30 Reasons Your AC Runs Nonstop But The House Won't Cool

You set the thermostat to 70. The display says 74. It just won’t cool enough. The AC has been running so long you can’t actually remember the last time you heard it cycle off, and the bedroom is still warm at bedtime. Sound familiar?

South Florida summers challenge your AC and expose every weak spot in your home. A 95° day with 80% humidity finds every gap, every worn seal, every forgotten penetration in your walls and starts pushing heat and moisture inside, 24 hours a day. Your system can be working perfectly and still lose the fight, because the house itself is sabotaging it.

The frustrating part is that most of these leaks are invisible. You don’t see them. You just feel the muggy spot in the family room, notice the bedroom never quite cools down, and eventually see the power bill (which is its own special kind of pain). But the comfort problem comes first, and it’s usually the sign that something on this list is happening in your home right now.

We’ve been pulling AC permits and servicing systems across Miami-Dade, Broward, and Palm Beach for over 55 years. The same hidden culprits show up over and over. Here are 30 of them, grouped by where they hide, plus what to do about each one.

Doors: The Most Obvious (and Most Overlooked) Leaks Contributing to Why Your AC Won’t Cool

1. The gap under your front door

Stand inside on a sunny afternoon and look at the bottom of the door. If you can see daylight, conditioned air is pouring out and hot, humid air is pouring in. The fix is a $15-$25 door sweep from any hardware store, plus a quick check that the threshold underneath hasn’t worn flat.

2. Worn weatherstripping around the door frame

Run your hand around the inside of the door frame while the door is closed. If you feel warm air at any point, the foam or rubber seal has compressed, dried out, or torn. In South Florida the salt air ages these seals about twice as fast as inland climates, so they may need replacement every 3 to 5 years instead of the 7 to 10 the manufacturer advertises. The fix: peel-and-stick replacement weatherstripping from any hardware store (or Amazon), around $10 to $20 per door. Pull off the old stuff, wipe the frame clean, press the new strip into place. 20-minute job per door.

3. Doors that don’t close flush

If you have to lift, pull, or slam to latch the door, the door has warped or the hinges have loosened. A door that isn’t pulled tight against the jamb leaves a continuous gap along the strike side that you can’t see but the heat finds easily. The fix: tighten the screws on the hinges first. If they spin freely, replace the middle screw on each hinge with a 3-inch screw that bites into the wall stud behind the jamb. That usually pulls the door back into alignment. If the door itself has warped (common with older wood doors in our humidity), it needs to be planed or replaced.

4. The deadbolt and knob holes

When the deadbolt strike plate isn’t perfectly aligned, air whistles right through the hole in the jamb. Same with the knob latch. If you hear a faint whistle near the door on a windy day, that’s it. The fix: unscrew the strike plate, stuff the hole behind it with a small piece of foam backer rod or even rolled-up weatherstripping foam, then re-mount. If the latch isn’t seating fully, file the strike plate opening or reposition it so the bolt seats deeply and pulls the door tight.

5. Sliding glass and balcony doors

This is the single biggest offender in South Florida homes and condos. Five things to check on every slider, with the fix for each:

Dirty track: Vacuum the track thoroughly, then run a damp cloth along it. Built-up dirt is the #1 reason a slider doesn’t close all the way.
Crushed brush seal: Replacement pile/brush weatherstripping is sold by the foot at hardware stores. Pull the old strip out of its groove, slide the new one in. About $15 per door.
Worn felt at the meeting stiles: Same deal. Replace the felt strip where the two doors meet. Usually a 10-minute fix.
Misaligned latch: Adjust the strike plate position on the jamb so the latch pulls the door snug. Two screws to loosen and reposition.
Worn roller wheels: If the door sags or drags, the rollers need adjustment or replacement. There’s an adjustment screw at the bottom of most sliders (consult your model). Full roller replacement is a job for a handyman or door specialist.

If you stand inside a closed balcony door on a hot day and feel warm air anywhere along the edges, work through the list above. Fix the seals before you blame the AC.

6. Pet doors

Even the “insulated” magnetic flaps lose a tremendous amount of air, especially after a few years when the magnets weaken and the flap doesn’t seal flat. If the pet door faces west or south, you’re getting direct radiant heat through it on top of the air leak. The fix: replace the flap (most brands sell replacement flaps for $15 to $40), upgrade to a dual-flap or triple-flap insulated model, or for the best seal, switch to an electronic pet door that opens only when your pet’s collar tag is near it. If the pet door isn’t being used anymore, remove it entirely and patch the opening.

7. The door from your garage into the house

This one almost everyone forgets. Your garage on a July afternoon is 110° to 130°. The door between your garage and your living space needs to be sealed and weatherstripped exactly like an exterior door, because it effectively is one. Treat it the same way: install a door sweep on the bottom (around $15), replace any worn weatherstripping (item #2 above), and if your door is supposed to be spring-loaded but isn’t auto-closing anymore, adjust or replace the closer. Florida building code actually requires this door to be self-closing for fire safety, so it’s a code issue as well as an energy one.

8. Mail slots and old milk doors

Older homes sometimes have these built into the wall. If you have one and it’s not actively in use, foam-seal it from the inside.

Windows: Where the Heat Sneaks Through

9. Caulk gaps between the window frame and the wall

This one catches almost everyone off guard. Stucco walls and window frames don’t expand and contract at the same rate, and the South Florida sun cooks the caulk on the outside of your home into something brittle within a few years. Newer construction is especially prone to gaps as the building settles in the first 2 to 5 years. Walk around the outside of your house and check every window frame perimeter for hairline cracks or pulled-away caulk. Re-caulking is cheap. Heat loss through a poorly sealed window frame can be substantial.

10. Unlocked windows

Locked windows close noticeably tighter than unlocked ones. The lock pulls the sashes together against the seal. If a window is unlocked all summer, it’s leaking even when it looks closed. The fix is the easiest one on this list: walk through the house right now and lock every window. Free, zero effort, immediate improvement.

11. Old single-pane glass and aluminum frames

If your home was built before the 1990s and still has the original windows, the glass alone is a major source of heat gain, and the aluminum frame conducts heat like a radiator. The long-term fix is replacement with double-pane impact windows, which pay back in energy savings and add hurricane protection in one move. The short-term, low-cost fix is solar-control window film applied to the inside of the glass (around $1 to $3 per square foot DIY, or a few hundred dollars per window professionally installed). It reduces heat gain by 40 to 70 percent depending on the film. Cellular or honeycomb interior shades on west-facing windows also help significantly.

12. Hurricane shutter penetrations

Accordion shutter tracks, Bahama shutter brackets, and panel shutter bolt holes all create penetrations through your exterior wall. The holes for the bolts and tracks are supposed to be sealed with caulk or foam. Over years of sun and salt, those seals fail. Walk around your house and inspect every shutter mounting point. The fix: re-seal any cracked or missing caulk with an exterior-grade silicone or polyurethane sealant (sold at any hardware store, around $6 a tube). For larger gaps, use minimal-expansion exterior foam first, then cap with sealant. Pick a color close to your stucco.

13. Storm and casement window seals

The crank-out style windows that swing open like an oven door (awning and casement) depend on a gasket sealing all the way around the perimeter when closed. If the crank mechanism is worn or the latch doesn’t pull the window tight, it leaks. Check by closing the window and pushing on it from outside. Any wiggle is a leak. The fix depends on the cause: if the gasket is cracked or missing, replacement gaskets are sold by window brand (often less than $20 per window). If the crank or latch is worn out, hardware kits are available, or a window-repair specialist can replace the operator in under an hour.

14. Skylights and ceiling sun tunnels

The flashing and gasket around a skylight gets baked by the sun all day. Eventually it cracks. You won’t see the gap from inside, but you’ll feel the heat radiating down from it. The fix: get a roofer to inspect the flashing and re-seal as needed. If the skylight itself is older than 15 years, the seal between the glass panes has likely failed (look for fogging or condensation between the panes), and the whole unit needs to be replaced. For non-opening skylights, a solar shade or interior cellular blind cuts radiant heat dramatically as a stopgap.

The Attic and Ceiling: The Hidden Heat Reservoir

Your attic in a South Florida summer hits 130° to 150°. Anything that connects your living space to that attic is a heat highway.

15. The attic access hatch

Climb up on a ladder and look at the underside of your attic hatch. If there’s no foam insulation glued to the back and no weatherstripping around the frame, you have an uninsulated rectangle of drywall sitting between your hallway and an oven. This is one of the single biggest passive heat leaks in most homes and one of the cheapest to fix. The fix: glue a piece of 2-inch rigid foam board (cut to size, around $20 from any home improvement store) to the back of the hatch with construction adhesive, then run self-adhesive foam weatherstripping around the perimeter of the frame so the hatch seals tight when closed. Total cost under $40. Total time under an hour.

16. Recessed can lights

Older “non-IC-rated” recessed lights have vent holes that connect your ceiling directly to your attic. Hot attic air falls straight down into your living room through every one of them. LED retrofit kits with sealed gaskets fix this and save energy at the same time.

17. Bathroom exhaust fan dampers

The flapper that’s supposed to close when the fan is off often gets stuck open, especially in older homes. Hot attic air falls straight down into your bathroom 24 hours a day. Same problem with your kitchen range hood and clothes dryer vent dampers. The fix: turn the fan off, look up into the grille with a flashlight, and check whether the damper sits closed. If it’s stuck, dust and lint are usually the cause. Remove the grille (usually clipped or screwed in place), wipe the damper clean, and it’ll seat again. If the damper is broken or missing entirely, replace the whole vent assembly (around $25 to $60). Same approach for the kitchen range hood damper and the exterior dryer vent hood: clean or replace.

18. Attic pulldown stairs

If you have a pulldown ladder for attic access, the panel itself is usually uninsulated plywood with a gap around the perimeter. The fix: install an attic stair insulated cover (often called an “attic stair tent” or “attic ladder insulator”). They look like a small zippered tent that sits over the opening from the attic side. Sold at home improvement stores and online for $80 to $150, fits standard openings, and installs in 15 minutes with no tools. Also add foam weatherstripping around the perimeter of the ladder door for a tighter seal.

19. Ceiling fan and light fixture boxes

The electrical box for every ceiling fan and light fixture penetrates the drywall up into the attic. If those penetrations weren’t sealed during installation, hot air is leaking down past the box. The fix is best done from the attic side: locate each electrical box from above, and run a bead of fire-rated caulk or use a small amount of expanding foam around where the box meets the drywall. This is one of those jobs where if you’re not comfortable in the attic in summer (it’s brutal), it’s worth bundling into an electrician visit when you’re already paying for other work.

Outlets, Switches, and Wall Penetrations

20. Electrical outlets and switches on exterior walls

Take a cover plate off an outlet on an exterior wall in summer. Put your hand near the opening. You’ll often feel a draft. Cheap foam gaskets that go behind the cover plate (a few dollars for a pack of 20) cut this leak immediately. Do every outlet and switch on every exterior wall.

21. Cable, internet, and TV mount holes

Same problem, often worse because the holes are larger. Any place where a cable was routed through your exterior wall is a potential leak point. Cover plates and exterior caulk are the fix.

22. The AC line set penetration

Where your refrigerant lines come through the exterior wall from your outdoor condenser, there’s a hole in the wall, usually packed with insulation and caulk. That caulk degrades. Walk around outside and inspect the penetration where your AC lines enter. Re-seal with exterior-grade caulk if you see gaps.

23. Outdoor faucets, hose bibs, and dryer vent hoods

All of these are penetrations through your exterior wall. Check the seal around each one. The dryer vent specifically should have a working flapper that closes when the dryer isn’t running. If yours hangs open, replace the vent hood.

24. Plumbing penetrations under sinks and behind toilets

Open the cabinet under your kitchen sink. Look at where the drain pipe and water lines come through the cabinet floor or back wall. If you see a gap around the pipes, that gap usually leads into a wall cavity that connects to the attic or crawlspace. Hot air comes down through these gaps. Foam sealant in a can fixes this in minutes.

The HVAC System Itself

25. The forgotten air filter

A clogged filter restricts airflow across the evaporator coil. The system runs longer to move the same volume of air, the coil can freeze, and efficiency drops fast. In South Florida, change disposable filters every 30 to 60 days during summer, not the “every 3 months” the package suggests.

26. Closed or blocked supply vents

Closing vents in unused rooms feels like it should save energy. It doesn’t. Your system is balanced to move a certain volume of air. Closing vents raises duct pressure, which causes more leakage at duct seams in the attic and makes the blower work harder. Open every supply vent and move furniture off of them.

27. Blocked return vents

The big intake grilles on your wall or ceiling need clear airflow. A couch pushed against the return is making your system work twice as hard for half the airflow. The fix: walk through your home and identify every return vent (they’re the big ones without adjustable louvers). Pull furniture at least 12 inches away. Don’t drape blankets, towels, or rugs over them. If a return is in a closet (some older homes have this), leave the closet door slatted or undercut for airflow, and never block it with stored items.

28. Duct leaks in the attic

Florida building code allows ductwork to be installed in the unconditioned attic, and most homes are built this way. If those ducts have any leaks, you’re paying to cool your attic. Industry studies estimate 20% to 30% of conditioned air is lost through leaky ducts in typical homes. A duct leakage test from a qualified HVAC contractor will tell you exactly where you stand.

29. The outdoor condenser unit

Walk outside and look at your condenser. Is landscaping crowding it? Is the top covered in leaves, palm fronds, or debris? Is direct afternoon sun beating on it for six hours without any shade? Your condenser needs 2 to 3 feet of clear space on all sides and unobstructed airflow above. A condenser that can’t shed heat is a condenser that runs longer and costs more.

30. Running your fan on “ON” instead of “AUTO”

This one surprises people. When your thermostat fan is set to ON, the blower runs continuously, even after the cooling cycle ends. In South Florida humidity, that means air is constantly blowing across the wet evaporator coil and re-evaporating the moisture your AC just removed, sending it back into your house. Set the fan to AUTO. Your house will feel cooler at the same temperature.

Bonus: The Habits That Sabotage Everything

These aren’t air leaks, but they make your AC work harder and could be why your home, or certain rooms, just won’t cool:

Curtains and blinds open on west and south windows during peak sun. Close them from noon until evening. Direct sun through a window adds the equivalent of a small space heater per room.
Running the dryer mid-afternoon. Dryers vent hot, humid air outside and suck conditioned air from inside the house as makeup air. Run it after sunset.
Long hot showers without the bathroom exhaust fan. That moisture has to be removed by your AC.
Turning the AC completely off when you leave for the day. Humidity climbs while you’re gone, and the system has to remove all of that moisture when you get home, which takes hours and runs the system hard. Set it 4 to 6 degrees higher instead.
Ceiling fans running in empty rooms. Fans cool people, not rooms. An empty fan adds heat from the motor without cooling anyone.
Old incandescent or halogen bulbs. Each one is essentially a small heater. Swap to LED.

When the Equipment Itself Cannot Keep Up

Everything above is about the building. If you have worked through the leaks, the vents, and the habits and the house still will not hold temperature, the problem has moved from the house to the machine. These are the mechanical reasons a system runs all day and still loses ground, and they are worth knowing before you call so you can describe what you are seeing.

The evaporator coil is frozen

Ice on the indoor unit or on the copper line set, weak airflow at the registers, and water around the air handler once it thaws. A frozen coil comes from restricted airflow (a clogged filter, a dirty coil, a weak blower) or from low refrigerant. The system keeps running, but ice is an insulator, so almost no heat moves. Switching the thermostat to Fan Only for a few hours thaws it and checking the filter tells you whether the filter was the cause. If a fresh filter does not fix it, the restriction is further in and needs a technician.

The indoor coil is dirty

The evaporator coil sits in the airstream and collects dust and biofilm, and in humid Florida air it grows a coating that acts like a blanket. A coated coil cannot pull heat out of the air no matter how long the system runs. The signs are gradual: longer run times, weaker cooling, and a musty smell from the vents. We can chemically clean the coil, which is maintenance rather than a repair, and in this climate it is worth doing every two to three years.

The outdoor coil is caked over

Reason 29 covers the clearance around your condenser. This is about the coil surface itself. After a season or two it packs with lawn debris, pollen, dryer lint, and palm fiber. The fan still spins and the compressor still runs, but the unit cannot dump heat outside, so capacity drops and the compressor labors. Skip the garden hose. High pressure water bends the aluminum fins and makes the airflow problem permanent. A proper chemical cleaning is the fix.

The blower motor is failing

The thermostat is calling for cool and the outdoor unit is running, but little or no air arrives at the registers. Sometimes there is a loud hum or buzz from the air handler closet. A weak blower also starves the coil of airflow, which is one of the ways coils freeze. If you smell anything burning, shut the system off at the breaker and call.

The compressor is failing

The compressor pumps refrigerant through the system, so when it goes the cooling stops even though the fan keeps blowing air around the house. Typical signs are an outdoor unit that hums louder than usual and trips its breaker, or one that sits silent and will not start even though the capacitor and contactor test fine. Those two parts are the more common culprits and are covered in detail in the five reasons an AC stops cooling in Florida heat.

The system is undersized for the house

If the AC runs continuously and the house never gets below 78 on a 92 degree afternoon, the equipment may simply be too small for the load. This shows up after an addition, after a replacement was sized by guesswork rather than a load calculation, or in homes where the original calculation was never done properly. Oversizing is its own problem, since a system that is too large short cycles and never runs long enough to pull humidity out. Getting the size right takes a load calculation, not a rule of thumb.

The system is simply near the end of its life

Most residential systems in South Florida give 10 to 15 years. Salt air, humidity, and a duty cycle that runs most of the year all shorten things compared with cooler parts of the country. Past year 10 it is common to see several components fail in a single season, and that pattern, rather than any one failure, is the signal to start planning. ENERGY STAR puts most of the efficiency gain of a newer system in exactly this scenario. If the unit runs on R-22, the calculation shifts further, because production of that refrigerant ended in 2020 and refrigerant side repairs are now expensive enough to be worth weighing against replacement. The full picture is in our R-22 phase-out explainer.

If any of this sounds like your system, request service and describe what you are seeing. Ice on the line set, no air at the registers, and a unit that hums but will not start are three different visits.

When to Call a Pro

A lot of what’s on this list you can check, fix, or seal yourself in an afternoon. Some of it (duct leakage testing, refrigerant levels, condenser cleaning, AC line set inspection, ductwork repairs) needs a licensed HVAC contractor.

If your system is running constantly and your house still feels warm or muggy, get a pro out before the deepest part of summer. Total Repair Pros has been licensed in HVAC, electrical, and plumbing in South Florida since 1970, which means we can address most of the items on this list (AC, dryer vents, outlet sealing, plumbing penetrations) under one roof. Get a free service quote or call us at 954-900-6767.

Your AC is doing its job. The question is whether your house is letting it.

Frequently Asked Questions

Why is my AC running constantly but the house still won’t cool?

In South Florida, the most common reason is hidden heat gain and air leaks the AC has to fight constantly. Doors, windows, attic penetrations, recessed lights, electrical outlets, and duct leaks let conditioned air escape and outside heat in. When the load on the system exceeds what it was sized for, it runs nonstop and still cannot hit the thermostat setpoint. Fixing the leaks lets the AC actually catch up.

What are the most common air leaks in a South Florida home?

The biggest offenders are exterior door weatherstripping, garage doors that double as living space access, window seals and single-pane glass, attic hatches, recessed (can) lights, plumbing and wiring penetrations into the attic, electrical outlets on exterior walls, and duct leaks in the attic. Many of these are hidden behind drywall or above ceilings, which is why homeowners rarely notice them until utility bills spike.

Does attic insulation really affect how cool my house feels?

Yes, dramatically. A South Florida attic can hit 130 to 150 degrees on a summer afternoon. Without adequate insulation (R-30 or higher) and a proper radiant barrier, that heat conducts straight down through the ceiling into your living space. Underinsulated attics can add several tons of cooling load to a home and are one of the biggest reasons a properly sized AC system still cannot keep up.

Can leaky ducts cause my AC to run all the time?

Yes. Duct leaks in the attic can lose 20 to 30 percent of conditioned air before it ever reaches a register. That air dumps into the attic instead of cooling the house, which means the AC runs longer to compensate while simultaneously creating negative pressure that pulls hot, humid outdoor air into the home. Sealing ducts with mastic or aeroseal often produces immediate, measurable improvements in cooling and humidity.

How do I find air leaks in my house without special equipment?

On a hot day with the AC running, walk room to room and feel along door frames, window sashes, attic hatches, recessed lights, and outlets on exterior walls. Anywhere you feel warmth, humidity, or moving air is a leak. A stick of incense or a lit candle shows air movement clearly. Professional energy auditors use blower-door tests and infrared cameras for a complete picture, but a manual walk-through finds most of the major leaks.

When should I call an HVAC technician versus fixing it myself?

Call a pro when the AC is short-cycling, freezing up, blowing warm air, tripping breakers, or making unusual noises. Also call if the home does not cool below 78 degrees on a 90-degree day even with the system running constantly, which suggests undersizing, low refrigerant, or a failing compressor. Sealing weatherstripping, caulking outlets, and adding attic insulation are DIY-friendly. Anything inside the equipment, refrigerant lines, or ductwork should be a licensed technician.

How long should a residential AC system last in South Florida?

Most residential systems in South Florida last 10 to 15 years. Salt air, humidity, and a duty cycle that runs most of the year shorten the lifespan compared with cooler climates. After year 10, it is common for several components to fail in a single season, and that pattern is usually the signal to start planning a replacement rather than any single breakdown.

Is it worth repairing an AC system that runs on R-22 refrigerant?

Usually not, when the failure is on the refrigerant side. R-22 production in the United States ended in 2020, and remaining stock is expensive enough that most refrigerant side repairs approach the cost of replacing part of the system. Repairs on the non-refrigerant side, such as capacitors, contactors, and electrical parts, are still reasonable on an R-22 unit.