It's actually a very common problem between boilers and water heaters. A rinnai certified technician should have been able to fix the problem for you. The heating parameters must be set in the controls. It the unit continues to work without the settings all kinds of problems arise, like breaking the fan motor, the air flow switch or the temperate sensors. Your builder or plumber probably installed the heater, turned it on and walked away. The parts fail because of the constant on and off during the heating cycle.
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