A.O. Smith BFM - 100, BFM - 120 manual Water temperature protection, Pressure switch, Flame probe

2

Working principle of the appliance

•the Water temperature protection;

•the Flame probe;

•the Pressure switch.

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2.4.2Water 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

Protection	Description
Frost thermostat	When the frost thermostat sensor (46) measures a temperature of 20°C or
	less, the heating cycle (2.3 "The appliance's heating cycle") will start.
High-limit thermostat	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.
Safety thermostat	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.

2.4.3Pressure 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	Opening pressure difference
	difference
BFM	> 200 Pa	< 170 Pa
30
BFM	> 270 Pa	< 240 Pa
50
BFM	> 255 Pa	< 225 Pa
80
BFM	> 610 Pa	< 580 Pa
100
BFM	> 220 Pa	< 190 Pa
120

2.4.4Flame 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.

14 Instruction manual BFM