Greg - yes I'm very happy with my stoker, just trying to tweak a stove for the garage.
If I understand downdraft correctly, you are actually burning the coal at the bottom of the coal bed, and since the draft is coming from above, the "new" coal above the lower burning coals is slowly ignited, and the volatiles are forced through the hot coals below it.
I figured to line the firebox with firebrick, but I didn't think the heat exchanger would need lined. Definitely want air intake above the coal (but upstream according to draft direction) But maybe some adjustable air inlets in the ash pan door would be good.
If you look at the website regarding the Dunsley stoves, it might help explain it. (see page 3 of the following pdf)http://www.dunsleyheat.co.uk/images/yor ... 0stove.pdf
Maybe I'm all wrong on this concept - that's why I'm bouncing it off you all.
---While writing this post - 2 others submitted posts - guess I hit a nerve. I'll have to take time to digest the 2 posts above, but still enjoying the discussion. Not really dying to build another stove, would love to tweak the one I have, just getting ideas. Thanks