Some clarification is due here:
The IRC does not specifically approve of PVC or any kind of plastic venting. The code recognizes listed venting, which means listed to UL 1738, such as AL29-4c stainless steel or per mfrs. instructions. Now, most mfrs simply say you can use Sch 40 pvc. They don't get into specifics of joining, support, etc. So, where do these regs. for purple primer come from? Local ordinances. The inspectors want to CYA. This became more of an issue in the wake of the Lofgren carbon monoxide case in Aspen, CO where PVC was not properly joined.
FYI, Charlotte pipe DOES require primer on Sch 40 PVC. Download the installation instruction and see page 89. The primer softens the plastic for a better, more uniform bond.
As for the 'ugly' stains, that's a reflection of workmanship. I've posted on this before but here's the trick: wrap the pipe with blue painter's tape revealing a 1/8"-1/4" uniform band. Prime to the tape, apply cement, join and pull the tape. You get a pretty purple ring everytime and only takes an extra few seconds.
If something goes wrong and you are called in, you'd better be following the mfrs instructions on joining.
Back to the 'approval' issue for venting: None of the major mfrs. have 'approved' their pipe for combustion venting. There is no ASTM std. to test their pipe to for venting. This is a major reason we have not been able to agree on a new listing std. on the UL Standards Technical Panel the way the Canadians have with their S636 listing. We see too many problems with their listing.
With a max. operating temp. of 140F, you'be do well to perform combustion analysis to ensure it is not too hot for the venting. BTW, I called Fernco and they say the same as Charlotte pipe: 140F max continuous and not approved for venting even though their couplings are used in this application and even sold to Bradford White for their TTW water heaters.