Code 79 = Combustion fan fault. The control board commanded the fan to start (or run at a specific RPM) and either didn't receive the expected RPM feedback signal, or the fan didn't respond.
Diagnostic sequence
1. Listen for the fan
When the unit attempts to fire, you should hear the combustion fan ramp up (audible whir from inside the cabinet). If silent, the fan motor is dead, the power lead is disconnected, or there's a PCB output failure.
2. Inspect air inlet
If the fan is heavily loaded (intake blocked), it may not reach commanded RPM and throws Code 79. Clean the intake filter and inspect the intake vent termination.
3. Check wiring
Power harness from PCB to fan motor — loose connector or damaged pin pulls voltage. Visual inspection plus a multimeter check on the harness.
4. RPM feedback wire
Many Rinnai fan assemblies include a Hall-effect RPM sensor on the fan shaft. If the sensor or its return wire fails, the fan may spin correctly but the PCB still throws Code 79 because it can't confirm RPM. Continuity test the sensor wire pair.
5. Fan motor
If the fan itself is bad, replacement is dealer-level. The fan assembly (fan + motor + housing) is typically a single SKU. Cost: $150-300 parts.
Reset procedure
- Power cycle the unit
- Listen carefully for the fan during the next ignition attempt
- If silent, escalate to wiring / motor diagnosis
- If audible but Code 79 returns, RPM sensor or feedback wire
Common scenarios
- Code 79 only in winter: intake icing — check vent termination clearance from snow line
- Code 79 + grinding/noisy fan: bearings failing, replace fan assembly
- Intermittent Code 79: loose harness connector — re-seat and add dielectric grease
Bottom line
Code 79 is hardware — the fan is the suspect or the PCB output to the fan. Cleaning the intake and re-seating connectors resolves a meaningful fraction. Persistent Code 79 = installer call for fan replacement. See customer service.