The capacitor in your outdoor AC or heat pump unit is a cylindrical component that stores and releases electrical energy to start and run single-phase motors. The compressor and condenser fan are both single-phase motors—they need a phase-shifted current to start rotating. The capacitor provides that shift.
Without a working capacitor, a single-phase motor either can’t start (it hums and draws high current) or runs weakly and overheats. In either case, the compressor trips on thermal protection and the AC blows warm air.
Run capacitor vs. start capacitor
Run capacitor — stays in the circuit while the motor runs. It maintains a phase shift in the auxiliary winding, improving motor efficiency and torque. This is the standard type in residential AC systems. Rated in microfarads (µF or MFD) and voltage (370V or 440V).
Start capacitor — connects only during starting (briefly, via a relay) to boost starting torque. Much higher µF value than a run capacitor. Used on hard-starting compressors or when the run capacitor alone can’t get the motor moving. Common on heat pumps in cold climates and older or worn compressors.
Dual-run capacitor — the most common type in residential outdoor units. One cylindrical can contains two capacitor sections: one for the compressor and one for the condenser fan motor. Typically marked C (common), HERM (compressor), and FAN. The two sections share one can but have independent µF ratings (e.g., 45+5 µF means 45 µF for the compressor, 5 µF for the fan).
| Type | When it’s in the circuit | Typical µF range |
|---|---|---|
| Run capacitor | Continuously, while the motor runs | Motor-specific, e.g. 35–60 µF (compressor) |
| Start capacitor | Briefly, only during startup | Much higher than a run cap — often 100+ µF |
| Dual-run capacitor | Continuously — one can, two sections | e.g. 45+5 µF (compressor + fan) |
Common capacitor brands include Titan (Packard), AmRad, Supco, MARS/JARD, and Totaline (Carrier’s OEM parts brand). Reputable brands are UL-listed and built to the EIA-456-A capacitor standard published by the Electronic Industries Alliance — ask for that documentation the same way you’d verify any other electrical component’s rating.
Reading the capacitor label
Every capacitor is labeled with:
- µF (MFD): capacitance rating. Match this within ±6–10% when replacing.
- VAC: voltage rating. Match or exceed (e.g., 440V can replace a 370V spec; 370V cannot replace 440V).
- HERM / FAN / C (on dual-run): terminal labels.
A 45+5 µF, 370V dual-run capacitor has the compressor section at 45 µF and the fan section at 5 µF. Replacing with a 45+5 µF, 440V unit is acceptable; replacing with a 40+5 µF unit is not.
Signs of a failed capacitor
Compressor hums but won’t start: The most common presentation. The contactor pulls in, 240V reaches the compressor, but the rotor can’t start turning. You’ll hear a loud hum from the outdoor unit, then the compressor trips on internal thermal protection after ~5–30 seconds.
Fan starts but compressor doesn’t (or vice versa on dual-run): One section of a dual-run capacitor can fail while the other remains good. Fan running + no compressor = HERM section failed. Compressor running + no fan = FAN section failed. A dead fan motor can cause the compressor to overheat and trip even if the capacitor is fine—check both.
AC runs but blows warm: A weakened capacitor (not fully failed) lets the compressor start but run at reduced torque. The unit draws more current, runs hotter, and doesn’t cool as efficiently. Capacitance measuring below 10% of spec is a clear replacement indicator.
Capacitor is visually bulging or leaking: The top of the can should be flat. Bulging indicates internal pressure from failing electrolyte—replace immediately.
How to check a capacitor
Safety: Capacitors store charge even after the unit is powered off. Discharge the capacitor before touching its terminals: with power off at the disconnect, use a resistor (e.g., a 20kΩ, 5W resistor) or a commercially rated capacitor discharge tool across each terminal pair. Do not short with a screwdriver—a direct short can damage the capacitor or the tool. Verify discharge with a multimeter before touching terminals.
- Turn off the outdoor unit breaker and pull the disconnect block
- Wait 5 minutes for stored charge to dissipate, or discharge manually
- Photograph the wiring before removing any leads
- Disconnect the capacitor leads and remove the capacitor
- Set a multimeter to capacitance mode (µF)
- Measure each section: HERM-to-C for compressor section, FAN-to-C for fan section
- Reading within 10% of the rated µF is acceptable; outside 10% indicates replacement
Replacement basics
Capacitors cost $10–30 and are available at HVAC supply houses or online. Match µF exactly, and match or exceed the voltage rating. The repair is straightforward:
- Power off, discharge the old capacitor
- Label each wire lead before removing
- Install new capacitor in the same bracket; torque the clamp snugly
- Reconnect leads to matching terminals (HERM to HERM, FAN to FAN, C to C)
- Restore power and test
This is one of the most common DIY AC repairs. The total part-plus-labor cost if done professionally is typically $150–350.
Related glossary terms
- AC contactor — the other common outdoor unit failure; different symptom (pitted contacts vs. weak starting)
- Compressor — what the capacitor is helping start
- Y wire — the thermostat terminal that initiates the cooling call, which the contactor + capacitor serve
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FAQ
Q: My AC worked yesterday and now just hums. Did the capacitor fail overnight? Yes—capacitor failures are often sudden, especially after a hot summer (heat accelerates electrolyte degradation). Yesterday’s successful operation doesn’t rule out a capacitor that was already at the edge.
Q: Can I use a higher µF capacitor than the spec? No. Using a higher µF rating changes the motor’s phase angle and can damage the motor windings or cause overheating. Stay within ±6% of the rated value.
Q: My capacitor measures fine but the AC still doesn’t cool. What else could it be? Check the contactor (pitted contacts), the refrigerant charge (a tech job—requires gauges), and the compressor itself (if the capacitor is fine and the contactor pulls in but the compressor doesn’t start, the compressor windings may be failed—a much bigger repair).
Q: How long do capacitors last? Typically 10–20 years. High ambient temperatures shorten life—capacitors in hot attic-mounted units or in very hot climates fail faster. If your system is over 10 years old and having starting problems, the capacitor is the first thing to test.
For what this repair actually costs installed and the questions to ask before you approve it, see AC capacitor replacement cost.
