In my case, the error code 21 was caused by the failure of the vent fan to run-on after shut off (easily verified by listening to the unit). The problem was not resolved by swapping either the fan or the control unit with another heater. I talked to a local service tech who had a couple of suggestions. One was to move DIP switch #6 to the upper position. After doing thisit will be necessary to re-initialize the controll by cycling power off and on. This fixed the problem. I emailed Takagi tech support to ask if I should consider this as permanent fix, or if there was something else I should do. Since I got no reply, I'll pass on the tech's other suggestion: buy a Rinnai.
SOURCE: Takagi Jr Flash Water Heater
explain your problem.
Takagi is a great tankless heater, very few problems...
need more info
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