Heat recovery systems, recirculate interior air mixed with fresh outdoor air, to
lower costs of having to heat all fresh outdoor air, while exhausting old air through building ventilation.Insulated sliding shutter doors is simply an attempt to increase the R - Value of the weak
spots (doors and windows) either when you're not at home, or when you're going to
sleep and those windows serve only to leak heat out.Geothermal heating / cooling depends on your geography.