and reduce 10% for shaded rooms. A 5,000 BTU unit in a 400 sq ft south-facing living room will lose every battle in July.
9. Inspect the Drain Pan and Slope
Window units are designed to tilt slightly downward toward the outside so condensate drains away. If the unit sits level or tilts inward, water pools, the coils stay damp, and humidity in the room climbs. Use a small level. Adjust the brackets so the back drops about 1/4 inch lower than the front.
When It Is Time to Replace
If the unit is over 8 years old, the compressor is failing, or refrigerant has leaked, replacement is usually the smart call. A solid 8,000 BTU window unit from LG, Frigidaire, or Midea runs $230 to $380 and pulls roughly 25% less power than units sold a decade ago. Look for an Energy Star label and a CEER (Combined Energy Efficiency Ratio) of 12 or higher.
Pro Tip: The 15-Minute Annual Tune-Up
Once a year, before the first hot week, pull the unit out of the window or remove the front grille. Vacuum the coils, rinse the condenser, wash the filter, and check that the drain holes are clear. A 15-minute service in May saves the panic call in July.
FAQ
Why is my window AC running but not cooling?
Most often a clogged filter or dirty coils restricting airflow. Second most common is a stuck compressor that runs only the fan. Check the filter first, then listen for the low compressor rumble.
Can I add refrigerant to a window AC myself?
No. Window units are sealed systems and require EPA-certified technicians to recharge. Most are also not designed to be recharged in the field — leaks usually mean replacement.
Why does my AC blow cold for 10 minutes then warm?
Classic sign of icing on the evaporator coil, usually from a dirty filter or low refrigerant. Turn the unit off, run the fan only for 90 minutes to thaw, then clean the filter and coils before restarting.
How long should a window AC last?
With basic seasonal maintenance, 8 to 12 years is typical. Units run hard in dirty environments — pet hair, smoke, kitchen grease — top out closer to 5 to 7 years.
