HVAC Capacitor Testing and Replacement Reference

Why this matters

Capacitors are the single most common HVAC failure part. A weak or failed capacitor explains most "compressor won't start," "blower hums but doesn't run," and "condenser fan won't spin" calls. The test is a 30-second procedure with a capacitance meter; the part is inexpensive. Skipping the test and condemning a motor (5-10× the cost) is the highest-frequency unnecessary part swap in the trade.

Two types of capacitors in residential HVAC

Run capacitor - permanently in the motor circuit. Provides continuous phase-shifted current to the start winding for proper running torque. Used in:

• PSC (Permanent Split Capacitor) blower motors
• Condenser fan motors
• Compressor motors (often combined as "dual run" with the fan cap on the same physical unit)

Start capacitor - only in circuit during starting (centrifugal switch or potential relay disconnects it after speed). Provides high torque boost for starting heavy loads. Used in:

• Some larger compressors
• Heavy-duty pumps
• "Hard start" kits (start cap + potential relay added in parallel with the run cap)

Dual run capacitor - physical unit containing two capacitors in one shell. Three terminals: HERM (to compressor), FAN (to condenser fan), C (common). Common in residential AC units; reduces parts count.

Capacitor ratings

A capacitor is rated by:

Microfarads (μF or "mfd"): the capacitance value. The tolerance is usually ±6% (or ±10% on some). Critical to match the OEM rating closely.

Voltage: AC rating, typically 370 V or 440 V. Use 440 V - works on either 230 V or 240 V system, more margin against transients.

Type: AC motor run (the most common HVAC type); not interchangeable with DC capacitors used elsewhere.

Dual run capacitor markings: look like "45/5 μF / 440V" - meaning 45 μF compressor side, 5 μF fan side, 440 V rating.

Common capacitor sizes

Always match the existing rating from the unit's data plate or the part-number cross-reference. Don't guess.

Symptoms of a failed capacitor

Run capacitor failure (most common):

• Motor hums but doesn't spin (starts with hand help, then runs)
• Motor runs slowly or weakly
• Motor draws excess current
• Motor overheats and trips on thermal overload
• Compressor short-cycles on overload

Start capacitor failure:

• Hard-starting motor doesn't start at all
• Tripped overload after a few seconds of locked rotor
• Sometimes the centrifugal switch sticks too - combined failure

Visual failures (obvious):

• Bulged top (rounded dome) - failed; replace immediately
• Oil leaking from can - failed; replace
• Sides bulged outward - failed
• Discoloration / scorching around terminals - failed
• Cracked case - failed

If you see any of these, don't bother testing - replace.

Testing procedure

Tools:

• Multimeter with capacitance (μF) function (Fluke 116, Klein MM700, Fieldpiece HS35, or similar)
• Insulated screwdriver for safe discharge
• Safety glasses, gloves

Critical safety: capacitors store charge even after power-off. ALWAYS discharge before testing.

Discharge procedure:

1. Power off the unit (disconnect, breaker, AND lockout)
2. Wait 5-10 minutes for residual to drain
3. Use insulated screwdriver to short across the capacitor terminals: place one tip on each terminal, hold contact briefly (you may see a spark on a charged cap)
4. For dual run: discharge HERM-C and FAN-C separately

Test procedure:

1. Disconnect the wiring from the capacitor terminals (note positions)
2. Set multimeter to μF (capacitance) range
3. Touch meter leads across the capacitor terminals
4. Read the measurement
5. Compare to label value
6. Within tolerance (±6% for premium, ±10% for standard) = OK
7. Outside tolerance = replace

Example: Capacitor labeled 45 μF / 440V.

• Read 44.2 μF → within 2% → OK
• Read 38.5 μF → 14% low → replace
• Read 47.5 μF → 5.6% high → marginal; some shops replace, some don't
• Read 0 or OL → open / failed → replace
• Read shorted (near 0 with no charge buildup) → shorted → replace

Dual run capacitor: measure HERM-C and FAN-C independently. Each side must be within tolerance.

Why borderline capacitors should be replaced

A capacitor at 88% of rated value still works - but barely. Symptoms:

• Motor draws higher current than rated (heat / wear)
• Hard-starting in cold weather
• Eventual complete failure (when stressed by extreme heat or thermal cycling)

Replacing a borderline cap during annual maintenance prevents the emergency callback in July when it finally fails. The customer accepts an inexpensive capacitor + 15 min labor much more readily than a full emergency service call.

Replacement procedure

Tools:

• New capacitor (matched μF, matched or higher voltage)
• Insulated screwdriver
• Wire labeling pen or paper labels
• Phone for photo before disconnecting

Steps:

1. Power off and discharge (per above)
2. Photograph the wiring before disconnecting anything
3. Label each wire with paper flag (HERM, FAN, C for dual capacitors)
4. Remove the mounting strap (usually one or two screws)
5. Note the orientation (some have specific mounting; most don't)
6. Install the new capacitor in the same orientation
7. Reconnect wiring per photo and labels: brown wire(s) typically to FAN, yellow or other to HERM, black or other to C - verify against the photo
8. Snug terminal screws (don't crank - terminals can break)
9. Restore power and test

Cycle the unit:

• Listen for normal motor starts (no humming or chattering)
• Verify compressor runs smoothly
• Check amperage if you have a clamp meter (should be at or below nameplate FLA)

Capacitor brands and quality

Mars / Packard / Genteq: standard residential brands; good quality.

TitanPro / TURBO Series: premium, run cooler, longer life claims.

Generic / no-name: sometimes acceptable, often fail early. Avoid for paid service work.

The cost difference is small. Stick with reputable brands.

When the capacitor keeps failing

A new capacitor that fails again within 1-2 years suggests:

• High ambient heat: capacitor near a hot motor or in a sunny enclosure
• Voltage spikes / surges: intermittent overvoltage damages capacitor
• Wrong voltage rating: 370 V cap on a system that runs slightly high
• Motor mechanical issue: weak bearings cause motor to draw more current, capacitor stresses
• Cheap aftermarket capacitor: premature failure

References

• AHRI 540 (motor capacitor standards)
• UL 810 / UL 1995 (capacitor safety)
• Manufacturer service manuals (Carrier, Trane, Lennox, Goodman, Rheem) for specific capacitor part numbers
• Mars / Packard / Genteq / TitanPro technical datasheets