Capacitor
A component that stores and releases electrical energy to start and run single-phase motors. Capacitors hold a charge after power is removed, which is one reason opening an electrical compartment is technician work rather than a homeowner check.
Also called run capacitor, start capacitor, dual capacitor
What does a capacitor do in an HVAC system?
A capacitor stores and releases electrical energy to start and run single-phase motors. It holds a dangerous charge after the power is switched off, which is why the compartment is technician only.
Capacitor: how it works
A single-phase supply cannot start a motor turning on its own, so the capacitor shifts the current in a second winding out of phase with the first and creates the rotating field the motor needs. A run capacitor stays in circuit and keeps that field efficient; a start capacitor is larger, gives a bigger kick, and drops out once the motor is turning.
Where you meet the term
- In an outdoor unit that hums but does not start, which is the description a technician associates with the starting circuit.
- On a repair invoice, where a capacitor is a low cost part with a high safety requirement attached to fitting it.
- On the part itself, where the microfarad and voltage ratings printed on the case decide the replacement.
How to check it
- Describe the sound rather than opening the panel. A hum with no rotation, or a fan that starts when nudged, are the observations that matter, and both can be reported without removing anything. Owner check
- Isolate, discharge, then measure capacitance. The capacitor is discharged through a resistor before testing, and the measured microfarads are compared against the rating printed on the case. Technician
Capacitor: what it is not
- Switching the breaker off does not make a capacitor safe. It stores energy and has to be deliberately discharged first.
- A capacitor that looks intact can still be out of tolerance. Bulging is a clear failure, but a flat case proves nothing without a measurement.
Capacitor questions
Why did my outdoor unit hum and then stop?
A hum with no rotation describes a motor receiving power but unable to start turning, which puts the starting circuit, the capacitor within it, and the motor itself on the shortlist. Diagnosing which needs a live measurement.
What is a dual run capacitor?
One case containing two capacitors, one for the compressor and one for the outdoor fan, sharing a common terminal. The terminals are marked HERM, FAN, and C.
Can I replace a capacitor myself?
It is not an owner task. The compartment holds live mains terminals and the capacitor itself retains a charge after isolation, so discharging it safely is part of the job.
Where this definition comes from
Written from manufacturer support article material held in the HVAC Bench evidence record. Where a document limits a definition to certain models, that limit is repeated here rather than generalised across a brand.
- Mini-Split Compressor Not Turning OnTrane ductless outdoor operation, breaker stop points, airflow observations, and compressor service limits.