Ruud tankless water heater provides hot water that turns cold intermittently or temperature fluctuates.

Safety first: Water temperature swings can cause discomfort or scalding. Test carefully.

• Step 1 – Ensure adequate flow: Open hot water faucets more fully to maintain flow above the unit's minimum activation rate.
• Step 2 – Clean inlet filter: Turn OFF water and clean the cold water inlet filter screen on the Ruud tankless unit.
• Step 3 – Descale unit: If scale is suspected, perform a full descaling of the heat exchanger and flow passages.
• Step 4 – Check for plumbing crossover: Close the cold supply to the heater and open a hot faucet. If water still flows, there is a crossover (e.g., faulty single-handle faucet or mixing valve) that must be repaired.

If fluctuations persist, a technician can check gas modulation, sensors, and recirculation settings.