T-D2-IN / T-D2-OS Service Manual
Ver. 1.00
66
CONFIDENTIAL
12-14. Freeze protection system
There are two features to the T-D2-IN or T-D2-OS’s freeze protection system: the automatic firing system
and the ceramic heating blocks. The automatic firing system allows the T-D2-IN or T-D2-OS to briefly
fire on for about 4 seconds and the ceramic heating blocks will heat up whatever portion of the heat
exchanger the blocks are strapped to. The firing system mainly focuses on providing freeze protection
around the heat exchanger pipes. The three heating blocks focus on protecting the inlet and outlet
piping inside the T-D2-IN or T-D2-OS, as well as the front drum pipe. The conditions to activate either
feature are different from each other. However, the two features are not mutually exclusive, because
they focus on different areas of the T-D2-IN or T-D2-OS. As long as each particular feature’s activation
requirements are met, it is possible for both features to operate at the same time.
Automatic firing system
Activation conditions will depend on the on whether or it is an indoor / direct-vent or outdoor
installation. Gas and electrical power are required for this feature to operate.
T-D2-IN installation:
After 5 minutes have elapsed since the T-D2-IN or T-D2-OS’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-D2-IN or T-D2-OS, which is mixed with water from the bypass valve). The
automatic firing system will not activate at all unless these 5 minutes have elapsed.
When the computer checks for these temperatures, the freeze protection firing system will activate if:
T
in
<
37°F and T
out
<
50°F OR T
out
<
37°F
where T
in
is the temperature of inlet thermistor and T
out
is the temperature of the output thermistor.
