The Rheem RTEX-18 is a compact electric tankless water heater designed to deliver on-demand hot water. When it fails to heat, homeowners often encounter frustrating cold showers or inconsistent temperatures. This guide provides practical, step-by-step troubleshooting focused on common causes, safe electrical checks, and practical fixes to restore hot water. It covers electrical, water flow, sensor, and control issues, emphasizing safety and when to seek professional help.
Common Symptoms And Quick Checks
Before diving into deeper troubleshooting, identify the symptoms. A lack of hot water, fluctuating temperatures, or a noticeable error indicator on the unit are typical signs. Quick checks include verifying the unit is powered on, confirming the circuit breaker is ON, and ensuring the device’s display shows normal status. If the display is blank or shows an error code, note the code for reference when consulting manuals or a professional.
Electrical System And Breaker Inspection
The RTEX-18 requires a dedicated electrical supply. A disrupted power source is a frequent cause of no-heat conditions.
- Confirm the unit is connected to a dedicated 240V circuit as specified by the manufacturer.
- Check the main breaker and any branch fuses for tripped status. Reset if appropriate, and test to see if hot water returns.
- Inspect wiring for signs of damage, loose connections, or corrosion at the terminal blocks. Loose connections can cause intermittent heating or no heat.
- Ensure the proper gauge conductors are used. Inadequate wiring can trip breakers or prevent the unit from reaching the required kW output.
- If the unit has a thermal cutout or internal fuse, verify these safety components are intact and not degraded.
Safety note: Always power off the electrical supply before inspecting wiring. If you’re unsure, consult a licensed electrician.
Temperature Setting And Calibration
Incorrect settings can lead to cold water despite heating elements functioning properly.
- Check the remote dial or internal temperature setting. Ensure it’s set to a temperature appropriate for your climate and preferences (typically 120–140°F for most homes).
- If the unit uses a sensor-based control, verify that the sensor is properly seated and not damaged.
- After adjusting, allow several minutes for the system to reheat. Some units require a short reset after changing settings.
Note: A setting that is too low or a faulty temperature sensor can mimic a “not heating” condition even when the heater is operational.
Water Flow Rate And Incoming Water Temperature
Tankless heaters depend on adequate water flow and inlet temperature to reach heat output.
- Confirm the hot water faucet is opened sufficiently. Very low flow can prevent the unit from starting or maintaining heat.
- Measure the incoming water temperature. Cold supply requires higher wattage to reach target output; if the incoming temperature is too cold, the unit may struggle to deliver hot water at higher temperatures.
- Check for downstream restrictions such as blocked filters, faulty aerators, or closed valves that reduce flow.
- If multiple fixtures are drawing water, consider reducing simultaneous demand or upgrading to a higher-capacity unit if recurrences persist.
Tip: Clean or replace inlet filters if equipped, since clogged filters reduce flow and can trigger protective shutoffs.
Sensor And Heating Element Diagnostics
Failures in sensors or heating elements are common non-heating causes in electric tankless heaters.
- Inspect temperature sensors for corrosion or displacement from their mounting. A misread sensor can prevent heating.
- Test heating elements for continuity with a multimeter. A burned-out element will show no continuity and require replacement.
- Check for mineral buildup on heating elements and around the heat exchanger area. Scale can insulate and hinder heat transfer, causing under-heating or no-heat conditions.
Due to the electrical and heating-element nature of RTEX-18, some diagnostics require service tools. If you’re not comfortable testing components, call a licensed technician.
Error Codes And Diagnostic Procedures
Many RTEX-18 units display error codes to aid troubleshooting. Common codes may include generic fault indicators or heater element warnings.
- Consult the user manual for a code-by-code interpretation. Note the exact code and any accompanying LED indicators or display messages.
- Power cycle the unit: turn off power at the breaker, wait 30 seconds, and restore power to reset the controller. Recheck for the code after reboot.
- Record the circumstances when the error occurs (flow rate, temperature setting, and water usage) to help identify patterns.
If codes persist after a reset, professional diagnostics are advised to safely replace sensors, elements, or control boards.
Preventive Maintenance And Routine Care
Regular maintenance reduces the odds of a “not heating” event. Implement simple routines to sustain performance.
- Periodically inspect electrical connections for tightness and signs of wear.
- Flush the unit to remove mineral scale and debris, following manufacturer guidelines for safe flushing procedures.
- Clean inlet filters and screens to maintain adequate water flow.
- Verify venting, wiring, and mounting hardware remain secure and undamaged in installations that are subject to vibrations or building movements.
When To Call A Professional
Most gas- or electric-powered tankless heater issues can be resolved at the consumer level, but certain problems require licensed professionals.
- Persistent no-heat conditions after performing basic checks and resets.
- Frequent tripping of the circuit breaker or blown fuses, indicating electrical faults beyond standard wiring.
- Repeated error codes pointing to sensors, heating elements, or control boards.
- Visible signs of water leaks around electrical connections or the unit itself.
Professional service ensures electrical safety, correct component replacement, and proper reprogramming of temperature and flow settings.
Replacement Considerations And Long-Term Outlook
If the RTEX-18 frequently fails to heat or cannot meet hot-water demand, replacement considerations may arise.
- Assess household hot-water needs: a higher- or multiple-unit setup may be necessary for larger homes or high-demand scenarios.
- Consider efficiency and energy use: modern units offer improved energy efficiency and better temperature stability.
- Factor in installation costs, warranty terms, and potential rebates for energy-efficient upgrades.
Upgrading to a model that better aligns with peak demand can reduce maintenance frequency and improve reliability over time.
Key Takeaways
Rheem RTEX-18 not heating usually stems from electrical supply issues, incorrect temperature settings, insufficient water flow, sensor or element faults, or clogged passages. Systematically check power, breakers, wiring, and settings first. Ensure adequate water flow and incoming temperature. Inspect sensors and heating elements, and review any error codes. When in doubt, consult a licensed electrician or certified technician to perform advanced diagnostics and safe replacements. Proper maintenance can minimize future heat-loss events and extend unit life.