You are forgetting some heat sources already present that can have a big effect: Humans - yes humans From http://aheatingguysblog.blog.com/2009/09/23/human-bo

You are forgetting some heat sources already present that can have a big effect: Humans - yes humans From http://aheatingguysblog.blog.com/2009/09/23/human-body-btu-output/ http://aheatingguysblog.blog.com/2009/09/23/human-body-btu-o... : "The human body at rest will produce about 250 BTU while sleeping, At idle about 400 BTU, doing light work about 650 BTU, and with heavy work up to 2400 BTU. That means 25 people dancing in a 2000 square foot home with temperatures of -10 degrees outside would never need a furnace. This is why bars and misc other places tend to have swings in temperatures more frequently." Electronics and lighting Incandescent and halogen lighting while dropping in usage is still huge. Computers or any other machinery. Insulation Not a heat source but slows down the heat transfer.