Rinnai REU-2018W Service Manual page 22

5. Combustion Fan
Air for combustion is supplied by a centrifugal fan
driven by a DC motor. After a pre-purge period of
0.2 seconds, the fan speed is controlled by the
PCB to provide the correct volume of air for
combustion. The calculation for the fan speed is
based upon incoming water temperature, water
flow and the temperature selected on the remote
controls.
The actual speed of the motor is continuously
monitored by a magnetic pulse sensor.
This sensor emits 4 pulses per rotation of the fan.
This is the fan feedback or confirmation data processed by the PCB and used for 2 operations.
1. The fan speed is constantly corrected to provide optimum combustion conditions.
2. To determine the opening degree of the modulating gas valve, so that the gas rate always
matches the volume of air for combustion, as well as the input required to heat the water.
The reason for controlling the opening degree of the modulating gas valve based upon data from
the combustion fan is that the gas valve is able to react much more quickly to a change in control
signal than the combustion fan. Controlling the gas valve based upon data from the combustion
fan means that combustion remains satisfactory, even if there are sudden changes in input
conditions.
6. Water Flow Sensor and Water Flow Control device:
Water flow is detected by a turbine/magnetic pulse generating device. Water flows through the
turbine/magnetic sensor providing information to the PCB by generating a pre-determined
number of pulses in proportion to the water flow. These pulses are counted by the PCB - no pulse
indicates no water flow. The frequency of the magnetic pulses increases as the water flow
increases, this enables the PCB to calculate the exact water flow, and determine the water flow
in litres/minute. As soon as the required water flow is detected, the PCB activates the combustion
fan. The combustion fan speed is monitored by a magnetic pulse sensor. The output from this
sensor is processed by the PCB which opens the gas modulating valve to a degree proportional
to the fan speed. See above for further details on the combustion fan.
The water flow control consists of a plug and barrel valve which is rotated by a motor to increase
or decrease the volume of water passing through the heat exchanger.
Water flow control device.
Infinity REU-2425W - 19 - Issue 2 - 5/07/04 ©Rinnai