Swollen AC Capacitor: Signs, Symptoms, and Replacement Guide

Common Symptoms

• Outdoor unit compressor hums for 1–3 seconds then shuts off — thermal overload trip
• AC breaker trips immediately at startup — compressor drawing locked rotor amps without capacitor assistance
• Condenser fan starts but compressor won't start, or compressor starts but runs slowly
• System cools poorly — runs continuously but house temperature barely decreases
• Visible dome or bulge on the top of the cylindrical capacitor can
• Oil residue, dark fluid staining, or dried oil crust around the capacitor base or terminals
• Capacitor case discolored (brown, black) or visibly cracked
• High amp draw on the compressor — clamp meter reading above nameplate RLA when running

Most Likely Causes

1. 1

   Thermal Degradation — Heat Cycling Breaks Down Dielectric Film (Most Common)

   AC run capacitors use polypropylene film as the dielectric between rolled aluminum foil conductors. Every start cycle and continuous operation at elevated temperatures degrades the film. Inside the condenser cabinet on a 95°F summer day, capacitor operating temperature reaches 140–165°F. Manufacturer ratings typically assume a maximum operating temperature of 70°C (158°F); sustained operation near or above this threshold halves capacitor life with each 10°C increase (Arrhenius rule). As the dielectric breaks down, the capacitor loses microfarad value and eventually shorts internally. Internal shorts cause gas generation inside the sealed can — the internal pressure is what causes the visible dome or bulge on the top of the case. A domed capacitor is a failed capacitor — replace immediately.

2. 2

   Overvoltage or Voltage Spikes — Dielectric Puncture

   Capacitors are rated for a maximum working voltage (typically 370VAC or 440VAC for residential units). Voltage spikes from lightning, utility switching events, or a failing transformer can puncture the dielectric film at weak spots, creating a partial short. Partial dielectric failure reduces microfarad value without always causing the visible dome — this is the 'failed but not obviously failed' capacitor that shows up on a multimeter test as 20–25 µF on a 35 µF rated capacitor. Always use a 440VAC-rated capacitor as a replacement (even if the original was 370VAC) — the higher voltage rating provides a larger safety margin against voltage transients.

3. 3

   Age and Capacitor Chemistry — End of Service Life

   Residential dry-film run capacitors have a typical service life of 5–10 years under normal conditions. After 8–10 years, microfarad value degradation to below the ±6% AHRI tolerance is expected even without visible physical damage. A capacitor that measures 29 µF on a 35 µF unit (17% low) will cause the compressor to run hotter and draw more amps on every cycle, accelerating compressor bearing wear. Proactive replacement at 8–10 years (or any time capacitance drops below 10% of rated value) extends compressor life by reducing thermal stress. The cost of proactive capacitor replacement ($20–$35) is orders of magnitude less than a compressor replacement ($800–$2,500).

4. 4

   Run vs. Start Capacitor — Different Failure Modes

   The run capacitor is always present on a residential AC unit — it is a single cylindrical can (or dual-run can with three terminals: HERM for hermetic/compressor, FAN for condenser fan, C for common). It is in the circuit continuously during operation, not just at startup. The start capacitor (usually found only on hard-start kit installs like Supco SPP6 or MARS 32456) is a higher-microfarad, lower-duty-cycle component that is only in the circuit for 300–500 milliseconds at startup. A start capacitor is physically larger in MFD value (88–108 µF or 161–193 µF) and uses a potential relay to remove it from the circuit once the compressor reaches about 75% of running speed. Start capacitors fail differently — often an open circuit rather than a dome — and their failure shows as startup difficulty specifically (longer startup time, humming on first start of the day) rather than continuous running problems.

Quick DIY Checks

1. 1Visual inspection — identify capacitor condition before touching anything: Turn off the outdoor unit at the disconnect box. Open the side access panel of the condenser (4–6 screws). Locate the capacitor — a cylindrical aluminum can, typically 2–4 inches in diameter and 4–6 inches tall, with terminals on top and a wiring harness. Inspect the top of the can: a flat or slightly concave top is normal. A convex dome, bulge, or top that has lifted higher on one side than the other = the capacitor has internally gassed and is failed. Inspect the base and terminals for oil residue, dark staining, or crystallized deposits. Any visible dome, leak, or bulging = replace immediately, no further testing needed. If the capacitor looks physically normal, proceed to the electrical test.
2. 2Safe discharge procedure — MANDATORY before touching terminals: Wait 5 minutes after removing power at the disconnect for passive discharge through the bleed resistor (most modern capacitors have a 15k–20kΩ bleed resistor that discharges them within 1–2 minutes, but confirm by testing). After 5 minutes, hold an insulated-handle screwdriver by the plastic handle only. Touch the metal blade of the screwdriver across the HERM and C terminals simultaneously — a small spark may occur, which is normal. Then bridge the FAN and C terminals the same way. Then bridge HERM and FAN. After all three pairs have been shorted, the capacitor is fully discharged. Verify by setting your multimeter to DC voltage mode and measuring across HERM-to-C — it should read less than 5VDC. A properly discharged capacitor is safe to handle. Do not skip this step — a 440VAC capacitor can retain enough charge to cause ventricular fibrillation if you contact both terminals simultaneously.
3. 3Microfarad (MFD/µF) capacitance test procedure: After safe discharge, disconnect the wires from the capacitor terminals one at a time (photograph the wiring before disconnecting — mixing up HERM and FAN wires causes the compressor or fan to run on the wrong capacitance section). Set your multimeter to capacitance mode (usually indicated by a capacitor symbol or F/µF setting). For a dual-run capacitor: connect probes to HERM and C terminals — this measures the compressor capacitor section. The reading should be within ±6% of the labeled HERM rating (e.g., a 35 µF capacitor should read 32.9–37.1 µF for AHRI ±6% tolerance; within ±10% is often used as the practical field replacement threshold). Then connect probes to FAN and C — should be within ±6% of the FAN rating (typically 5 µF, so 4.7–5.3 µF). Any reading more than 10% below rated value = replace. A reading of 0 µF or OL = capacitor is open, definitely replace.

1. 4Selecting the correct replacement capacitor: Match three values from the old capacitor label: (1) Microfarad ratings — for dual-run, both HERM and FAN values (e.g., 35/5 µF); (2) Voltage rating — use 440VAC rated replacement even if the original was 370VAC, for better transient protection; (3) Pole configuration — a 3-terminal dual-run (HERM/FAN/C) or 2-terminal single-run. Round capacitors (oval cans also exist) — match the terminal configuration. Capacitors are available at HVAC supply houses, electrical supply stores, and online. Using a capacitor with higher voltage rating (440V vs 370V) is correct and preferred. Using a capacitor with different MFD ratings is not acceptable — the HERM and FAN sections must match the compressor and fan motor specifications. A 35/5 µF cannot be substituted with a 40/5 µF for the compressor section without risking motor overheating.
2. 5Installation and verification: Reconnect wires to the new capacitor in the same configuration as the original — HERM wire to HERM terminal, FAN wire to FAN terminal, C wire to C terminal. Verify all connections are tight (no loose push-on terminals). Restore power at the disconnect. Set the thermostat to cooling mode and observe startup: the compressor should start within 1–2 seconds of the contactor pulling in, without humming or hesitation. Use a clamp meter on the compressor wire to verify current — it should drop from the brief startup peak to the nameplate RLA (Running Load Amps) within 3–5 seconds. Any sustained hum without compressor starting after capacitor replacement indicates a separate problem (locked rotor compressor, contactor issue, or wiring error). Upload a photo of the new capacitor installation to /diagnose for AI verification before closing the panel.

Repair vs Replace

Recommended Tools & Parts

• Dual Run Capacitor 35/5 MFD 440VAC

  Most common residential dual-run capacitor for 2–5 ton split systems. HERM section 35 µF for compressor; FAN section 5 µF for condenser fan. 440VAC rated provides superior transient protection vs 370V. Round 3-terminal (HERM/FAN/C) configuration. Replace if MFD reading is more than 10% below labeled value.

  $15–$30

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• Supco SPP6 Hard Start Capacitor Kit

  Start capacitor and potential relay kit for residential AC compressors. Reduces startup current draw by 50–70%, protecting the contactor and breaker. Recommended when the compressor has hard-start history or when replacing a run capacitor on a system over 8 years old. Installs in 15 minutes.

  $20–$40

  Buy on Amazon →

• Fieldpiece SC260 Dual Display Clamp Multimeter

  HVAC-specific clamp multimeter with capacitance mode (µF), AC current clamp for RLA verification, and AC/DC voltage. Industry standard for capacitor MFD testing in the field. Auto-ranging. The capacitance function is more accurate than general-purpose multimeters for values above 20 µF.

  $80–$130

  Buy on Amazon →

Links are Amazon affiliate links (tag: fixitfastai-20). Prices are estimates.

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