Rinnai RH180 Installation Fundamentals page 58

Maintenance Codes – No Code displayed.
Further explanation of conditions that might cause no hot water and/or No error
codes:
Since the hot water supply to the building comes from the tank and not directly
from the tankless engine, opening a fixture will not flow water through the engine
and will not automatically trigger it to fire.
• The tankless engine will fire only when it detects water flow and since the flow
is provided by the built in circulation pump, no code or operation could be the
result of the pump not turning on. The issue could be the pump is not being
commanded to turn on. If the tank thermistor is faulty or damaged and not
reading the correct temperature, it may not be triggering the pump to turn on.
• If the pump is being signaled to turn on and the unit is not sensing flow (due to
the pump not operating or not flowing water or due to a damaged flow turbine,
flow sensor, disconnected flow sensor, or clogged filter) you should get a code
63 since the flow reading would be "zero". Code 63 is posted if the flow rate is
below 1.3 GPM.
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