Thermocouple
Common pilot and ignition part on gas water heaters.
Rinnai CBU Combi Boiler Error Codes — What They Mean
The Rinnai CBU (CBU199iN, CBU199eN) is a condensing combination boiler providing space heating and domestic hot water in a single unit. It uses PVC venting, a stainless steel primary heat exchanger, and a brazed-plate secondary heat exchanger for DHW production. Fault codes are displayed on the wired remote controller (MC-91-2) or built-in display.
Rinnai CBU Error Code Reference
| Code | Meaning |
|---|---|
| 02 | Bypass control valve fault |
| 10 | Exhaust temperature sensor fault |
| 11 | Ignition failure |
| 12 | Flame loss during operation |
| 14 | Thermal fuse tripped |
| 16 | Outlet water overtemperature |
| 25 | Condensate drain fault |
| 31 | Inlet air temperature sensor fault |
| 33 | Heating supply temperature sensor fault |
| 34 | Heating return temperature sensor fault |
| 52 | Modulating gas valve fault |
| 57 | Combustion fan fault |
| 61 | Heating pump fault |
| 65 | Water flow control fault |
| 71 | Gas valve solenoid fault |
| 72 | Flame sensor fault |
| 80 | Heating system pressure low |
| 81 | Heating system pressure high |
| 84 | Heating temperature too high (safety limit) |
Common Causes by Code
- Code 11 — Ignition failure — Low gas pressure, blocked combustion air intake, or a failed igniter/gas valve. The CBU requires a minimum gas pressure of 3.5” W.C. at the inlet — check this under firing conditions, not static.
- Code 14 — Thermal fuse — One-time safety device. A tripped thermal fuse indicates the exhaust pathway has exceeded safe temperatures. Causes include a blocked exhaust vent, severely scaled heat exchanger, or abnormal combustion. Do not simply replace and restart without identifying the root cause.
- Code 25 — Condensate drain — The CBU condensate drain must flow freely at all times. In cold climates, the drain line should be insulated to prevent freezing. A blocked drain causes condensate backup into the heat exchanger, which can quench the flame or cause Code 12 (flame loss).
- Code 61 — Heating pump — The internal circulation pump for the heating circuit has failed or is not responding to the control board signal. Check pump power supply and confirm the pump is not air-locked.
- Code 80 — Low heating pressure — The CBU heating circuit requires a minimum pressure of approximately 12 PSI. If the pressure drops (visible on the pressure gauge), add water through the fill valve. Repeated pressure loss indicates a leak.
- Code 81 — High heating pressure — Expansion tank has failed (lost pre-charge pressure) or the fill valve is stuck open. Test the expansion tank bladder by pressing the Schrader valve — if water comes out instead of air, the bladder has failed.
- Code 84 — Heating overtemp — The heating circuit return temperature is too high, or there is insufficient flow through the heating circuit. Check the heating pump, confirm all zone valves are open, and verify adequate water flow.
Step-by-Step Fix
- Read the remote controller — The MC-91-2 shows the error code. Hold the INFO button to view current system pressures, temperatures, and flow.
- For Code 11 — Confirm gas at the meter, check other appliances. Inspect both the combustion air intake and exhaust vent at the exterior termination. If venting and gas are confirmed, proceed to igniter and gas valve check.
- For Code 25 — Trace the condensate drain from the unit to the drain outlet. Clear any blockage with warm water. If the drain line runs through an unheated space, confirm it is insulated.
- For Code 61 — Locate the heating pump on the CBU (typically at the left or bottom of the unit). Listen for pump hum during a heating call. No hum = power issue. Hum without flow = pump airlock. Bleed the pump using the bleed screw on the pump body.
- For Codes 80 / 81 — Check system pressure gauge on the unit front. For Code 80, add water through the fill valve. For Code 81, check the expansion tank — press the tank Schrader valve; if water exits, replace the tank.
- For Code 14 — Test the thermal fuse for continuity. Replace only after descaling the heat exchanger and confirming the exhaust vent is fully clear.
- Reset — Press ON/OFF on the remote controller to reset after correcting the fault.
Parts Often Needed
| Part | Notes |
|---|---|
| Thermal fuse | One-time safety device; investigate first |
| Expansion tank | For Code 81; check pre-charge before replacing |
| Heating pump (circulator) | For Code 61; bleed airlock first |
| Condensate drain trap | Replace if cracked or blocked |
| Flame sensor | For Code 72; clean first |
| Gas valve | For Code 71; confirm 24V signal first |
Need Replacement Parts?
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When to Call a Pro
Rinnai CBU combi boiler diagnosis involves both the refrigerant/heat circuit and the gas combustion system. Thermal fuse replacement, gas valve work, and heat exchanger descaling should be performed by licensed professionals. Contact Rinnai technical support (1-800-621-9419) for warranty service.