T-M50 / T-M50 ASME Service manual
Ver. 1.05
26
CONFIDENTIAL
10-11. Hi-limit switch
Unit Part #
#403
AOS Part #
EKN34
Checkpoint
C1
Function
-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" or "121" error
codes will display.
Failure event
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".
Effects on the water
heater if hi-limit switch
fails
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
112
Error codes when hi-limit
switch fails
The T-M50 doesn't have the "141" error code that was used in our previous
models. This error code is now replaced by the "111" and the "121" error
codes.
Diagnostic
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.
Color/Number of wires
Blue-Blue
Less than 1.0 Ω
When temperatures exceed 194˚F (90˚C), the reset button trips and switch goes to OFF mode.
ON mode:<194˚F (90˚C)
OFF mode: to reset switch back to ON mode, press the reset button
Reset Button
9
0
"9 0" m a r k
1
-
3
/
8
"
(
3
5
m
m
)
5 / 8 " ( 1 5 m m )
3 / 4 " ( 1 8 .4 m m )
