Rheem Water Heater No Hot Water — What’s Happening
When a Rheem water heater produces no hot water, the unit has stopped heating because the control board detected a problem with combustion, flame sensing, water flow, venting, or temperature. This is a symptom, not a single fault code. Rheem tankless and gas units will display specific error codes (such as code 11 for ignition failure at startup or code 12 for flame loss during operation) that point to the underlying issue.
Your first job is to identify the exact model and read the fault code from the unit’s display or LED pattern. Rheem’s diagnostic process starts with the code, then walks through checks for gas supply, water flow, venting, ignition components, and sensors. Common causes include empty LP bottles, blocked exhaust or air inlets, flow rates below the 2 to 3 liters per minute minimum needed to keep a continuous-flow heater running, scale buildup in the heat exchanger, and failed ignition or flame sensors.
Most Likely Causes
- No gas or low gas supply An empty LP cylinder, closed gas valve, or low supply pressure will prevent ignition or cause the burner to drop out during operation.
- Insufficient water flow through the unit Flow restrictors, low-flow taps, or a clogged inlet filter can drop flow below the 2 to 3 liters per minute minimum, keeping the heater from firing or staying on.
- Blocked or restricted venting and combustion air Obstructions at the flue inlet, exhaust termination, air inlet filter, or closed ventilation openings will trigger combustion or ignition faults.
- Faulty ignition or flame sensor A worn ignitor, dirty or corroded flame sensor, or damaged wiring prevents the unit from lighting or causes it to shut down after a few seconds.
- Scale or mineral buildup in the heat exchanger Hard water deposits restrict water passages and can cause over-temperature or flow-related shutdowns.
- Failed temperature sensor or over-temperature sensor A bad inlet, outlet, or overheating sensor will give false readings and shut down the burner to protect the unit.
How to Diagnose and Fix
- Read and record the fault code displayed on the unit or note the LED flash pattern, then consult your model’s service manual for the code definition.
- Confirm gas supply to the appliance by checking that the gas valve is fully open and, for LP units, that the cylinder has fuel and adequate pressure.
- Open a hot-water tap fully and measure flow at the fixture to verify at least 2 to 3 liters per minute reach the heater, and remove any flow restrictors or clean the inlet filter if flow is low.
- Inspect the flue inlet, exhaust termination, air inlet filter, and all ventilation openings for blockage, nests, or debris, and clear any obstructions you find.
- Check the ignitor, flame sensor, and all related wiring for corrosion, cracks, or loose connections, and clean the flame sensor rod with fine steel wool if accessible.
- Test or inspect the inlet and outlet temperature sensors and the overheating sensor, looking for damaged wiring or sensor faults indicated by the code.
- Flush the heat exchanger with a descaling solution if scale buildup is suspected or if the unit has not been serviced in over a year.
- After correcting the fault, close all hot taps, cycle power off and back on at the breaker or service switch, then open a hot tap and observe whether the unit ignites and stays running.
Parts You Might Need
| Part | Notes |
|---|---|
| Rheem ignitor kit | Amazon | Match to your exact model number. |
| Rheem flame sensor | Amazon | Confirm part number from the wiring diagram on the unit. |
| Rheem inlet or outlet temperature sensor | Amazon | Order by model and code definition. |
Related Error Codes
If your appliance also shows a code on the display, these match this problem:
When to Call a Pro
Call a licensed plumber or gas technician if you see a persistent fault code after checking gas supply and water flow, if you are not comfortable working with gas connections or combustion components, or if the fault returns immediately after a reset. Ignition, flame sensing, venting, and gas-pressure work on tankless and gas water heaters require specialized tools and knowledge of combustion safety. A pro will diagnose the exact code, test gas pressure and flow rates, inspect venting for code compliance, and replace sensors or control boards as needed. For gas line, burner, or igniter work, or if you ever smell gas, stop and call a licensed technician.