We recently had a Quietside tankless HWH installed. I noticed that the unit cycles on (but does not fire) when there is a cold water only demand.
Is the unit most likely detecting water flow within the house on the inlet side and cycling on even though there is no demand for hot water? If so, is an inlet side check valve the solution? Thanks.
Is it possible you have air in some hot water lines still after installation?
If you used some cold water, the cold line should see some pressure drop and the air would expand slightly pushing some water back through the HWH and the opposite direction when you turn the cold off and pressure returns to normal.
I'd flush all hot water taps just to rule that out.