well, if your heater is near the window - and it probably is - then you can put one side on the window, and the other over the heater, then dump what you generate into a battery. and, the colder it is outside, the better it will work.
you could also potentially dig up your floors..
it's true that you'll lose a little of the heat, but the point is that you're wasting oodles of it in the first place.
if you could potentially set up a system that takes a few watts an hour from a dozen different sources, you could salvage enough heat to charge your batteries. ten sources at two-three watts each would be twenty-thirty watts an hour.
so, these are the two ways to salvage excess energy from around your home - light and heat. a potential third source would be sound, but it probably is too low efficiency to bother with.
if you don't use a lot of energy to begin with, i think you should be able to reduce your consumption to the big appliances, and please understand that i'm not proposing you can get off of the grid using these methods. you will need something like a solar panel outside to run your stove and your fridge, and your lights and your heaters in the first place.
but, i've piqued my own curiosity, here.