Dual-Run Capacitors vs. Single-Run Capacitors — What Every Technician Should KnowCapacitors in HVAC condenser units come in two main configurations: single-run and dual-run. Confusing them leads to incorrect diagnosis, improper replacement, and callbacks.Single-Run CapacitorsA single-run capacitor has two terminals and serves one motor — either the compressor or the condenser fan motor. They are oval or round canisters labeled with a single µF rating (e.g., “10 µF / 370V”).When to use: older or simpler systems with separate capacitors for each motor.Dual-Run CapacitorsA dual-run (or “dual” or “combo”) capacitor has three terminals — HERM, FAN, and COMMON — and serves two motors from a single can. The HERM side powers the compressor (hermetic) motor; the FAN side powers the condenser fan motor. The label shows two µF ratings (e.g., “45+5 µF / 440V”), where 45 µF is for the compressor and 5 µF is for the fan.Why This Matters for DiagnosisWhen a dual-run capacitor fails, either the compressor, the fan motor, or both may fail to start. Before condemning a motor, always test the specific section of the capacitor that serves it:Fan won’t start → test FAN–COMMON terminalsCompressor won’t start → test HERM–COMMON terminalsBoth fail → test all three terminal pairsReplacing a Dual-Run Capacitor with Two SinglesIf the exact dual-run replacement is unavailable, it is acceptable to substitute two separate single-run capacitors — one matching the HERM rating, one matching the FAN rating — wired in parallel at the COMMON wire. This is a valid temporary or permanent fix, but ensure both caps are properly mounted and insulated.Voltage Rating Notes370V capacitors are found in older and lower-efficiency systems.440V capacitors can replace 370V caps universally — the higher voltage rating is simply a measure of the capacitor’s insulation, and running at lower than rated voltage is not harmful.Never install a lower voltage-rated cap in place of a higher-rated one.