tankless turns on when I turn on the cold water
I recently renovated my house, and installed a Takagi T-K3 tankless water heater. I now find that when I turn on my cold water in the tub, flush my toilet, etc. the tankless water heater fires up briefly, then shuts off. When I shut off the cold water, the heater again fires up briefly, then shuts off. Is this expected, and if not, does anyone know what the problem might be?
I do have a dedicated recirculation line running back to the tankless, but I don't think that is the problem. I also have a check valve on the cold water supply to the tankless as the manufacturer suggests.