Rinnai Water Heater Water Too Hot - Causes & Fix

Rinnai Water Heater Water Too Hot — What’s Happening

A “water too hot” complaint on a Rinnai tankless water heater means the unit is detecting an over-temperature condition, either at the outlet or inside the heat exchanger. Rinnai LC fault codes specifically warn that scale buildup in the heat exchanger is restricting heat transfer and causing the unit to run hotter than normal.

This is not an ignition or venting fault. The heater is firing but cannot shed heat properly, so outlet water temperature climbs above setpoint or internal sensors trip. The condition can also result from low water flow causing short-cycling or from blocked combustion air and exhaust passages that prevent proper heat release.

Jump to Fix

Most Likely Causes

• Scale and mineral buildup in the heat exchanger Rinnai associates LC codes directly with scale fouling the heat exchanger fins, which blocks heat transfer and forces the unit into over-temperature protection.
• Low water flow through the unit Restricted flow from a clogged inlet filter, partially closed valve, or undersized recirculation loop causes the heater to short-cycle and overheat the small volume of water passing through.
• Blocked combustion chamber or exhaust piping Debris in the burner chamber, vent terminal, or exhaust run reduces airflow and prevents the heat exchanger from cooling properly.
• Restricted air intake or vent terminal Lint, leaves, or snow blocking the fresh-air intake starves combustion and disrupts the temperature balance inside the unit.
• Faulty temperature sensor or damaged wiring A drifting or shorted outlet temperature sensor can report false high readings, or a failed sensor may prevent the control board from modulating the burner correctly.
• Heat exchanger damaged by long-term overheating If scale buildup has been ignored for too long, the exchanger itself can warp or crack and become unrecoverable even after flushing.

How to Diagnose and Fix

1. Note the exact fault code displayed on the controller and measure actual outlet water temperature at a nearby fixture to confirm the complaint.
2. Check water flow rate through the heater by running a tap at full hot and verifying flow is strong and steady, not pulsing or weak.
3. Remove and inspect the inlet water filter for debris, then check that all isolation valves upstream and downstream are fully open.
4. Inspect the vent terminal and air-intake opening outside for blockage, then look inside the combustion chamber viewing port for soot, scale flakes, or foreign material.
5. Perform a manufacturer-approved heat-exchanger flush using a descaling pump and vinegar or commercial descaler if an LC code is present or scale is visible.
6. Inspect the outlet temperature sensor and its wiring harness for corrosion, pinched insulation, or moisture, and test sensor resistance if your model’s service manual provides a reference value.
7. If the unit still faults after flushing and all flow and venting checks pass, consult a licensed technician to test the control board and consider heat-exchanger or sensor replacement.
8. After any repair, run the heater through a full heat cycle and verify outlet temperature stays at setpoint without cycling off on high limit.

Parts You Might Need

When to Call a Pro

Call a licensed plumber or gas technician if you see an LC code and are not comfortable performing a chemical flush, if the heat exchanger shows visible damage after flushing, or if the over-temperature fault returns after you have confirmed good water flow and clear venting. Any work that involves disconnecting gas lines, replacing the heat exchanger, or diagnosing control-board and sensor circuits should be handled by a Rinnai-trained service provider. If your unit is still under warranty, professional documentation of the flush procedure may be required for any subsequent heat-exchanger claim. For gas line, burner, or igniter work, or if you ever smell gas, stop and call a licensed technician.