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What is typical cycling of tankless unit in closed-loop system

Full disclosure, I'm that guy; the one that is hell-bent DIY. I also saw the other posts lamenting the use of Takagi units by amateurs...guess what, I used a Takagi 😊 So, for those that are still reading, here is my situation. The unit comes on as it should when the pump begins to circulate the water, outlet temp rapidly builds to 120 F (or whatever I set it at), and as the inlet temp reaches approx 92 F the unit shuts down and does not come back on until inlet and outlet temps drop to around 85 F. I've never had a tankless before, but shouldn't it run continuously, as long as the circulator pump is on? Its not showing any error codes, so I am rather stumped.


  • Jamie Hall
    Jamie Hall Member Posts: 23,270
    Not sure on the Takagi's control logic -- but most of them are not real happy with hot (which 92 is) input water. They are designed to take a relatively small flow of water at usually no more than 50 to 60, and boost it to usually no more than 120, but that takes a lot of heat input. You are asking it to go from 90 to 120, which is only half as much heat, and I suspect the thing is getting confused.

    The solution, of course, if you really want to use the Takagi, is to let it feed a nice big buffer tank and use the buffer tank to feed the system.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England