Rinnai REU-2007W, Service Manual

Page: 36 / 69

Infinity REU-2007,2008,2408,2424 - 31 - ©Rinnai
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The preset temperature is selected at one of the remote controls (where fitted). Where no remote
control is fitted, the default temperature is 40 C, 43 C, 50 C, 55 C, 60 C, 65 C, 75 C or 85 C
(Heavy Duty only) depending on the position of dip switch numbers 3 to 6 (see page 13).
When the unit is first plugged into 240 Volts, the PCB assumes an incoming water temperature of
25 C, this prevents the appliance starting on HIGH, and producing very hot water the first time it
is used.
The data used to determine the outgoing water temperature initially is incoming water flow, and the
remote control pre-set temperature.
From the incoming water flow and remote control pre-set temperature data, the CPU is able to
determine a suitable gas rate to kick the appliance off, once a hot water tap opens.
This calculation of temperature rise and water flow is called simulation feed-forward.
The water heater calculates incoming water temperature by subtracting the theoretical temperature
rise from the outgoing hot water temperature, to establish the correct gas flow.
When a hot water tap is opened, water begins to flow through the appliance. The turbine in the water
flow sensor begins to revolve. The revolution speed is proportional to the water flow. A sensor
located inside the device relays information in the form of magnetic pulses to the main PCB to
determine whether or not water is flowing, and also, the volume of water flowing. When a pre-
determined water flow is sensed, the ignition sequence begins.
The combustion fan pre-purges the combustion chamber. A rev counter on the combustion fan
indicates the fan rpm to the main PCB. Once the pre-purge cycle is completed, the PCB controls the
fan rpm by varying the DC voltage to the fan motor. This maintains the correct air/gas ratio
throughout the time the water heater is in use, to ensure good combustion.
The gas is ignited by direct electronic spark and the flame is sensed by the flame rod. The opening
degree of the modulating valve is determined by the combustion fan speed.
The changeover valve directs gas to one side or both sides of the burner. At the point where the
changeover valve opens or closes, the modulating valve is instantly re-adjusted by the PCB to
compensate for the change in the number of burners in use. From the information provided by the
water flow sensor and the water temperature thermistor, the PCB determines how much gas is
required to heat the water to the temperature selected on the remote control.
The PCB is programmed to provide the maximum volume of water possible at a given temperature
rise. As the water flow from the tap is increased, the PCB increases the gas and air flow to the burner.
In the case of the 2008 and 2408, once the maximum gas rate is reached the PCB begins to control
the water flow through the appliance in order to maintain the preset temperature. This is achieved
by the PCB turning the valve within the water flow control device by means of a stepping motor.
The water flow control device operates at high water flows and/or high temperature rises. When the
temperature rise is low, or the water flow is restricted by the hot water tap, then the device may not
be required to operate. [See section on “Main Components” on page 20, for clarification].
There is no direct connection between the mechanical water flow control device with water flow
sensor and the PCB, therefore, there could be occasions when the input required to heat the water
exceeds the capacity of the appliance, and water at a temperature lower than that requested on the
remote control is output. In this case, manually reducing the water flow at the tap will increase the
temperature.
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