PEX and Tankless - what distance matters?

[itaki]

I just bought a Manabloc manifold and am using it and PEX on this job for the first time. I have mounted the Manabloc about 5 feet off the ground in a closet and am hoping to use a tankless water heater mounted either above or below in between the same stud framework. Is this safe? I see that the recommended distance of 36 inches verticallly or 18 inches horizontally, but does this change with tankless? I was considering actually mounting the heater above the manifold, which would eliminate heat rising into it anyway.

I was considering using this heater:
Eemax Series 2
and running the pipes straight down from the heater into the manifold with a clearance of about 18 inches.

Any ideas on whether or not this is possible?

Thanks for your help,
Michael