A.O. Smith BFC - 28 - Manual (Page 15)

Protection for the appliance
Release R.1.3 UK 15
gis
2.4.1 Water temperature protection
With temperature sensor T1 g and temperature sensor T2 m, the
ThermoControl monitors three temperatures which are important for safety.
Table 2.1 explains the functioning of the temperature sensors.
2.4.2 Gas control
The ThermoControl opens the gas control so that gas can be supplied to the
burner. As a safety measure, the gas control has two valves. Both valves shut
off the gas supply.
Smooth ignition is achieved by opening the gas control with a delay (‘softlite’).
2.4.3 Fan
The fan provides an optimum air supply when there is a heat demand. A safety
aspect is that the fan ensures that any gases present in the combustion
chamber get removed, both before and after combustion. We refer to this as
pre- and post-purge.
The fan speed is continuously monitored by the ThermoControl. The
ThermoControl takes control if the speed of rotation varies too much from the
value set.
2.4.4 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
standard position of the pressure switch is open. When sufficient pressure
differential is reached, the pressure switch closes. However, in the event of a
fault, the pressure switch will be tripped open, and the heating cycle will be
interrupted.
Table 2.1 Temperature protection
Protection Description
Against frost
(T
1
< 5°C or T
2
< 5°C)
The frost protection cuts in. The water is heated to 20°C.
For maximum water temperature
(T
1
> 85°C or T
2
> 85°C)
The high-limit safeguard serves to prevent overheating and/or excessive
formation of scale in the appliance. Should the high-limit safeguard be
activated, the heating is halted. This causes the water in the tank to cool down.
Once the water has cooled sufficiently (T
1
< 78 °C), the ThermoControl resets
the appliance.
For extra safety
(T
1
> 93°C or T
2
> 93°C)
A lockout error of the water heater controller takes place. The controller must
be manually reset before the appliance can resume operation (see paragraph
’6.3 Error conditions’ ). The reset may only be performed once T
1
< 78°C.