On the Navien NR-21DU remote, press the Lock button for two seconds to activate or deactivate Temperature Lock. The lock icon appears when active. The manual still permits error or alarm resets and temperature changes from the heater's front panel. This is a remote-button safeguard, not whole-system access control or protection from hot-water scalding.
How do you activate or deactivate it?
Identify the remote before using this procedure. A lock icon on one Navien controller does not establish the same button sequence or limits on another. This answer applies to the NR-21DU remote described in its own manual, connected to compatible equipment.
The NR-21DU owner instructions say to press the Lock button for two seconds to activate or deactivate Temperature Lock Mode. When active, the lock icon appears on the remote display.
Record the intended water-temperature setting before enabling the feature. Verify the icon afterward rather than assuming a brief tap activated the lock. If the controller or button does not behave as documented, use the exact model’s support instructions instead of trying combinations meant for another remote.
Do not enter commercial-temperature settings, installer menus or factory resets merely to obtain a lock icon. Those functions have different purposes and can change more than access to an operating button.
What does the lock leave available?
The manual describes the ordinary buttons as locked except for Lock, then explicitly preserves error/alarm resetting and settings changes from the water heater’s front panel. The remote lock therefore does not prevent every person with access to the heater from changing its settings.
Keep that limitation in the household’s plan. It can help prevent accidental remote adjustments, but it should not be described as a tamperproof heater, a gas shutoff or a repair for an active fault. A locked remote can coexist with a displayed error.
Does it protect against scalding?
The NR-21DU manual warns to check the set temperature before using hot water. Navien’s NPE A2/S2 user manual separately warns that higher settings increase scalding risk and addresses supervision and temperature-limiting protection. Those safety requirements remain relevant when the remote is locked.
Do not change a setting while someone is using hot water. Follow the actual heater and fixture instructions for safe temperature control; the lock icon does not measure delivered tap temperature or replace needed protection.
How is this different from clearing an error?
Navien error codes identify reported faults. Error-history controls concern saved diagnostic information. Neither task is the same as locking the NR-21DU’s operating buttons.
If a screen is blank, the display checklist addresses that separate observation. Keep the remote model, lock state, intended setting and any error recorded for service, rather than treating an unlocked controller or successful reset as proof that the heater is ready to operate.