Rheem Tankless Water Heater Flow Sensor Replacement Guide — What This Part Does
The flow sensor (also called a flow meter) detects water movement through your Rheem tankless heater and signals the control board to fire the burner. When water demand reaches the minimum threshold (typically 2 to 3 liters per minute on Rheem continuous-flow units), the sensor sends a voltage signal that permits ignition. If the sensor fails or cannot read flow correctly, the board locks out the burner to prevent unsafe operation.
Flow sensors fail from mineral buildup coating the turbine or Hall-effect transducer inside, from scale partially blocking the flow chamber, or from wiring corrosion at the connector. Because tankless heaters cycle hot water hundreds of times per day, hard water deposits accumulate faster on the sensor than on most other parts. A clogged inlet filter or low supply pressure (below 15 PSI in some troubleshooting references) can also cause flow-sensor faults by starving the unit of water, but true sensor hardware failure requires replacement of the sensor assembly itself.
Signs It Needs Replacing
- Error code 24 on the display Rheem troubleshooting documentation lists code 24 as a flow-sensor fault, meaning the board is not receiving the expected flow signal.
- Unit does not ignite even with hot tap fully open The control board blocks ignition because it does not detect the minimum flow rate required to fire the burner safely.
- Intermittent shutdown during a shower or wash cycle A failing sensor sends erratic flow readings, causing the heater to cycle off mid-demand even though water is still flowing.
- Hot water works only at very high flow rates A fouled sensor may register flow only when water velocity is much higher than the normal 2 to 3 liter-per-minute minimum.
- No error code but heater never fires If the flow sensor is completely dead or disconnected, the board receives no flow signal at all and will not permit burner operation.
- Clicking or relay sound but no flame You hear the ignition sequence start, but without a valid flow signal the gas valve remains closed and the burner does not light.
How to Replace It
- Turn off the gas supply valve upstream of the heater and shut off the main water supply valve feeding the unit.
- Disconnect electrical power by switching off the dedicated circuit breaker or unplugging the heater if it uses a cord.
- Close the two isolation valves on the inlet and outlet water lines, then open a nearby hot-water faucet to relieve line pressure.
- Remove the front cover or service panel by unscrewing the retaining screws (location varies by model, consult your manual).
- Locate the flow sensor, usually a cylindrical or rectangular assembly mounted inline on the cold-water inlet manifold with a two- or three-pin connector.
- Photograph or label the wiring harness, then unplug the connector from the sensor and set the harness aside.
- Use an adjustable wrench or the correct size socket to unscrew the sensor body from the manifold (some models use a threaded brass fitting, others use retaining clips or screws).
- Remove the old sensor and inspect the O-ring or gasket on the threads; replace the seal if it is flattened, cracked, or hard.
- Install the new flow sensor, threading it into the manifold by hand first to avoid cross-threading, then snug it with a wrench (do not overtighten or you may crack the plastic housing).
- Reconnect the wiring harness, making sure the connector seats fully and locks into place.
- Reinstall the service panel, then open the isolation valves and main water supply and check all fittings for leaks.
- Restore electrical power and reopen the gas supply valve, then open a hot-water tap to purge air and verify the heater ignites and holds flame without error codes.
The Part You Need
| Part | Notes |
|---|---|
| Rheem tankless water heater flow sensor (flow meter) | Amazon | Part number varies by model. Check the data plate on the right side or inside the cabinet door for the full model and serial number, then search “Rheem [model] flow sensor” or call Rheem customer service with your serial to confirm the exact replacement part. |
| Flow sensor O-ring or gasket | Amazon | Often included with the new sensor. If sold separately, verify thread size and material (typically Buna-N or silicone for hot-water service) against your model’s parts diagram. |
Related Error Codes
If this part is failing you may also see one of these codes:
When to Call a Pro
If you are not comfortable working with gas piping, call a licensed plumber or gas technician to handle the gas shutoff, sensor replacement, and leak-check on restart. Similarly, if you replace the flow sensor and still see error code 24 or no ignition, the fault may lie in the wiring harness, control board, or a restriction elsewhere in the water path (clogged heat exchanger or inlet filter), all of which require systematic electrical testing and descaling that most homeowners prefer to leave to a professional. Any work that involves opening gas connections or testing gas-valve voltage should be performed by a qualified technician to meet local codes and warranty requirements. For gas line, burner, or igniter work, or if you ever smell gas, stop and call a licensed technician.