My friend had that in a ceiling from the 70's. He had blacksnakes living above it, it kept them warm all year long. Imagine two copulating blacksnakes falling out from under the fireplace mantle and onto the family room floor on Thanksgiving day. There is a price for great heat. (I'm not talking about electric bills).
Apparently these are a bit rare.
The system was mostly nonfunctioning. The scary part is that most of the panels were disintegrating and they were wired 240 volt but only switched on one leg. All those wires were hot all the time. Not a problem unless someone touched one. No snakes though.