Components Reference

What each HVAC component does and how to test it — capacitors, contactors, motors, boards, valves and sensors, explained for the field.

16 guides

• Capacitors: Run vs Start vs Dual-Run (Component Reference)
  A field reference on the three capacitor types you meet in HVAC — run, start, and dual-run — covering what each does, how to identify and size a replacement by microfarads and voltage, and how each one fails.
• Pressure Switches: High, Low, and Air-Proving (Component Reference)
  A field reference on the pressure switches techs meet — refrigerant high-pressure and low-pressure cut-outs and the furnace air-proving (draft) switch — covering cut-in/cut-out, manual vs auto reset, and how to test each.
• Relays: Definite-Purpose vs General-Purpose and Pin Numbering (Component Reference)
  A field reference on HVAC relays — definite-purpose (fan relays) vs general-purpose (ice-cube) relays, how to read coil voltage and contact configuration, normally-open vs normally-closed, and how to pick a replacement.
• The Condensate Float Switch and Condensate Pump (Component Reference)
  A field reference on the condensate float (safety) switch and the condensate pump — the switch types and how they're wired into the control circuit, pump operation, and how each fails.
• The Contactor: Types, Poles, and Sizing (Component Reference)
  A field reference on the contactor as a component — single vs two vs three pole, definite-purpose ratings, and how to size a replacement by pole count, amp rating, and coil voltage.
• The Control Transformer (Component Reference)
  A field reference on the HVAC control transformer — primary/secondary voltages, VA sizing, multi-tap wiring, and why transformers burn out, plus how to test one.
• The ECM / Variable-Speed Motor (Component Reference)
  A field reference on the electronically commutated motor (ECM) — constant-torque (X13) vs constant-CFM variable-speed, the motor module, how to identify and test one, and what to know before replacing it.
• The Flame Sensor and Hot Surface Igniter (Component Reference)
  A field reference on the flame sensor (flame rod) and the hot surface igniter as components — how flame rectification works, healthy microamp and resistance values, the two igniter materials, and how to identify and test each.
• The Furnace Control Board / Integrated Furnace Control (Component Reference)
  A field reference on the integrated furnace control (IFC) board — what it manages, how to read its diagnostic LED, the relays and timings it controls, and how to confirm a board is actually bad before replacing it.
• The Gas Valve: Single-Stage, Two-Stage, and Redundant (Component Reference)
  A field reference on the furnace gas valve — single-stage, two-stage, and redundant designs, the built-in regulator and manifold pressure, coil testing, and natural-gas vs LP setup.
• The Heat-Pump Defrost Board (Component Reference)
  A field reference on the heat-pump defrost board — how time-and-temperature and demand defrost work, the defrost thermostat/sensor, how to force a test defrost, and how to confirm the board is the fault.
• The Limit Switch and Rollout Switch (Component Reference)
  A field reference on the furnace high-limit and flame-rollout safety switches — what each protects against, auto vs manual reset, temperature setpoints, and how to test them without defeating the safety.
• The PSC Motor (Component Reference)
  A field reference on the permanent-split-capacitor (PSC) motor — how it uses a run capacitor, how to read horsepower/voltage/speed-tap/rotation off the nameplate, and how to size and replace one.
• The Reversing Valve Solenoid (Component Reference)
  A field reference on the reversing valve's solenoid (pilot) coil — what it controls, O vs B logic, coil voltage and resistance, and how to test the coil separately from the valve body.
• The Shaded-Pole Motor (Component Reference)
  A field reference on the shaded-pole motor — the simplest small AC motor, where it's used (inducers, small fans), why it has no capacitor, how to identify rotation, and how to replace one.
• TXV vs EEV: The Metering Devices Compared (Component Reference)
  A field reference comparing the thermostatic expansion valve (TXV) and the electronic expansion valve (EEV) — how each meters refrigerant and controls superheat, how to identify and size one, and how each fails.

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