Rinnai Tankless Error Code 65: What a Water Flow Control Fault During Bath Fill Means for East Cooper Homes
If your Rinnai tankless flashes Code 65, the unit is saying the water flow control valve (the servo that meters and stops flow during Bath Fill) didn't close when it was supposed to. Bath Fill is meant to stop at a set volume. When that valve won't shut, the heater locks out instead of letting the tub keep filling unsupervised.
Rinnai lists it as "Water Flow Control Fault." In plain English: during Bath Fill, the board told the flow-control valve to close, and the valve didn't. Hot water can keep coming when it shouldn't. That's the code doing its job — and Rinnai's own guidance is blunt: turn off the water, stop using Bath Fill, and get a licensed tech.
This is related to, but not the same as, Code 03 (power interruption during Bath Fill). Code 03 is "power died mid-fill." Code 65 is "the bath-fill valve failed to close." Same neighborhood. Different problem.
What usually trips Code 65
Around Charleston, Daniel Island, Mount Pleasant, and the rest of East Cooper, the usual suspects look like this:
A sticky or failed water flow control servo — the valve that opens and closes for Bath Fill wears, binds, or won't drive closed.
Loose or damaged wiring at the flow-control harness — vibration, a bumped connector, or a terminal that worked loose over heat cycles.
Debris or scale in the valve path — less common than a dead servo, but coastal hard water and sediment don't help.
Less often, a board-side drive issue after other electrical problems on the circuit.
This is a Bath Fill flow-control fault. It isn't a "flush and hope" code like scale (LC), and it isn't condensate (25) or ignition (11/12). Don't treat it like a filter rinse.
Safe checks you can do
Keep it simple and stop before you open the cabinet or touch gas components:
Turn off the water to the heater (or at least shut every hot fixture) and discontinue Bath Fill on the controller. Don't keep trying to fill.
Write down the code and whether it came up mid-fill, right when Bath Fill should have stopped, or after a reset.
Power-cycle once — turn the unit off at the controller, wait a minute, turn it back on. If 65 returns immediately, don't keep resetting.
Note recent work — someone using Bath Fill, a recent service visit, storm power blips, or work near the heater. That history helps.
If you're smelling gas, hearing a hiss, seeing uncontrolled flow you can't shut from the fixtures, or the heater is doing anything that feels unsafe: leave it, get everyone out if needed, and call from outside. Code 65 is not a DIY valve job.
When to call a pro
Call when:
Code 65 comes back after one reset
Bath Fill won't stop cleanly or you're afraid the tub will overfill
The code sits alongside other faults
You're not set up to check the servo harness, resistance, or replace the water flow control assembly safely
A tech will typically check the wiring harness from the water flow servo back to the PC board, power-cycle to confirm the code, verify the valve is failing to close during Bath Fill, and replace the water flow control servo/assembly when the numbers say so. That's textbook Rinnai guidance — not guesswork.
Local note for East Cooper homes
A lot of tankless units out here sit in garages, closets, or utility niches. Controllers with Bath Fill get used on big soaking tubs in Mount Pleasant and Daniel Island homes, and coastal humidity plus the occasional bumped wire when someone stores bikes or Christmas boxes don't help a delicate servo harness. If you've already dealt with Code 03 (power interruption during Bath Fill), you know the display is pointing at the Bath Fill subsystem. Code 65 points at the water flow control valve failing to close. Fix that path; don't chase the whole heater.
Bottom line
Code 65 = water flow control valve failed to close during Bath Fill. Safe home steps are limited: shut the water / stop Bath Fill, note the code, one reset, then a qualified tech for harness and servo work. Don't force resets and don't open the gas or water valve assembly yourself.
I'm Christian with Avery & Sons Plumbing + Tankless. We work Rinnai tankless every week across Charleston, Daniel Island, Mount Pleasant, and East Cooper. If 65 is on your controller and you want it sorted, call (843) 305-5555 or visit averysplumbing.com.
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