Field pattern: tankless water heater goes cold mid-shower with no code (flow, inlet filter and scale first)
A tankless heater only fires above a minimum flow rate and drops out when flow falls near that line. Clogged inlet screens, scaled heat exchangers, low-flow showerheads and aerators, and mixing valves that cut hot flow are the usual causes. The wrong turn is calling it a gas or ignition fault and replacing parts when the real issue is water flow.
Cross-checked ~60 min moderateBrands: Rinnai, Navien, Noritz, Rheem, Takagi, All
Before you start
- Turn off electrical power and close the water isolation valves before removing the inlet filter
- Gas supply, gas valve, burner, combustion and venting work are BLOCKED: refer to a licensed gas technician or plumber
- Hot water can scald; check temperature before working on fittings
Diagnostic steps
Unlock all 7 steps with expected values
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