Error Code 94

EC94 or Error Code 94 (EC941 or EC942 on display window units)

An error code 94 means the exhaust temperature is too high (condensing units). One of the benefits of installing a high efficiency tankless is the low exhaust temperatures that allow for the use of PVC, CPVC or PP venting (check local code to see if there are any additional restrictions).

A non-condensing tankless usually has exhaust temperatures above 300 degrees F and thus requires the use of category III stainless steel venting. A condensing unit uses a secondary heat exchanger to capture as much of that heat energy in the exhaust and put it into the water resulting is exhaust temperatures usually below 150 degrees F. As an additional safety measure our modern high efficiency tankless units have an exhaust temperature thermistor to monitors the temps and shut down the unit if they get too high.

Exhaust temperature thermistor at the top center of the exhaust box of an NRC98DV.

Common Causes of the EC94:

  1. Scale build up. Excessive scale in the heat exchanger acts as an insulator and reduces the heat transfer into the water. This may also result in other overheat related error codes like the EC20 or EC14. A good indication of scale build up is the copper heat exchanger surface starts to develop dark grey coloring on the surface.
  2. Restrictions in exhaust or intake venting. If the heater is unable to effectively pull in fresh air or remove the hot exhaust, the exhaust temps will rise. Make sure vent termination screens are clear of any debris or build up.
  3. Dirty fan. Similar to restrictions in the venting, if the fan has excessive build up, it will not be able to move as much air into or out of the unit.
  4. Incorrect manifold gas pressures. If the internal manifold gas pressures are significantly out of spec it could result in poor/abnormal combustion and in some cases create too much heat. Checking and adjusting manifold gas pressures requires the use of a digital manometer and is usually something left to a professional.
Example of dark spot on heat exchanger indicating excessive scale build up.

On smaller units that feature a display window on the bottom right side you might see an Error Code 941 or Error Code 942. These are both error codes related to the exhaust temperature being too high. EC941 is more of a warning code when the detected exhaust temp is over 182 degrees F and it will restrict BTU use to 77k or less. EC942 will shut down the heater when detected exhaust temp is over 191 degrees F. Common causes above still apply to these 3 digit error codes.

For further technical assistance call 866-766-7489 or use our Contractor Locator to find a local service representative.

Updated on July 14, 2022

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