Navien tankless water heater code E740: meaning and exact next steps

Model-specific

What is true of this model and not of its siblings

Outdoor reset

An outdoor reset curve lets a heating system lower its water temperature as the weather warms, which saves energy. It needs an outdoor sensor, and the code appears when that sensor reads open or short.

Navien's actions

Ensure the curve is configured properly and check the sensor wiring connection.

Weather

Outdoor wiring suffers from moisture and sun, so a seasonal fault is common.

Why an outdoor sensor

Outdoor reset adjusts the supply water temperature to the weather: colder outside, hotter water for the heating loop, and the reverse. The sensor outside tells the heater which way to go. If its wire is cut, shorted or wet, the heater cannot follow the curve.

Open or short

Sub-code one means the circuit reads open, as with a broken wire. Sub-code two means it reads short, as with water in the connection. Either points to the sensor and its cable, which are exposed to weather.

When you do not need it

If you do not use outdoor reset, the setting can be turned off by a technician, and the code will not appear.

Checking the sensor outside

Look at the outdoor sensor and its cable for chewed or cracked insulation, corrosion at the terminals and water inside any splice. A small dab of dielectric grease and a weatherproof connector often cures an intermittent reading.

Setting up the curve

If you use outdoor reset, a technician sets the curve so that the supply temperature follows the weather in a way that suits your radiators or floor. A badly set curve wastes energy even when the sensor works, so ask for it to be checked at the same visit.