Which capacitor do I need?

Safety first

To discharge it, use a capacitor discharge tool or a resistor rated for the job, with insulated handles. Touch it across the terminals for a few seconds. Not comfortable doing this? Stop here and call a licensed HVAC tech.

1Read the label on the old one

Every capacitor has a printed label. It shows the µF rating and the voltage. µF is also written as MFD or uF. They mean the same thing.

Take a photo of the label and of the wires before you remove anything.

2Match the µF exactly

The µF rating must match. A dual capacitor shows two numbers, like 45 + 5 or 45/5. The big number is for the compressor (HERM). The small number is for the fan (FAN).

The label also shows a tolerance, like ±5% or ±6%. That is the normal range. It does not mean you can pick a different size.

3Volts: same or higher

Most AC capacitors are rated 370 V or 440 V. You can always replace a 370 V part with a 440 V part. Never go lower. A 370 V part cannot replace a 440 V part.

4Dual, single or start

• Dual run: one can, three terminals (HERM, FAN, C). Runs the compressor and the outdoor fan.
• Single run: two terminals. Runs one motor, like a fan or blower.
• Start capacitor: used in hard start kits. It shows a range, like 88 to 108 µF. It is not a run capacitor. Do not swap one for the other.

5Round or oval

Shape does not change how it works. Round and oval capacitors with the same ratings do the same job. Pick the shape that fits your bracket and lets the wires reach.

Signs of a bad capacitor

• The outdoor fan hums but does not spin.
• The unit tries to start, then stops.
• The top of the capacitor is domed or swollen.
• Oil is leaking from the can.

A swollen top is a clear sign. A capacitor can also fail and still look fine. A tech can test it with a meter that reads µF.

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