Hi-Limit Switch - A.O. Smith Water Heater 520 Direct Vent Indoor Service Manual

10-14. Hi-limit switch

Unit Part #
Function
Failure events
Effects on the water
heater if hi-limit switch
fails
Error codes when hi-limit
switch fails
Diagnostic
Color/Number of wires
When temperatures exceed 194˚F (90˚C), the reset button trips and switch goes to OFF mode.
ON mode : <194˚F (90˚C)
#432 AOS Part #
-Based on bi-metal thermal expansion.
-Detects excessively high water temperature (more than 194˚F or 90˚C) in pipes
of the heat exchanger.
-After detection, communication between the computer board and gas valves are
severed, shutting down the water heater instantly. The "111", "121"or "711"
error codes will display.
1. Unable to detect excessively high water temperatures if switch fails "closed".
2. Continuous detection of excessively high water temperatures (regardless of
what the actual water temperature is) if switch fails "open".
1. Unable to shut down the water heater if the water temperature from the heat
exchanger exceeds 194˚F (90˚C).
Note: The mixing and output thermistors always act as backup hi-limit
detectors to detect excessively high water temperatures in the heat
exchanger.
2. The water heater is always shut down immediately after the ignition process,
and either the "111" or "121" error code will display.
111
The 520 Indoor / 520 Outdoor don't have the "141" error code that was used in
our previous models. This error code is now replaced by the "111",the "121"
and "711" error codes.
1. Visual inspection: connection/breakage of wires. Possibility also includes
scale deposits inside the heat exchanger.
2. Check voltages and resistance: proper range of values shown below.
Blue-Blue
Less than 1.0 Ω
OFF mode: to reset switch back to ON mode, press the reset button
Reset Button
520 Indoor / 520 Outdoor Service Manual
319143-095
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CONFIDENTIAL
Ver. 1.00
Checkpoint C1
711