If you are dealing with AC running but not cooling in Florida summer, welcome to the most common call we get from June through September. The compressor is humming, the fan is spinning, air is coming out of the registers - but the thermostat reads 80 and your house feels like it. Here are the seven causes, ranked roughly by how often we find them, and how to test each one yourself before you call.
Step one: the 20-degree rule
Before anything else, measure your supply-versus-return temperature. An infrared thermometer is $25 at any hardware store. Aim it at the air coming out of a supply register near the air handler, then at the return grille. Subtract.
- 16 to 22 degrees of differential = system is producing cooling normally. The problem is heat gain or duct delivery, not the equipment.
- 10 to 15 degrees = system is producing partial cooling. Refrigerant, coil, or airflow issue.
- Less than 10 degrees = system is barely cooling. Almost always low refrigerant or a frozen coil.
This one number tells you which half of the list to focus on.
Cause 1: dirty filter
The most common cause. A clogged filter chokes airflow across the indoor coil. The coil overcools, ices over, and now you have a block of ice where airflow should be. The system runs constantly because it never reaches setpoint.
- Pull the filter and inspect it. If you cannot see light through it, replace it.
- If the indoor coil is iced, shut the system off and run fan-only for 2 to 4 hours to thaw. Then restart with a clean filter.
Cause 2: dirty outdoor condenser coil
The second most common, especially after a windy week or for any home within 5 miles of the Gulf in Punta Gorda. A clogged condenser coil cannot reject heat. Refrigerant pressures climb, the system loses capacity, and the house creeps up.
- Walk outside and look at the unit. Palm fronds, leaves, grass clippings, dryer lint? Pull them off.
- Look at the fins. If they are matted with debris or coated in grime, a hose rinse helps but a chemical coil cleaning is the real fix - especially after a hurricane or in coastal homes.
- Confirm 2 feet of clearance. Shrubs and stacked storage starve airflow.
Cause 3: low refrigerant (a leak)
If the system was working last summer and is not now, something changed. The most common change in older systems is a small refrigerant leak - usually at a Schrader valve, a flare fitting, or a corroded coil. Low charge means the indoor coil cannot absorb heat properly. Symptoms:
- Supply temp differential below 14 degrees.
- Frost on the larger copper line at the outdoor unit (the suction line).
- Hissing sound at the indoor coil or outdoor unit.
- System runs continuously and never reaches setpoint on hot afternoons.
Refrigerant work is federally regulated (EPA Section 608) and not DIY. Anyone who tells you to "just add a can of R410A" is wrong - you have to find and fix the leak first, then weigh in the correct charge.
Cause 4: dirty evaporator coil or blower wheel
The indoor coil and the blower wheel both accumulate dust and biofilm over years. A coated coil insulates itself from the airstream. A dirty blower wheel moves 20 to 30% less air than it should. Both reduce capacity in ways the homeowner cannot see without pulling the access panels. This is a common finding on systems that have not been serviced in 3+ years.
Cause 5: leaky duct
If your air handler is in the attic - which most Florida systems are - the supply ductwork runs through 130-degree attic air. Leaks in the supply trunk or branches dump cold air into the attic and pull hot attic air into the return. We have seen 25% capacity losses on systems with bad duct connections.
- Open the attic access on a hot afternoon while the AC is running.
- Listen for hissing at the duct joints.
- Look at the duct insulation - if it is sagging, separated, or has moisture stains, the duct underneath may be leaking.
Duct leakage testing (a duct blaster) and aerosol sealing (Aeroseal) are real fixes when the leakage is bad enough.
Cause 6: undersized for a new addition or change
Did you finish a Florida room? Add insulation? Install hurricane impact windows? Each of those changes the cooling load - sometimes for the better, sometimes for the worse. A system that was right-sized in 2012 may be undersized in 2026 if you added 400 sq ft of conditioned space. Symptoms: the system runs constantly, especially in late afternoon, and supply temps are normal but the house never catches up.
Cause 7: hot attic and poor envelope
Sometimes the AC is fine and the house is the problem. A 130-degree attic above R-19 insulation pours heat into the ceiling all afternoon. Single-pane windows facing west are radiative space heaters at 4 PM. The AC cannot keep up because the heat gain is genuinely massive.
- Attic insulation upgrade from R-19 to R-38. Often $1.50 to $2.50 per sq ft of blown cellulose. Measurable summer savings in North Port and Cape Coral homes.
- Window film or shutters on west and south exposures.
- Attic ventilation - powered, soffit-and-ridge, or both. Drops attic temps by 20+ degrees in summer.
Quick diagnostic flow
- Measure supply-vs-return temp. Below 14 degrees? Refrigerant or airflow.
- Check the filter. Dirty? Replace and re-test.
- Check the outdoor unit. Blocked? Clean and re-test.
- Look for ice on the indoor coil or suction line. Iced? Shut down, thaw, suspect refrigerant or airflow.
- Still bad? Service call.
When to call us
If you have measured the differential, swapped the filter, rinsed the condenser, and the house still will not cool below 78, that is what a $99 diagnostic visit is for. We work in Punta Gorda, Punta Gorda Isles, Port Charlotte, North Port, and Cape Coral. Call (941) 205-6331 or request service. License CAC1824348.