Is it okay to use three-eighths PEX pipe for a tankless water heater manifold system?
It depends on the length of the run. 3/8 pipe tends to be a
little small and will restrict the flow. Use the 1/2 if you can.
It's not that much extra but it will give you much better water
delivery. Of course, between the manifold and the water heater, use
3/4 inch or larger copper.
I would have to agree with the previous contributor. 1/2" PEX is
indeed the preferred size for running the water to the plumbing