What is the difference between foam core and solid core. Are both of them fire rated. Why do the high efficiency boilers require solid core. Is solid core more fire resistant and if not does it make a difference on vertical. Because I know that foam core is softer and good for plumbing dwv application.

Foam core is also known as open cell which has small air pockets as opposed to closed cell which is solid. When used for venting you must use closed cell or solid PVC. If the rating on the pipe is ASTM F891 it cannot be used for venting.