Laundry room with a tank-type water heater beside the washer
4 components covered

Field manual · Water Heater

Gas tank water heater (atmospheric and power-vent): diagnostic and electrical-side manual

Covers: Residential standing-pilot FVIR tanks with thermopile electronic gas controls, and 120 V power-vent tanks; diagnosis and electrical/water-side only

Cross-checkedBrands: A.O. Smith, State, Rheem, Ruud, Bradford White, Whirlpool4 components

Gas tanks heat with a burner under the tank and vent combustion products up a central flue. Atmospheric models vent by natural draft; power-vent models add a 120 V blower, pressure switch and ignition control. Everything on the gas side (valve, burner, pilot, gas pressure, venting, combustion) is blocked: our role is to read status codes, check the 120 V and water side, and refer.

Safety first

  • Gas smell: evacuate and call the gas utility
  • No procedure for gas valve, burner, pilot, gas pressure, venting or CO: refer to a licensed plumber/gas technician
  • 120 V checks on power-vent units: caution
  • Permits and licensed installers are commonly required for gas water heater work

How it works

Standing-pilot models: the pilot heats a thermopile/thermocouple that powers the gas control. The control's LED reports status by flash count.

FVIR (flammable vapor ignition resistant) design: a sealed combustion chamber with a flame arrestor and a thermal switch that shuts the heater down if vapors ignite or air is starved.

Power-vent models: on a call for heat the blower purges, a pressure (air) switch proves draft, then ignition is attempted and the flame is proven. Failure at any stage leads to retries and lockout, reported by LED flash codes.

Components & tests

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Official manufacturer documentation

Always confirm values against the tech sheet for the exact model.

OnCall Tech is a reference tool for trained technicians and can make mistakes. Always disconnect power and confirm with the manufacturer's documentation before you work. © 2026 OnCall Tech. Original content; not a reproduction of any manufacturer document.Generic part photos (not of this brand): Laundry room with a tank-type water heater beside the washer by Alex Tyson, Pexels License

Diagnostic guides for this appliance

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Heat-pump (hybrid) water heater: slow recovery, noise, errors

Hybrids need enough room air and a clean air filter, a working condensate drain and the right mode. Many complaints are mode settings or a dirty filter. Refrigerant faults are sealed-system work.

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Tankless water heater descaling (flush)

Scale insulates the heat exchanger, causing overheat codes and fluctuating temperature. Flush with a pump and descaling solution through the isolation valves.

5 steps ~75 mineasy
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Hot water recirculation pump problems

Check timer/settings, check valves, air in the loop and the pump itself. On tankless units with built-in pumps, a recirculation code may point to air or a leak in the loop.

5 steps ~60 minmoderate

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Gas tank water heater (atmospheric and power-vent): diagnostic and electrical-side manual · OnCall Tech