A.Mike Casey, a licensed plumber in Connecticut and California and coauthor of Code Check Plumbing, responds: Yes, both electric and hydronic radiant systems can get hot enough to melt wax, leading to failure of the toilet's wax ring. While neither the IRC nor the Uniform Plumbing Code specifically addresses this issue, both do state that installation of radiant systems should comply with manufacturers' instructions. And every instruction manual I've read notes that it's important to keep a heat source — such as an electric heating mat or hydronic tubing — 4 to 6 inches away from the toilet-floor waste penetration. Some even recommend insulating around the penetration to further prevent heat transfer, particularly with metal drain pipes.
A concerned installer could provide additional insurance by using a neoprene ring in lieu of the standard wax ring — but in my experience, the neoprene rings tend not to seal as well as wax.
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