Rheem Tankless Code 00: Why It Appears and How to Reset It

Rheem tankless water heaters are designed to heat water in bursts. When one runs without stopping for more than an hour, it shows code 00 as a warning.

The heater is usually fine. The question is what is keeping it running.

Rheem lists code 00 on its earlier tankless models. If your unit is newer and shows 00, check the manual for your model, because code meanings differ between Rheem generations.

How to reset code 00

Close every hot water tap in the house. When flow stops, the code clears. If the unit is connected to a recirculation pump, turn the pump off to test.

Common causes

A recirculation pump running constantly

The most frequent cause. A pump set to run nonstop, or installed on a unit not set up for recirculation, keeps water moving through the heater.

A running fixture

A dripping hot tap, a toilet fed by the wrong line, or a fixture left open in a guest room, garage or outdoor shower.

A plumbing crossover

A faulty single-handle faucet or mixing valve can let water flow between the hot and cold lines, which the heater sees as constant demand.

Genuinely long use

Filling a large soaking tub or running several showers back to back can trigger it occasionally.

When to call a plumber

If code 00 keeps returning with every tap closed and no pump running, a plumber can track down a crossover or a hidden leak on the hot water side.

Common questions

Is Rheem code 00 serious?

Not usually. It is a warning that the unit has run for over an hour straight. Find out what is drawing hot water, because constant firing wears the heater and wastes gas.

Why does my Rheem show 00 at night?

Something is using hot water while the house is quiet, most often a recirculation pump or a leaking hot water fixture.

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)