Rinnai tankless repair: common problems and who fixes them

Safety first

If you smell gas, do not reset anything. Leave on foot, avoid anything that can spark, and call the gas utility and 911 from a safe place^[4].

The checks on this page stay outside the cover. Rinnai's own manual lists items that only a trained and qualified professional should handle, and anything involving gas, venting, wiring or parts inside the cabinet belongs to a licensed plumber or a Rinnai-trained technician.

Rinnai splits its line into SENSEI condensing models (RX, CX, RE, RSC), non-SENSEI models (V-series, RL, RU, RUR, RUC and commercial units) and the RH180 hybrid^[1]. Most codes mean the same thing across families. Our full table is at Rinnai error codes.

Common problems by symptom

Symptom	Code family	Common causes	Safe owner check	Pro work
No hot water, burner will not light	11	Gas valve closed, gas service off, empty propane tank, low gas pressure	Gas valve open at the heater and meter; propane level; power cycle once	Gas pressure test, igniter or flame rod
Burner lights, then goes out	12	Low or unstable gas, undersized gas line, venting, dirty flame rod	Note if it fails when the furnace or dryer runs, and tell the pro	Pressure test under full load, flame rod, vent inspection
Unit stops, often after snow or wind	10	Blocked vent terminal, full or frozen condensate line	Clear snow, leaves or nests from the terminals; check the condensate line drains	Vent and heat exchanger inspection
Unit stops on a condensing model	25	Condensate backup	Check the condensate line for kinks, clogs or ice	Condensate system repair
Unit shuts down hot	14	Restricted flow, scale, wrong gas setting	Do not keep resetting; shut it off	Root-cause diagnosis
Display shows LC or SS	LC, SS	Scale building up, or a time-based flush reminder	Book a flush	Flush and descale
No heat in a freeze	02	Freeze protection ran but the burner could not fire, often a gas supply problem	Confirm gas is on	Gas supply diagnosis
Code naming a sensor, fan or valve	32 to 41, 52, 61, 65 and similar	Failed part	None beyond one power cycle	Diagnosis and part replacement

Code meanings are our paraphrase of Rinnai's diagnostic code guide, not Rinnai's text^[1]. Rinnai singles out code 14 as one to treat seriously, and LC is the only code that lets the unit keep running, as a warning that scale is building up^[1].

What a homeowner can safely check

1. Gas, water and power. The gas valve at the heater open, gas service on, the cold water valve open, and power to the unit.

2. The inlet water filter. Rinnai's manual describes closing the isolation valves, catching water in a bucket, unscrewing the filter at the bottom of the unit, rinsing it and reinstalling it^[2]. If you are not comfortable working the isolation valves, ask a plumber to do it during the next flush.

3. Vent terminals and the condensate line. Rinnai says to keep the area around the vent terminal and the condensate drain line free of snow and ice, and to call a qualified professional if the vent is blocked and cannot be easily cleared^[2].

4. One power cycle. Turn the unit off and on once. If the code returns, stop.

Leave these to a pro. Rinnai's air filter on SENSEI models sits behind the front panel, which is held by screws^[2]. Rinnai's manual also says testing the pressure relief valve should only be done by a trained and qualified professional because the discharge can cause severe scald burns^[2].

Hard water, LC codes and flushing

Scale is the most preventable Rinnai repair. Rinnai's water treatment table in the SENSEI manual sets these guidelines^[2]:

Water hardness	Grains per gallon	Rinnai's guidance	Flush frequency
Soft, 0 to 60 mg/L	0 to 4	No treatment	None listed
Slightly to moderately hard, 61 to 160 mg/L	4 to 9	Softener or scale control recommended	Residential once a year; commercial twice a year
Very hard, 161 to 200 mg/L	9 to 12	Softener or scale control strongly recommended
Extremely hard, over 200 mg/L	12+	Softener or scale control required

Rinnai warns that failing to flush will damage the heat exchanger and that scale damage is not covered by the warranty^[2]. Scale builds faster at high temperature settings^[2]. See how to descale a tankless water heater for what the flush involves.

Freezing

Rinnai's SENSEI RX manual says the unit protects itself from freezing down to -22 F indoors and -4 F outdoors, out of the wind, but only while it has 120-volt power^[2]. If power or gas is lost in freezing weather, the unit should be drained, and freeze damage is not covered by the warranty^[2]. An optional drain-down kit can drain it automatically^[2].

Warranty basics

Terms vary by model, so read the certificate for yours. For Rinnai's RSC residential models, the October 2024 warranty sets these periods from the date of purchase^[3]:

Item	Residential	Commercial
Heat exchanger	15 years or 12,000 operating hours	8 years or 12,000 operating hours
Other parts	5 years	5 years
Reasonable labor	5 years if registered within 90 days, otherwise 1 year	1 year

Other conditions in the same document^[3]:

• Coverage ends if the unit is moved or reinstalled at a new location.
• Repairs must use genuine Rinnai parts and be done by a qualified professional trained for that repair.
• Rinnai recommends installation by a professional who has attended a Rinnai installation class, and says improper installation may void the warranty.
• Rinnai can be reached at 1-800-621-9419 for the name of a trained professional.

Use our Rinnai age decoder to estimate the unit's age before you call.

Costs

HomeGuide's national ranges (August 2025): tankless repairs $300 to $1,300, heat exchanger replacement $600 to $1,300, and a flush and descale $75 to $300^[5]. If a repair approaches half the price of a new unit, compare it with replacement on our repair or replace page.

When to call a pro

• Code 14, or any ignition or flame code that returns after one power cycle with gas confirmed on.
• Vent or condensate codes that return after the terminals and drain line are clear.
• Any sensor, fan, valve, board or internal leak code (79).
• A flush on a SENSEI model, where the descaling mode is set from buttons behind the front panel.