In article , Michael Chare
wrote:
Is there a little box that I could buy which would have an Ethernet port
(and IP) that I could attach to my home network and would allow me to
use my computer to monitor the status of the mains voltages circuits (of
my central heating system)?

You are sticking your toe into very deep water ... Google for "home
automation" and you'll find more "little boxes" than you could probably ever
wish for.

X10 is another relevant search term.

Cheers,
Daniel.