The Rheem RTEX-18 tankless water heater is designed for on-demand hot water with compact efficiency. When it fails to heat, homeowners may encounter cold showers or insufficient hot water. This article provides practical, safe troubleshooting steps and actionable guidance to identify and resolve common RTEX-18 heating issues. The focus is on findings you can verify and fixes you can implement or communicate to a professional if needed.
Overview Of The RTEX-18 Not Heating Issue
The RTEX-18 uses gas ignition, an electronic control unit, and various sensors to regulate water temperature. A failure to heat can stem from power or gas supply problems, ignition or flame sensing faults, temperature or pressure sensor issues, mineral buildup, or a faulty control board. Understanding where to start helps in quickly diagnosing the root cause and preventing unnecessary service calls.
Common Causes Of No Heat Or Low Heat
- Gas supply disruption: Insufficient gas pressure or a closed gas valve reduces flame intensity or prevents ignition.
- Vent or exhaust blockage: Poor venting can trigger safety protections, reducing heat output or shutting down the heater.
- Ignition system fault: A weak or failed igniter may fail to ignite the gas flow.
- Flame sensor or thermistor issues: A dirty or faulty flame sensor or temperature sensor can prevent proper flame detection or temperature regulation.
- Water flow sensor problem: If the unit does not detect adequate flow, it may not engage heat production.
- Electrical supply or control board fault: Power irregularities or a failing control board can stop heating cycles.
- Scale buildup: Mineral deposits on heat exchanger or sensors reduce efficiency and heat transfer.
- Incorrect temperature setting: A thermostat setting that is too low can feel like a lack of heat.
Step By Step Troubleshooting
Follow these steps in order, ensuring safety and compliance with local codes. If a step requires professional service, discontinue further attempts and contact a licensed technician.
- Check water temperature setting and incoming cold water temperature. Verify that the unit is set to a higher temperature than the baseline household demand. A setting that is too low can produce insufficient hot water even when the unit is functioning correctly.
- Inspect gas supply and valve position. Ensure the gas shutoff valve is fully open and that the gas line shows consistent pressure. If you smell gas or hear a hissing sound, evacuate and call your gas provider or emergency services.
- Confirm ignition status and flame appearance. Listen for the ignition sequence and observe the flame through the flame inspection window if available. A persistent clicking or no ignition indicates an ignition or valve issue.
- Examine venting and exhaust for blockages. Lint, debris, or bird nests can restrict venting and trigger safety shutdowns. Clear any obstruction and ensure proper vent routing per installation guidelines.
- Inspect the flame sensor and igniter. A dirty flame sensor or failed igniter can prevent reliable ignition. If accessible, follow manufacturer guidance to clean the sensor with a soft, dry cloth or recommended cleaner. If the igniter shows signs of wear, replacement may be required.
- Check water flow sensor and scale buildup. Ensure there is adequate water flow when a hot water tap is open. Remove any trapped debris in the inlet filter or screen. Consider descaling the heat exchanger if mineral buildup is evident or if the unit operates intermittently at low flow.
- Evaluate electrical connections. Inspect the power supply cord, circuit breaker, and connections at the control board for signs of looseness, corrosion, or damage. Reset the breaker and reseat connections if safe to do so.
- Read error codes on the unit’s display. RTEX-18 units often display fault codes that point to specific components (igniter, flame sensor, gas valve, flow sensor, etc.). Refer to the user manual’s fault code table to interpret the issue.
- Test with a known-good water demand. Temporarily increase hot water draw to see if the unit responds with heat. If it heats correctly under higher demand, the issue may be related to flow sensing or installation constraints rather than the heat source itself.
- Assess mineral scale and perform a professional descaling if needed. Hard water can accumulate deposits on the heat exchanger, reducing efficiency. A licensed technician can perform a proper descaling procedure and check for bypass valve integrity and flow restrictions.
Safety Considerations And When To Call A Pro
- Gas safety: If you suspect a gas leak, do not operate the unit. Evacuate, avoid creating sparks, and contact the gas company or emergency services.
- Electrical safety: Working around live wiring can cause injury. If you are not comfortable with electrical testing, contact a licensed technician.
- Vent integrity: Any signs of exhaust backflow, corrosion, or improper venting should be addressed by a professional to prevent carbon monoxide risks.
- Persistent error codes: If fault codes persist after basic troubleshooting, professional diagnostic equipment is usually required to pinpoint electronic or sensor failures.
Maintenance To Prevent Future RTEX-18 Heating Issues
- Schedule regular descaling: For homes with hard water, descaling every 12–24 months can maintain heat transfer efficiency and prolong unit life.
- Flush and clean filters: Periodically clean inlet screens and filters to prevent debris from entering the flow sensor or heat exchanger.
- Inspect venting annually: Have a professional inspect venting for obstructions, corrosion, or improper slope that could impair exhaust flow.
- Test sensors and control board: If the unit starts behaving erratically, have a technician test the flame sensor, thermistor, and control board for accuracy and reliability.
- Water quality management: Use a water treatment approach to reduce mineral content in the supply, which helps preserve heat exchanger performance.