Bosch error code EA on a Greentherm or Greenstar tankless indicates a flame loss after successful ignition — the burner lit, ran briefly, then the flame was lost. Distinct from C7 (no initial ignition), EA points to issues that develop during operation rather than at startup.
What triggers an EA
The flame sensor confirmed ignition successfully, but during operation lost the microamperage signal it should receive from the burning flame. Causes include: gas pressure drops under load, condensate flooding the burner area, blocked exhaust restricting combustion, or a degrading flame sensor that worked at startup but failed under sustained operation.
Diagnostic steps
- Gas pressure under load: the single most common EA cause. Static gas pressure can be fine at 11" WC but drop to 7" WC or lower when the burner is at full fire. This is often a gas line sizing issue — common when a tankless replaces an old tank, since tankless requires significantly more gas flow at peak. Manometer reading at the unit's inlet under load is required to diagnose.
- Condensate path: high-efficiency condensing Bosch units produce significant acidic condensate. A blocked condensate drain can flood back into the burner area. Verify the condensate trap is clear and the drain line is unobstructed.
- Exhaust restriction: a partially blocked exhaust vent (insect nest, debris in concentric vent termination) restricts combustion airflow. Inspect the exterior vent termination and the path.
- Flame sensor degradation: oxidized or carboned sensors can give a signal at initial ignition that fades during operation. Clean with fine abrasive.
Gas line sizing
If a Greentherm replaces a 40-gallon tank water heater, the gas line was likely sized for the tank's ~35,000 BTU input. The Greentherm at high fire can demand 199,000 BTU — six times more. Even if the 1/2" line worked for the tank, it almost certainly won't supply the tankless under load. Upgrade to 3/4" minimum is typical; verify with the Bosch install manual based on total gas line length from meter to unit.
When to call a pro
Gas pressure diagnosis with a manometer, condensate trap inspection (acidic — proper PPE needed), or any combustion-related issue. Bosch authorized service techs can test gas pressure under load and verify combustion analysis.
Documenting the call
If EA recurs, document the conditions: ambient temperature, simultaneous gas appliance use (range running, dryer on), and roughly how long after ignition the EA appears. These data points significantly speed diagnosis when you contact Bosch service.