The problem w/PEX is it tends to get very brittle when exposed to heat and VO (and most likely B100). If your not putting much pressure in the PEX, it may be fine, at least for a season or 2. Take a peice of your PEX and leave it in the next couple batches and see what it looks/feels like!

I guess the worst case scenario is overflowing bio reactor w/water or ?? from boiler or contaminating the boiler fluid w/bio... either way doesn't sound too good for the boiler.

What about adapting to aluminum tubing or similar and putting that inside the drum? DON"T USE COPPER though!!

Why not wrap the outside of the drum w/PEX and then insulate over that?

How hot is the water from the boiler?