Rinnai B20 Service Manual download pdf (Page 30)

REU-2532W Series 30 Rinnai

17. Operation Principles

The preset temperature is selected at one of the remotes controls (where fitted). Where no remote control is

fitted , the default temperature can be set at 108, 120, 130, 140, 150, 160, 170, or 180 °F. To select one of

the above temperatures as your default setting, you MUST obtain written permission and training from

Rinnai. This function may not be allowed at temperatures above 120°F for safety reasons.

When the unit is first plugged into 120 volts, the PCB assumes an incoming water temperature of 77 °F.

This prevents the appliance from starting in “High fire” 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.

The 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 heater 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 predetermined 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 and ensures 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.

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Rinnai B20 Service Manual Page 30