My plan was to run water from my OWB directly to the slab using a mixing valve to reduce the temp to about 85 deg. The question is: since the PEX on a second floor apartment will be about five feet above the boiler which has a "pop off" that lets pressure out of the top of the tank at 2-3 psi., will it leak from the head pressure from the second story Pex? Mixing valves would allow me to run a different temp for under the second story sub floor. Is there any way to do this without a heat exchanger?

It's about 0.43 psi per foot of height, so yeah, if you have only 2-3 psi before the thing pops it's cork you're probably gonna leak, sooner or later. (5' x 0.42= 2.15 psi)

I'm surprised there's anything out there with such low pressure ratings, but I haven't looked at the specs for wood boiler stuff very closely.

No.

Thanks BB, thats what I needed to hear . Now I'm thinking a brazed plate heat exchanger of the proper size.
Thanks for the reply.