Can scale cause Rheem code 15?

Yes. Rheem lists sediment buildup in the heat exchanger among the causes. Flushing is the fix.

Rheem Tankless Code 15: Overheating and What Causes It

Code 15 is a serious overheating shutdown. Water near boiling inside a tankless heater means heat is not moving into the water as it should.

This page uses two Rheem sources: the current Rheem and Ruud tankless troubleshooting manual, and Rheem's error code list for earlier tankless models. Codes can differ between generations, so the meaning for each is shown where they differ. Rheem technical support: (800) 432-8373.

What it means by model generation

Rheem generation	Meaning
Current models	Hot water or heat exchanger temperature reached 207°F for more than 15 seconds
Earlier models	Boiling safety device activated

Common causes

Scale and sediment in the heat exchanger
Restricted water flow: a partly closed inlet valve or clogged inlet filter
Gas supply and venting problems that create hot spots
A faulty heat exchanger thermistor, which Rheem lists among the checks

What you can do

Make sure the cold water valve to the heater is fully open.
Clean the inlet filter.
If the unit has not been flushed recently, flush it; see our guide to descale a tankless water heater.
If code 15 returns, call a technician.

How to reset it

Close all hot water taps.
Turn the remote control off, wait about 10 seconds, then turn it back on.
Open a hot tap and watch whether the code returns.

Before you call

Have the model and serial number from the rating label ready, and note whether your unit is a condensing (high-efficiency) or non-condensing model. Tell the technician the exact code, when it appears and any recent work on the gas, water, venting or electrical supply. That often saves a second visit.

See every code on our Rheem error codes page.

Other Rheem codes

Code	Meaning
00	Continuous combustion for more than one hour
03	Communication failure between units, remote or manifold controller
05	Improper combustion detected by the flame rod (warning)
10	Air supply or exhaust blockage (hard lockout)
11	Ignition failure
12	Flame failure
13	Poor combustion detected by the flame rod (indoor units)
14	Overheat safety tripped
15	Heat exchanger or hot water temperature too high (boiling safety)
16	Outlet water temperature above the set point (over temperature warning)
21	Air intake filter switch fault (earlier models)
24	Remote control buttons held too long, or operational switch fault
29	Condensate not draining (condensing models) or heat exchanger temperature too low
31	Inlet water temperature sensor (thermistor) fault
32	Heat exchanger thermistor fault
33	Outlet (outgoing water) thermistor fault
34	Ambient or combustion air temperature sensor fault
35	Improper thermistor connection
51	Gas valve fault: flame detected after shutdown (current) / gas inlet solenoid valve fault (earlier)
52	Modulating gas valve (PGFR) fault
61	Blower (combustion fan) motor fault
65	Water volume control valve fault
66	Water bypass valve fault
71	Gas inlet solenoid valve fault
72	Flame rod fault: flame sensed before ignition
76	Remote control communication fault
79	Blower (fan) motor current fault
80	Gas cut-off failure: flame detected after the unit turned off
82	Control board program chip fault (current) / gas type data failure (earlier)
90	Vent blockage detected before ignition
92	Neutralizer needs replacing soon (warning, condensing models)
93	Neutralizer must be replaced; unit will not operate (condensing models)
1L	Lime (scale) buildup detected in the heat exchanger
99	Vent blockage detected after shutdown / blocked flue (hard lockout)
P1	Not enough water flow to fire the burner (warning)

Rheem code 15: overheating (boiling safety)