I remember a few years ago on ABC's New Inventors that someone came up with a system to heat your home, during the day, using the heat in your roof space.

Followed a google ad link on here and there is a mob in Victoria
who do the same, as well as cycle in the cool roof space air at night in summer (I just use my evap air con on "fresh clean air").

These aren't a "whole" heating and cooling solutions, but a cheap to run addition to your normal heating/cooling.

Anyone on here actually looked into this or got it?