A.O. Smith BFC Cyclone, Installation, User And Service Manual

Page: 49 / 118

6.3 Safety devices
6.3.1 Protection for the water heater
6.3.1.1 Water temperature protection
Using temperature sensors T
1
(7) and T
2
(13), the controller monitors three temperatures
that are important for safety. The table explains the functioning of the temperature
sensors.
Safety Description
Anti-frost
(T
1
< 5 ºC or T
2
< 5 ºC)
The frost protection cuts in. The water is heated to
20 °C.
Maximum water
temperature:
(T
1
> 88 ºC or T
2
>
88 ºC)
The high-limit safeguard serves to prevent
overheating and/or excessive formation of scale in
the appliance. If the high-limit safeguard is
activated, heating is stopped. This causes the
water in the tank to cool down. Once the water has
cooled sufficiently (T
1
< 81°C), the controller
resets the appliance.
For extra safety
(T
1
> 93 ºC or T
2
>
93 ºC)
A lockout error of the water heater controller takes
place. (see section 11) The controller must be
manually reset before the appliance can resume
operation. The reset may only be performed if T
1
<
81°C.
6.3.1.2 Gas control valve
The controller opens the gas control valve so that gas can be supplied to the burner. As a
safety measure, the gas control valve has two valves. Both valves shut off the gas supply.
6.3.1.3 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 controller (4). The controller takes control
if the fan rpm varies too much from the value set.
6.3.1.4 Pressure switch
The pressure switch only guarantees the supply of air during pre-purging of the water
heater. If the pressure difference during pre-purging is sufficient, the pressure switch
closes. Refer to the general and electric specifications in the appendices (see section 12)
for the trip point for each water heater.
Note
The trip point of the pressure switch is not adjustable.
6.3.1.5 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 controller uses this probe to detect the presence of a flame,
by means of ionisation detection. The controller closes the gas control valve the instant it
detects that there is a gas flow but no flame.
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