Two Capacitors, Two Different Jobs
Most air conditioners have more than one capacitor, and they are not interchangeable. Knowing which kind you are looking at is the first step in ordering the right replacement — and in understanding why a system with a bad capacitor behaves the way it does.
What a Run Capacitor Does
Single-phase motors — the compressor and the fan motors in a residential system — cannot start or run efficiently on their own. A run capacitor shifts the phase of the current in the motor's start winding, giving it the rotating field it needs to turn and then keeping it running smoothly. It stays in the circuit the whole time the motor runs, which is why it is called a run capacitor and why it wears out.
Dual Run Capacitors
A dual run capacitor is two capacitors in one sealed can, sharing a common terminal. The label shows both values — 45+5 µF, for example:
- The larger value (45 µF) serves the compressor, on the terminal marked HERM
- The smaller value (5 µF) serves the condenser fan motor, on the terminal marked FAN
- Both share the terminal marked C
You will find a dual run capacitor in the outdoor condensing unit of nearly every residential split system and heat pump. It is the capacitor that fails most often, because it lives beside the compressor in the hottest part of the system. CoralVolt dual run capacitors are stocked from 5+5 to 70+7.5 µF, all dual-rated 370/440 VAC.
Single Run Capacitors
A single run capacitor has one capacitance value and two terminals. It serves one motor. In a typical residential system you will find them:
- On the indoor blower motor in the furnace or air handler — commonly 5, 7.5 or 10 µF
- On some older or larger condensing units that use a separate capacitor for the compressor and another for the fan, instead of one dual can
Some replacement scenarios use two single capacitors in place of one dual. That works electrically if each value matches, but it means mounting two cans where one used to sit. Where the original is a dual, replace it with a dual.
How to Tell Which You Have
- Count the terminal clusters on top. Three (C, HERM, FAN) means dual run. Two means single run.
- Read the label. Two numbers joined by a plus sign (45+5) is a dual. A single number (7.5 µF) is a single.
- Note where it is. Outdoor unit beside the compressor — almost always dual. Inside the air handler cabinet — almost always a single for the blower.
Why the Two Ratings Matter
When a dual run capacitor weakens, it rarely fails both sections at once. A worn compressor section means the compressor hums but will not start, or trips on hot afternoons. A worn fan section means the outdoor fan spins slowly, needs a push, or stops while the compressor keeps running — which will overheat the compressor quickly. Testing each section separately tells you which is at fault; our multimeter guide shows how.
Replacing Either Kind
- Match the µF value exactly — both values, for a dual
- Match or exceed the voltage rating: 370/440 VAC replaces either 370 V or 440 V originals
- Round and oval are interchangeable; match the mounting strap
Safety
Every capacitor in the system stores a charge after the power is off. Isolate the unit at the breaker and the disconnect, and discharge the capacitor across its terminals before handling it. If any part of that is unfamiliar, call a licensed HVAC technician — a capacitor is inexpensive; a shock is not.
Servicing several units? Dual run sizes are available in multi-packs of 4, 6 and 10.