Gravity feed pellet burner, requesting help with heat exchanger

[Burnit0017]

Greetings, I created a gravity feed pellet burner. The pellet burner works very well. It has a 8 hour unattended burn time. My challenge now is to recover as much heat from the pellet burner and transfer that heat into the building. My first attempt using a stainless elbow positioned over the flame to heat the air inside the vent has failed. Basically the project is to have a fire box outside the building and then try recover as heat as possible and transfer that heat to inside the building. I am looking for ideas that are low cost and easy to fabricate. I found some videos on youtube showing heat recovery systems. If anyone has any other ideas please let me know.
Best regards.

[nomedia="http://www.youtube.com/watch?v=nfoDyKCA3xs"]YouTube - Gravity Auto-feed pellet burner[/nomedia]

[nomedia="http://www.youtube.com/watch?v=HTUxarUvb2E"]YouTube - Pellet Burner with Gravity Feed and Heat Exchanger[/nomedia]