Gas valve and venturi assembly
The component that meters gas into the air stream. The most expensive of the common tankless repairs, and never a homeowner job on any unit.
What it does
The gas valve is the boundary between your gas supply and the burner. On older units it is essentially a set of solenoids that open and close. On Rinnai’s SENSEI RX generation it is a Smart-Sense valve that continuously trims the air-to-fuel ratio within limits set per gas type, working with a venturi — the throat where air and gas mix — mounted to the fan motor.
That extra intelligence is why the RX generation has error codes that older Rinnai units do not have at all: code 18 when the valve runs out of adjustment range, code 22 when its adjustment mechanism or stored parameters are wrong.
Why it fails
Mechanically, from condensate or debris inside the valve body stopping a solenoid opening cleanly. Electrically, from a failed coil. And increasingly, not at all — the valve is fine and the fault is a parameter, a mispositioned reed switch, or a gas supply problem that only looks like a valve fault. A competent technician rules those out before quoting you for the assembly.
Why this is not a DIY part
It carries live gas. Replacing it means breaking a gas connection, proving the joint afterwards, and on a SENSEI RX entering gas valve parameters from Rinnai’s replacement instructions and running a reset sequence on the control board. There is no version of this that is safe to attempt at home, and in most US jurisdictions it is not legal to either.
What a replacement usually costs
The assembly is commonly $150-350 for the part, plus one to two hours of labor and the parameter work afterwards — which is why a gas valve job on a tankless unit typically totals $400-1,100.