520 Indoor / 520 Outdoor Service Manual
Ver. 1.00
84
CONFIDENTIAL
12-19. Freeze protection system
There are two features to the 520 Indoor or 520 Outdoor freeze protection system: the automatic firing
system and the ceramic heating blocks. The automatic firing system allows the 520 Indoor or 520
Outdoor to briefly fire on for about 3 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 drum. The five heating blocks focus on protecting the
inlet, outlet and drain piping inside the 520 Indoor or 520 Outdoor, 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 520 Indoor or 520 Outdoor. 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
Gas and electrical power are required for this feature to operate.
After 5 minutes have elapsed since the 520 Indoor or 520 Outdoor 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 520 Indoor or 520 Outdoor, 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
>
46°F and T
out
<
37°F
where T
in
is the temperature of inlet thermistor and T
out
is the temperature of the output thermistor.
