Bosch tankless water heater ignites but water temperature fluctuates; hot-cold-hot cycling during shower

Safety first: Turn OFF power and gas before internal service. Water temperature swings can cause scalding; use caution.

• Bypass mixing valves: Temporarily test with a simple two-handle faucet fully open on hot to see if temperature stabilizes. Pressure-balancing or thermostatic shower valves can cause cycling with tankless heaters.
• Check flow rate: Ensure showerheads and aerators are not excessively restrictive. Replace with higher-flow fixtures if necessary.
• Descale heat exchanger: Perform a descaling procedure to restore proper heat transfer and modulation response.
• Verify gas supply: Have a licensed technician check gas pressure and regulator sizing to ensure the heater can modulate correctly.
• Inspect outlet sensor: Check the outlet NTC sensor for secure mounting and replace if readings are erratic.