Rinnai FUA-E, Service Manual, Page: 13 / 67

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3. Thermal Fuse
The thermal fuse is an electric link which must be intact for the unit to operate. If the
thermal fuse reaches a set temperature it will melt and the unit will shut down. The
thermal fuse must be replaced if it melts. It is to protect against over heating and heat
exchanger splits where water may leak out and be superheated into steam.
4. Overheat Safety (Bi-metal Switch)
This Bi-metal Switch is fixed at the bend of the Heat Exchanger hot water outlet. If the
temperature outlet from the heat exchanger reaches 97
0
C the bi metal switch will open
and the solenoid valve circuit is broken. This will cease combustion in case of overheat.
5. Combustion Fan
The combustion fan supplies primary air into the wing burners and secondary air up
through the Bunsen style burners. The fan is DC low voltage and the speed is controlled
by the PCB depending on the hot water flow and temperature. The fan speed is
compared to the current required to attain that speed for air proving. If the fan current is
over or under the parameters for the given speed the unit will shut down on air proving.
6. Water Volume Servo with Water Flow Sensor
6.1 Water Flow Sensor
Water flow sensing is done with a small turbine that spins when water travels through it
in the correct direction. Each of the four fins on the turbine has a small magnet on it.
Outside of the valve there is a magnetic sensor that detects the speed that the turbine is
revolving. The revolution speed is input to the PCB which relates this speed to the water
flow volume and determines whether it is sufficient for ignition.
6.2 Water Volume Flow Servo
The water volume is adjusted by opening and closing the water flow valve with the servo
motor. This will limit the maximum hot water flow to 20 L/min, and will limit the water
flow further when the burner is at high fire to ensure the temperature setpoint it met.
Main Components