Takagi TK-3 turns on too often

[dh100m]

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

 

[Zl700]

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.