A general contractor is planning on putting floor tile heating pads under a tiled seat that will be inside a shower area. I have asked him for the details on the mfg for the heater but haven't got it yet. From memory, the heating systems with digital thermostats explicitly says in the manual that is must not be on a GFI. Further more, If the pads were in a shower area wouldn't they need to be GFI protected?
From the manufacturer's info I've seen on two such products, they are most adamant that the product be GFI protected - even away from the shower.
There are also numerous megger tests made at different points during the installation.
So ... checking the manufacturer's instructions would be the place to start. They might even advise against use in the shower at all.
These are generally custom products, ordered for specific jobs. Yet, I'm beginning to see the topic pop up on DIY forums, as folks try to modify, adapt, and hillbilly-engineer the things. I say: DON'T do it.