A.O. Smith 120 through 250 Series - Manual (Page 15)

Instruction manual BFC 15
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2.4.2 Water temperature protection
The electronic controller uses temperatures sensors T1 (7) and T2 (13) to
monitor three temperatures that are important for safety. The table explains the
functioning of the temperature sensors.
Temperature protection
2.4.3 Gas valve
The electronic controller opens the gas valve so that gas can be supplied to the
burner. As a safety measure, the gas valve has a double shut-off. The double
shut-off guarantees complete isolation of the appliance from the gas supply.
To help ensure smooth ignition, the gas valve opens gradually ('softlite").
2.4.4 Fan
The fan (18) provides an optimum air supply when there is a heat demand. As
a safety feature, the fan ensures that any gases present in the combustion
chamber are removed, both before and after combustion. We refer to this as
pre- and post-purge.
The fan speed is continuously monitored by the electronic controller (4). The
electronic controller takes control if the speed of rotation varies too much from
the set value.
2.4.5 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
Protection Description
Against frost
(T1 < 5°C or T2 < 5°C)
The frost protection cuts in. The water is heated to 20°C.
For maximum water temperature
(T1 > 85°C or T2 > 85°C)
The high-limit safeguard serves to prevent overheating and/or excessive
formation of scale in the appliance. If the high-limit safeguard is activated, the
heating is stopped. This causes the water in the tank to cool down. Once the
water has cooled sufficiently (T1 < 78°C), the electronic controller resets the
appliance.
For extra safety
(T1 > 93°C or T2 > 93°C)
A lockout error of the water heater controller takes place. The controller must
be manually reset before the appliance can resume operation (8.3 "Error
conditions") . The reset may only be performed if T
1
<78°C.
Appliance Closing pressure
differential Opening pressure differential
BFC
28
> 635 Pa < 605 Pa
BFC
30
> 855 Pa < 825 Pa
BFC
50
> 885 Pa < 855 Pa
BFC
60
> 1085 Pa < 1055 Pa