> How can a heating system be inefficient? Does it generate too much waste heat? Yeah, they do: 1) Heat that's present when people aren't is unnecessary heat.

> How can a heating system be inefficient? Does it generate too much waste heat? Yeah, they do: 1) Heat that's present when people aren't is unnecessary heat. 2) Heat that leaks out of the building is wasted heat. So isolate the building better...? Yes, of course... But you can never isolate 100%. And heat that's present while people aren't -- see 1) above -- is heat that can (and, to some extent, will!) leak out before ever warming anyone.