AO Smith Vertex GDHE-50 – Ignition Problems and Troubleshooting

Ignition problems on AO Smith Vertex GDHE-50 water heaters and how to troubleshoot? This troubleshooting guide will provide helpful repair tips, info on checking the proper operation, testing, diagnosing and fixing a water heater if there is no hot water due to ignition failu re and no flame at the burner.

To locate and identify the cause of the ignition failure problem, let’s first check out some of the main parts.

AO Smith Vertex GDHE-50 is equipped with a control system that regulates the water temperature inside the unit, heating cycle, ignition, and performs a diagnostic check during the water heating cycle. If there is a problem with the system, such as the mentioned ignition issues, the self-diagnostics test will detect the problem, display the message on the LCD screen, and stop the heating process.

AO Smith Vertex
AO Smith Vertex (photo:

The flame sensor is part of the burner assembly – together with the igniter, burner, and wiring harness. A gas blower and burner assembly must be removed before servicing the flame sensor and igniter.

The flame sensor is the metal rod partially enclosed in a ceramic insulator and connected to the control board with the wiring harness. The bare, metal part of the flame sensor is in contact with the burner flame and at a specified distance. The element is electronically controlled by measuring the flame sensing current, which ensures that the gas flows to the burner and is burning safely. The burner must be grounded if you want to have an uninterrupted current flow from the flame sensor to the burner.

The igniter type used to light the gas on the burner is the hot surface igniter type (HSI). The control system powers the igniter (120 VAC) and monitors the amperage through the element.

More the Amps higher the temperature of the igniter surface is.

The hot surface igniter is more reliable and can last longer than the pilot light, but its life expectancy depends on the usage.

Due to regular wear over time an igniter surface temperature decreases as the electrical current declines due to increased resistance. The igniter must be replaced if the testing shows that the resistance value is much below the design values (check the manual) or it is an open circuit with an infinite ohm.

The ignition failure is one of the problems related to the malfunction of the above elements. The water heater has failed to ignite the gas burner. The ignition failure fault message found on the LCD screen will be displayed if the flame sensor didn’t confirm the flame presence at the burner or the flame sensing current didn’t meet the requirement – equal or above 1.0 µA.

The control system will try to ignite the gas on the burner up to three times, and if nothing happens after that, the system will lock-out, and the “Ignition Failure” fault message will display on the LCD screen. The S1 dipswitch is factory set for 3 trials but can be configured between 1 and 3 trials.

Troubleshooting Ignition Problems On AO Smith Vertex: Repair Tips

To fix the problem, do the following;

  • Ensure that the main gas valve is open and that there are electricity and water flow.
  • Ensure that there is enough combustion air, that the air intake is not blocked, the vent pipe length does not exceed the recommended length, including the maximum number of elbows, and the vent pipe is not too small.
  • Make sure the correct gas type is used so as the orifice size (type).
  • Check if the burner is grounded. The water heater must be properly grounded and polarity correct, not reversed. Proper grounding is essential as the control system must perform flame verification.
  • Make sure the gas valve gets enough electricity.
  • Check if the gas pressure is correct and that it is freely flowing through the gas valve. Keep in mind that the Vertex water heater is certified for an altitude of up to 10,100 feet. Per the AO Smith manufacturer, the manifold offset pressure is 0.24″ W.C. (it can vary), minimum supply pressure is 3.5 W.C., and maximum supply pressure is 14″ W.C., for the natural gas. The manifold gas pressure is factory set. The ignition failure is noticed if there is a continuous drop in supply gas pressure of 1.5″ W.C. or more – it happens if the gas line is undersized.
  • The flame sensing current should be per specs.
  • Make sure there are no deposits on the flame sensor. Corroded flame sensor (rust deposits) is one of the most common causes of ignition failure. As the rust accumulates over time, the flame sensor should be checked and cleaned regularly. Also, check for signs of damages, cracks in the insulation, excessive wear and tear – and replace if needed.
  • You can check the current from time to time, and if it is below 8 µA, the flame sensor should be cleaned. For cleaning the corrosion , use ultra-fine steel wool, not the coarse grit sandpaper, to avoid damage.
  • See if the wiring harness is OK, that there is no damage, obstructions, and looseness. Ensure that the wires are crimped tightly in its connectors and that the connectors provide good contact with its mating part. For example, check out the 5-pin PWM signal plug that is properly installed and plugged in. Also, check if the flame sensor wiring is connected correctly and is not loose.
  • Make sure the gas burner is clean, so no dirt and debris can clog the element. Debris can be found on the burner as the result of vent pipe cutting during the vent installation.

Before servicing, things to check include proper installation of the water, gas, electrical and venting system, the gas pressure in the recommended range, correct polarity, and unit grounding.

Note: The above examples describe the leading causes and solutions of the ignition failure. Do not try to perform any of the work described above if you are not qualified for this work.

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