Rheem Tankless Gas Code 65
Water volume control valve malfunction. The valve that meters how much water passes through the unit is not behaving. It is one of the two ways a tankless heater controls outlet temperature — the other being how hard it fires.
Part replacementThe verdict
Replace a part
A moving part in the water path, moderate in price, replaced with the electrical, gas and water supplies off. The symptom while you wait is temperature that will not hold steady.
Typical cost: $200–$600
US estimate for parts and labor. Prices vary by region and by who does the work — see how we estimate costs.
What the manual says
The meaning above is transcribed from the manufacturer's diagnostic documentation for the Tankless Gas — not averaged across the brand.
Source: Rheem Tankless Gas Troubleshooting Pocket Manual (SVC820) (p. 76), read Aug 2026 — the manufacturer's own documentation for this exact series.
What you can safely check
- Note whether the temperature swings at the tap and at what flow rates.
- Book a service call.
Frequently asked questions
Is this the same as the bypass valve in code 66?
No. The volume control valve meters flow through the heat exchanger; the bypass valve routes water around it. Two valves, two codes, and they fail independently.
Don't confuse this with
5 other Rheem Tankless Gas codes point at a valve too. They are different components in different places, and ordering a part against the wrong one is the most common way this fault gets expensive: