The message
from The Natural Philosopher contains these words:

Pipes are ideally laid in a double spiral

That's important, that bit. Without it you'll end up with one end of the
floor getting a lot hotter than the other.

The spiral in then spiral back out again in between the incoming spiral
system means that the slab warms up evenly.

--
Skipweasel
Pay no attention to that man behind the curtain.