About four years ago my primary home received a bathroom remodel over a concrete slab.

The workers did not insulate the pipes and laid the flexible PVC down into a groove they jack-hammered into the slab. This was the first time I had ever used a contractor, so don't laugh. This 6x6 bathroom had insulation, subfloor, and ceramic tile laid down over-top of this groove. I.e. I can't access it.

For four years, the pipes have frozen. I let the faucets drip to prevent them from freezing. It is a secondary bathroom.

Is there something I can install DIY for under a couple hundred bucks that will exchange water both cold and hot to prevent the pipes from freezing? I've tried heat tape all the way up to the slab, but it is freezing the 5 foot section between the crawlspace and exit to the sink. Since it's a crawlspace, I don't want to install a heater or anything else other than heat tape to heat the water.

Thanks!