Takagi TK-3 turns on too often

My Takagi TK-3 works fine but even when the toilets flush it turns on for 5 sec or so then shuts off
I am on a well with average water pressure of 40lb The company did not respond to my inquiry
any help would be appreciated

The low pressure in the piping system causes the water in the hot side flowing backwards through the heater. After cold water draw stops, the piping system re-pressurizes including the hot water piping, causing brief flow through the heater. When this happens unit fires as designed since it senses flow through the tankless. A good spring loaded check valve on the cold water inlet of the tankless will solve this issue.