Rheem 66
Rheem code 66: water bypass valve fault
Short answer
Code 66 on current Rheem and Ruud tankless heaters is a water bypass valve malfunction. The bypass valve blends some cold water into the heated water to hold a steady outlet temperature. Rheem's steps are to reseat the connector, set the remote to 102°F so the valve activates, measure voltage, and replace the valve if it fails.
In this guide
The bypass valve helps the heater avoid temperature spikes by mixing in cold water. Code 66 means it is not working as expected.
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 | Water bypass valve malfunction |
What a technician does
- Removes and reinserts the valve connector; a loose connection is a common cause.
- Sets the remote to 102°F, which activates the bypass valve for testing.
- Measures voltage at the valve.
- Replaces the valve or control board based on the readings.
Related codes
Rheem lists the water bypass valve among the checks for code 16, where outlet water runs hotter than the set point.
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.
Common questions
Can I use hot water with Rheem code 66?
The heater may shut down or deliver uneven temperatures until the valve is fixed.
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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) |