Why does the tankless hot water heater cycle on even when no water is running?

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    Welcome to Stack Exchange! It's a bit difficult to answer your question when you give so few details. In theory, a tankless water heater should only cycle on when water is actually being drawn. Perhaps it's maladjusted, so that it isn't accurately detecting the water flow. More details would help people give you a useful answer. – Daniel Griscom Nov 5 '15 at 22:44
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    What's the make and model of the heater? Was it Installed according to the manufacturer's installation instructions? Is it newly installed, or an old unit that used to work properly? Did anything change, right around the time the problem started? – Tester101 Nov 6 '15 at 0:50

Tankless water heaters have a flow switch and temperature sensors. Flow switch activates the water heater when water is flowing and the temperature sensors control the amount of heat delivered. Perhaps the flow switch is stuck on.

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