A.O. Smith T-M50 ASME Service Manual page 78

Automatic firing system
Activation conditions will depend on the on whether or it is a direct-vent installation. Gas and electrical
power are required for this feature to operate.
T-M50 direct-vent installation:
After 2 minutes have elapsed since the T-M50 direct-vent's previous firing operation, the computer
will continually check the temperatures of the inlet and output thermistors (the output thermistor is
the one that checks the temperature of the water immediately leaving the heat exchanger, not the
water leaving the T-M50 direct-vent, which is mixed with water from the bypass valve). The
automatic firing system will not activate at all unless these 2 minutes have elapsed.
When the computer checks for these temperatures, the freeze protection firing system will activate if:
< <
T 39°F and T
in out
where T is the temperature of inlet thermistor and T
in
To sum it up in a chart:
Case 2 minutes have elapsed
since prior operation?
1 No
2 Yes
3 Yes
4 Yes
5 Yes
50°F OR
Inlet
temperature
Any
Above 39°F
Any
39°F and below
Any
T-M50 / T-M50 ASME Service Manual
<
T 39°F
out
is the temperature of the output thermistor.
out
thermistor Output
temperature
Any
Any
Above 50°F
50°F and below
39°F and below
78
CONFIDENTIAL
thermistor Will
firing be activated?
NO
NO
NO
YES
YES
Ver. 1.05
automatic