Freeze Protection System - A.O. Smith T-H3J-DV Service Manual

12-16. Freeze protection system

The 240 (T-H3J) models, 340 (T-H3S) models and 540 Outdoor (T-H3-OS) models freeze protection system
is comprised of the ceramic heating blocks. The ceramic heating blocks will heat up whatever portion of
the heat exchanger the blocks are strapped to.
There are two features to the 540 (T-H3) model's freeze protection system: the automatic firing system
and the ceramic heating blocks. The automatic firing system allows the 540 (T-H3) models 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 two heating blocks focus on protecting the inlet and outlet piping
inside the 540 (T-H3) models, 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 540 (T-H3) models. 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 : Only 540 Direct Vent Indoor (T-H3-DV)
Activation conditions will depend on the on whether or it is an indoor / direct-vent installation. There
is the DIPswitch No.8 (the upper bank) on the computer board to the "OFF" position. Gas and electrical
power are required for this feature to operate.
After 5 minutes have elapsed since the 540 Direct Vent Indoor (T-H3-DV) previous firing operation, the
computer will continually check the temperatures of the inlet and heat exchanger thermistors (the
heat exchanger thermistor is the one that checks the temperature of the water immediately leaving
the heat exchanger, not the water leaving the 540 Direct Vent Indoor (T-H3-DV), 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 < 32 °F
ex
where Tex is the temperature of exhasut thermistor.
To sum it up in a chart:
5 minutes have elapsed
Case
since prior operation?
1 No
2 Yes
3 Yes
240 (T-H3J), 340 (T-H3S) and 540 (T-H3) models Service Manial
Exhaust thermistor temperature
Any
Above 32 °F
32 °F and below
86
Page
CONFIDENTIAL
Ver. 1.00
Will automatic firing
be activated?
No
No
Yes