Takagi Condensing Code 651
Flow adjustment valve fault. The flow adjustment valve, which meters how much water passes through the unit, is not behaving. It is one of the two ways the 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 supplies off. Expect temperature that will not settle until it is done.
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 Condensing — not averaged across the brand.
Source: Takagi Condensing Models On-Demand Water Heater Troubleshooting Guide (p. 13), read Aug 2026 — the manufacturer's own documentation for this exact series.
What you can safely check
- Note whether the temperature swings and at what flow rates.
- Book a service call.
Frequently asked questions
Is this the same as 661?
No. 651 meters water through the heat exchanger; 661 is the bypass valve that routes water around it. Two valves, two codes.
Code 651 on other Takagi series
The same code number can mean something different on another series — make sure you're reading the page for your exact unit:
Don't confuse this with
4 other Takagi Condensing 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: