Takagi T4 starts with cold water use

[mlnk]

My new Takagi starts running and heating water when I turn off the cold water, like the garden hose. I installed a backflow preventer gravity valve then a spring type valve on the cold side feeding into the Takagi. Both did nothing. If I turn off the cold water slowly...there is no problem. The water pressure is 85 psi. How can I prevent it from coming on uselessly?

[LLigetfa]

I theorize there is an expansion tank somewhere absorbing the water hammer and the tankless is sensing the water movement.

[mlnk]

There is no expansion tank unless there is one inside the Takagi. I was thinking of installing one to reduce the abruptness when the water is shut off. Also I was thinking I could reduce the pressure to 50 lbs with a pressure valve, or is 85 psi OK?