Water flow servo and bypass flow valve
The motorised valves that meter how much water goes through the heat exchanger and how much bypasses it. They are how a tankless unit holds a steady outlet temperature.
What they do
A tankless heater controls outlet temperature two ways: by changing how hard it fires, and by changing how much water it sends through the heat exchanger. The water flow servo meters the flow. The bypass flow valve mixes heated water with water routed around the exchanger, which is what keeps the tap steady when demand changes.
On Rinnai’s SENSEI RX these are separate devices with separate error codes — 65 for the water flow control device, 66 for the bypass flow servo, the latter existing only on this generation.
Why they fail
They are motors with moving parts in a wet environment. Windings go open, gears bind, and connectors corrode. Rinnai’s first check on the bypass servo code is simply whether the harness and connector are wet — if they are, the real problem is water getting somewhere it should not.
There is a safety dimension on the flow servo specifically. Rinnai’s instruction when it fails to close during a bath fill is to turn off the water flow and stop using the bath fill function immediately: a valve that will not close means an unattended tub keeps filling.
Why this is not a DIY part
Replacement needs the electrical, gas, and water supplies all off, and the unit opened. Rinnai’s diagnosis also depends on resistance readings taken at specific pins on the control board — the readings are what separate a failed servo from a failed board.
What a replacement usually costs
$80-250 for the valve, typically $200-600 installed depending on which one and how much diagnosis it takes to tell servo from board.