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Navien NCB-240 cycling on and off when calling for hot water

I have an NCB-240 combi-boiler. It was installed 1.5 years ago. When I call for hot water from the kitchen sink, it continually cycles on and off. But when I call for hot water from the bathtub, it seems to work just fine. So is the problem wit the flow sensor? Because with a larger volume of water, it seems to work fine.


  • Shane_2
    Shane_2 Member Posts: 188
    I would start with the easy things first, check the filter on the cold inlet of the NCB-240. Check the faucet itself to make sure it has minimum flow for the tankless (.6 GPM, I believe). Faucet aerator? Check the set temperature for the NCB, if too high your mix temperature at the faucet might not let enough flow to activate.

    That new, if there is a problem with the unit- call Navien once you check the easy stuff.
  • Sethamin
    Sethamin Member Posts: 58
    (2.5 years later, but I thought I'd update this thread just in case anyone runs into the same issue.)

    It turned out to be a clogged faucet aerator. The flow of the faucet (when clogged) was just at the minimum flow cut-off, which was causing the tankless to cycle on and off. Once the aerator was cleaned, flow was much better and I had no issues with getting hot water.
    Larry Weingartenkcopp