Understand, diagnose, and identify failed AC capacitors — the single most common air conditioning repair in South Florida.
The capacitor is the most frequently replaced part in residential air conditioning systems, and South Florida's climate makes it worse. Capacitors are electrolytic or film-based components that store and release electrical energy to start and run your AC's compressor and fan motors.
In a climate where outdoor temperatures regularly hit 90–95°F and condensing units sit in direct sunlight, capacitor operating temperatures can exceed 150°F. Every degree above the rated temperature shortens the capacitor's lifespan exponentially. A capacitor rated for 10 years in a temperate climate may last 3–5 years in South Florida.
Same-day AC capacitor replacement in South Florida: AC Repair Today — Licensed FL Contractor CAC1824118, (305) 850-6810
Your AC system uses capacitors in two roles:
- Provides a burst of energy to get the compressor motor spinning
- Engages for 1–3 seconds during startup, then disconnects
- Higher capacitance (80–400 µF), lower voltage rating
- Failure symptom: compressor hums but won't start
- Maintains the electromagnetic field while the motor runs
- Stays in the circuit continuously during operation
- Lower capacitance (5–80 µF), often dual-rated (two capacitors in one can)
- Failure symptom: system runs but blows warm air, or fan spins slowly
Most residential systems use a dual run capacitor — a single cylindrical can with three terminals:
┌───────────┐
│ HERM │ ──→ Compressor
│ (C) │
│ COM │ ──→ Common (shared)
│ (C) │
│ FAN │ ──→ Condenser fan motor
└───────────┘
- HERM: Hermetric terminal — connects to the compressor
- FAN: Connects to the condenser fan motor
- COM (C): Common terminal — shared ground for both circuits
A typical dual capacitor is rated like 45/5 µF 440V — 45 microfarads for the compressor, 5 microfarads for the fan, rated at 440 volts.
| Symptom | What's Happening |
|---|---|
| AC hums but doesn't start | Start capacitor or compressor-side run capacitor failed |
| Fan spins slowly or not at all | Fan-side of dual run capacitor failed |
| System starts but shuts off after a few minutes | Weak capacitor causing compressor to draw excess amps |
| Clicking sounds from the outdoor unit | Contactor engaging but compressor can't start |
| Higher than normal electric bills | Weak capacitor forcing the motor to work harder |
| AC blowing warm air intermittently | Capacitor failing under heat, recovering when cool |
| Burning smell from outdoor unit | Capacitor venting — shut off immediately |
| Swollen or bulging capacitor can | Electrolyte breakdown — replacement needed |
Before testing with a meter, a visual check can identify obvious failures:
- Flat or slightly concave top
- No discoloration or rust on the casing
- Terminals are clean and tight
- No oily residue on the capacitor or surrounding area
- Bulging or domed top — internal pressure from electrolyte gas
- Oil leaking from the bottom or seams — dielectric fluid escaping
- Burn marks on the terminals or wiring — arcing from loose connections
- Cracked or split casing — severe failure, usually accompanied by a pop sound
- Rust or corrosion — moisture intrusion compromising the case
Safety Warning: Capacitors store electrical charge. Always disconnect power at the breaker AND discharge the capacitor before testing. Use an insulated screwdriver across the terminals to discharge safely.
- Turn off power at the disconnect box and breaker panel
- Photograph the wiring before disconnecting anything
- Discharge the capacitor using an insulated screwdriver across HERM↔COM and FAN↔COM
- Disconnect wires from the capacitor (label them if needed)
- Set multimeter to capacitance mode (µF symbol)
- Test HERM to COM: Should read within ±6% of the rated value (e.g., 42–48 µF for a 45 µF rated)
- Test FAN to COM: Should read within ±6% of the rated value (e.g., 4.7–5.3 µF for a 5 µF rated)
| Reading | Status | Action |
|---|---|---|
| Within ±6% of rated | Good | Capacitor is healthy |
| 6–10% below rated | Weak | Replace soon — nearing end of life |
| More than 10% below rated | Failed | Replace immediately |
| Zero or OL (open line) | Dead | Shorted or open — replace |
| AC System Size | Typical Compressor µF | Typical Fan µF | Common Dual Rating |
|---|---|---|---|
| 1.5 ton | 25–30 | 3–5 | 25/5 or 30/5 |
| 2 ton | 30–35 | 5 | 30/5 or 35/5 |
| 2.5 ton | 35–40 | 5 | 35/5 or 40/5 |
| 3 ton | 40–45 | 5 | 40/5 or 45/5 |
| 3.5 ton | 45–50 | 5–7.5 | 45/5 or 50/5 |
| 4 ton | 50–55 | 5–7.5 | 50/5 or 55/5 |
| 5 ton | 55–80 | 5–10 | 60/5 or 80/10 |
Always match the exact rating printed on the capacitor. Using the wrong capacitance can damage the motor.
| Document | Description |
|---|---|
| guides/when-to-diy-vs-call-pro.md | Safety assessment — what's safe to check yourself and when to call a technician |
| guides/extending-capacitor-life.md | Maintenance tips to maximize capacitor lifespan in hot climates |
Understanding why capacitors fail helps prevent repeat failures:
- Heat exposure: Operating above rated temperature is the primary killer. Shade structures or proper unit placement helps.
- Electrical surges: Lightning and power grid fluctuations (common in Florida summers) damage capacitor dielectric.
- Short cycling: Frequent on/off cycles from an oversized system or thermostat issues accelerate wear.
- Age: Electrolytic capacitors naturally degrade. 5–7 years is normal lifespan in South Florida conditions.
- Voltage mismatch: A capacitor rated at 370V running on a 440V circuit will fail prematurely. Always use the correct voltage rating or higher.
- Manufacturing defects: Some batches from certain manufacturers have higher failure rates. Name-brand capacitors (Amrad, TurboTech, MARS) last longer than generic imports.
Have corrections or additional diagnostic tips? Open an issue or submit a pull request. Real-world experience from HVAC technicians is especially welcome.
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AC not starting? A failed capacitor is the most likely cause. AC Repair Today offers same-day capacitor replacement across Miami-Dade, Broward, and Palm Beach counties. Call (305) 850-6810.