Why HVAC Run Capacitors Fail in Summer — and How to Catch Them Before the Motor Dies

18 juin 2026

Why Run Capacitors Fail in Summer — and How to Catch Them Before the Motor Dies

When Quebec hits its first 30°C day, your phone rings. The compressor won't start. The condenser fan is humming but not spinning. The blower is sluggish. Nine times out of ten, you'll find the same culprit: a weak or dead run capacitor.

Capacitors are the most common HVAC service call of the summer. They're also the most misdiagnosed component on the call. Techs replace motors that were never the problem — the dying capacitor was. Then the new motor burns out in three weeks for the same reason.

This guide covers why summer heat kills capacitors, how to read them, how to test them properly, and how to catch a marginal capacitor before it takes down a perfectly good motor.

What Summer Heat Does to a Capacitor

A run capacitor stores and releases electrical energy 60 times per second to keep a single-phase motor running. Inside is a thin dielectric film soaked in electrolyte. Heat is the enemy of both:

Sustained high temperatures — As ambient hits 30–35°C, the inside of an outdoor disconnect can reach 60–70°C. The electrolyte starts to dry out. Capacitance drops slowly at first, then accelerates.
Continuous run cycles — In spring, the unit cycles on and off. During a heat wave, it runs nonstop for hours. A capacitor that was marginal in May is dead by July.
Voltage spikes — Peak electrical demand causes voltage drops and brownouts. A capacitor rated for 370V exposed to 250V or sudden 440V spikes degrades faster.
Vibration — Hot motors vibrate more. Solder joints and lead wires fail.
Bulging or leaking — The classic dead capacitor sign. By the time the top is domed or you see oily residue, it's been dead for weeks.

Step 1: Know What You're Looking At — Run vs Start Capacitor

These get mixed up at the counter constantly. They look similar but do completely different jobs:

Run capacitor — Stays in the circuit continuously while the motor runs. Oil-filled, metal can, typically 5–80 MFD. Most HVAC condenser fan motors, blower motors, and compressor circuits use one. If this fails, the motor won't start or will draw high amps.
Start capacitor — Engages only during startup, then disconnects via a potential relay. Plastic case, much higher capacitance (100–400+ MFD), short duty cycle. Used on hard-starting compressors and some pump motors. If this fails, the motor hums but won't start.
Dual run capacitor — Two capacitors in one can. Three terminals: C (common), FAN, HERM (compressor). Common in residential split systems. If the fan side dies, the compressor side often still works — and vice versa.

Critical: A start capacitor cannot replace a run capacitor. The duty cycles are completely different. Putting a start cap in a run circuit will destroy it in minutes.

Step 2: Read the Spec Plate Correctly

Three numbers matter — get them all right or the replacement won't work:

MFD (microfarads) — The capacitance. Common values: 5, 7.5, 10, 15, 20, 25, 30, 35, 40, 45, 50, 55, 60, 70, 80 MFD. Must match within tolerance (usually ±6% or ±10%).
Voltage (VAC) — Common ratings: 370V or 440V. A 440V capacitor can replace a 370V — never the other way around.
Tolerance — Printed as ±5%, ±6%, ±10%. Tighter tolerance = better motor performance.

For dual capacitors, you'll see two MFD values (e.g., 45/5 MFD) — the larger is for the compressor (HERM), the smaller is for the fan motor (FAN). Both halves share one voltage rating.

Step 3: How to Test a Capacitor the Right Way

Most techs check capacitance and stop there. That misses half the failures. Here's the full procedure:

Discharge first — Use an insulated screwdriver across the terminals (or a 20kΩ resistor for a controlled discharge). A "dead" capacitor can still hold a painful jolt.
Disconnect both leads — Test out of circuit. In-circuit readings include the motor windings and give false numbers.
Set meter to MFD/capacitance mode — Not all multimeters have this. If yours doesn't, it's the one tool worth upgrading.
Read the value — Compare against the rated MFD. Acceptable range: within tolerance (e.g., 45 MFD ±6% = 42.3 to 47.7 MFD). Below tolerance = replace.
Check both halves of a dual cap — Test HERM-C and FAN-C separately. One side often fails while the other reads fine.
Look for physical damage — Bulging top, oil residue, swollen sides, corrosion on terminals. Any of these = replace, even if MFD reads in tolerance.

Pro tip: A capacitor that reads at the bottom edge of tolerance (e.g., 42 MFD on a rated 45) will continue to drift down with heat. Replace it now or you'll be back next month.

Step 4: Catch the Warning Signs Before Total Failure

A capacitor rarely dies all at once. These are the signs of one on the way out:

Motor hums but won't start — Or starts only after a 5–10 second delay.
Motor takes longer to come up to speed — Sluggish startup on the fan or compressor.
High amperage draw — Running amps creeping above nameplate FLA. Weak capacitance forces the motor to work harder.
Compressor trips on overload — Then restarts a few minutes later when it cools.
Reduced cooling capacity — Customer complains the system "isn't keeping up." Fan is moving less air or the compressor isn't pumping at full capacity.
Capacitor body is hot to the touch — A healthy run cap stays close to ambient. A hot one is dying.

Step 5: Why a Marginal Capacitor Will Kill Your New Motor

This is the call-back trap. You replace a humming condenser fan motor. The system starts up. Customer is happy. Three weeks later, the brand new motor is dead.

The original capacitor was already marginal. The new motor draws excessive starting current and runs slightly hot because the capacitance is below spec. Within weeks, the motor windings fail from thermal stress.

Rule: Replace the capacitor every single time you replace a motor. The capacitor costs $15. The callback costs you a day of labor and a customer's trust.

What to Stock on Your Truck for AC Season

The 80/20 rule of summer HVAC service: 80% of capacitor failures are covered by 20% of part numbers. Here's the truck minimum:

Dual run capacitors: 35/5, 40/5, 45/5, 50/5, 55/5, 60/5 MFD at 370V/440V
Single run capacitors: 5, 7.5, 10, 15, 20, 25, 30, 35 MFD at 370V/440V
Start capacitors: 88-108 MFD, 124-149 MFD, 161-193 MFD, 216-259 MFD at 250V/330V
Capacitor straps — Universal mount brackets for when the original is missing
Wire nuts and quick-disconnects — Most replacements need new terminals

Brands We Stock for Capacitor Replacements

At Motair, we keep a full range of run and start capacitors in stock across our three Quebec locations:

AmRad — Premium oil-filled run capacitors with tight tolerances
Mars / Motors & Armatures — Industry standard, broad MFD selection
Packard — Reliable run and start capacitors
Supco — Wide range of dual run capacitors
Diversitech — Capacitors, straps, and accessories

Don't Wait Until It's 35°C — Stock Up Now

The first heat wave is when the calls hit hardest. By then, every supply house in Greater Montreal is fielding the same demand and the common MFD values disappear from shelves first. Stock your truck before the rush.