A.O. Smith BFM - 100 Installation & Service Manual page 14

2
2.4.2
Protection
Frost thermostat
High-limit thermostat
Safety thermostat
2.4.3
2.4.4
14
Working principle of the appliance
• the Water temperature
• the Flame probe;
• the Pressure switch.
Water temperature protection
By means of the frost, high-limit and safety thermostats, the burner control
monitors three temperatures that are important for safety. The table explains the
working principle of the thermostats with sensors.
Temperature protection
Description
When the frost thermostat sensor (46) measures a temperature of 20°C or
less, the heating cycle (2.3 "The appliance's heating cycle")
When the high-limit thermostat sensor (39) measures a temperature higher
than 84°C, the high-limit thermostat will open. The heat demand is terminated
and the burner control halts the heating cycle until the high-limit thermostats
close once more. At that moment the burner control will reset the appliance and
the heating cycle will restart. The high-limit safeguard serves to prevent
overheating and/or excessive formation of scale in the appliance.
When the safety thermostat sensor (38) measures a temperature higher than
93°C, the safety thermostat will open. The heat demand is terminated and the
burner control will immediately halt the heating cycle. The burner control will go
into a lockout error state. This must be manually reset before the appliance can
resume operation.
Pressure switch
The pressure switch ensures the discharge of flue gases and the supply of
incoming air during the pre-purge and normal running of the appliance. The
default state of the pressure switch is open. When sufficient pressure difference
is reached, the pressure switch closes. However, in the event of a fault, the
pressure switch is tripped open, and the heating cycle is interrupted. The table
shows the trip point per appliance.
Note
The trip point of the pressure switch is not adjustable.
Pressure switch trip points
Appliance Closing pressure
difference
BFM > 200 Pa
30
BFM > 270 Pa
50
BFM > 255 Pa
80
BFM > 610 Pa
100
BFM > 220 Pa
120
Flame probe
To ensure that no gas can flow when there is no combustion, the water heater
is fitted with a flame probe (21). The burner control uses the ionisation-detecting
properties of this probe for flame detection. The burner control closes the gas
valve the instant it determines that there is a gas flow but no flame is present.
protection;
Opening pressure difference
< 170 Pa
< 240 Pa
< 225 Pa
< 580 Pa
< 190 Pa
Instruction manual BFM
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will start.