Navien E785: flow switch or flow sensor fault

In combination systems, hot water from the Navien also flows through an air handler to heat the home. Sensors confirm that water is moving through that loop. E785 means one of them is not reporting correctly.

The meaning and recommended action below come from Navien's User's Information Manual for NPE-2 condensing water heaters (NPE-180A2, NPE-210A2, NPE-240A2 and the S2 models). Navien technical support: 1-800-519-8794.

Navien notes this code applies when an air handler unit (AHU) for space heating is connected to the water heater, a setup used in combination water and space heating systems.

What can cause it

• Air trapped in the heating loop
• A closed valve in the loop
• A pump problem in the air handler circuit
• A failed flow switch or sensor, or loose wiring

What to do

Make sure valves in the heating loop are open and call your installer. Heating loops often need purging of air after service. For the water heater's own flow sensor fault, see Navien E439.

How to reset it

Navien says you can try resetting the water heater by pressing the Back button on the front panel. If the code returns, the cause is still there and the unit needs service.

Before you call

Have the model number and serial number from the rating plate on the side of the unit ready, along with the date it was installed and the exact code. Navien asks for this information when you contact technical support, and your installer should be your first call for warranty service.

See every code on our Navien error codes page.