Rinnai FUA-E, Service Manual

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J. Water Flow Sensor.
The water flow sensor is located inside the water heater immediately as the cold water
enters the unit. From that there are 3 wires (red, yellow, black) running to the pcb, there
is a connector plug in between the sensor and the pcb for testing. Measure the voltage
with the water heater power on and no flow. Following this measure the voltage
between Yellow and Black with water flow on. If there is a remote attached make sure it
was turned on before the water began running through the unit.
Black – Red
11 – 13 Vdc at all times
Yellow – Black appx 8.5 Vdc water off 4 – 7 Vdc water on
*note: This Yellow-Black voltage is not variable with flow rate.
If Black – Red is incorrect replace pcb
If Yellow – Black is incorrect the water flow sensor may be faulty. Before you replace the
water flow sensor be sure you have checked the filter (R) and for air locks (M).
If the voltage is incorrect and you believe the sensor may be faulty remove the water
valve and blow through the rotation turbine to make sure it is free to spin –
DO THIS
WITH THE GAS ISOLATED . You will be able to hear/see it spinning when you blow
through it. If it is spinning but the unit does not start the sensor is faulty. If it is not
spinning it may need freeing up. If its spins and the unit starts then there is a water flow
problem – no water is flowing through the unit. Check that the unit is not piped up with
cold flow into the hot outlet.
K. Thermistors.
Check all thermistors by unplugging thermistor lead and inserting meter ends into both
sides of thermistor plug and measuring resistance on the 20 k Ohms scale. Apply heat
to the thermistor bulb and watch the resistance decrease. Apply ice (or cold water) to
the thermistor bulb and ensure the resistance increases.
Thermistor values should read about:
15°C : 11.4 – 14 k Ohm
30°C : 6.4 – 7.8 k Ohm
45°C : 3.6 – 4.5 k Ohm
60°C : 2.2 – 2.7 k Ohm
If there is a break in the circuit the resistance reading will be over 1 M Ohm.
If there is a short circuit in the wire the reading will be less than 1 Ohm
If either of these occur the thermistor should be replaced.
Sometimes water thermistors become coated with scale. If the values obtained are
unexpected, or the outlet temperature is fluctuating remove the thermistor and check it
for scale. Look into the heat exchanger tube to check for a ‘scale pocket’ inside. Beware
when replacing the thermistor, there is an o-ring which usually sticks inside the pocket.
Remove it and put it on the thermistor before replacing.
Diagnostic Checks