Hello Lads,
Just wanted to share how I made a vented Propane furnace using a mini $20.00 BBQ from Canadian Tire. First off, gut it, like take everything out even the burner, but put aside, I then added a ban of sheet metal about 8 inches wide and the total length of the inside of the BBQ make it 2 inches longer so you can lap it and fasten it together, I bolted this metal following the shape of the BBQ 1 inch down leaving 7 inches sticking up. I added some heat shields to deflect the heat (but B4 you do this I advise putting the burner back in) I would stop each shield 2 inches from the end and have on a slight angle upwards to allow the heat to travel, I did this twice making the heat zig zag. Anyhow I cut a 3" hole at one end on the lid and bolted the lid to the ban of metal I used to make it sit higher, (it's a real pain reaching in and holding the nuts to bolt it together, but rivets will not work nor will TAP CONS) I then used HIGH Temp Silicon to get rid of any leaks. Now leave the bottom of the BBQ the way it is with the air vent holes, you can build a metal box around them and have a direct vent to the outside, but really it's not worth it. I then attached 3" metal chimney to the lid, when you cut your hole make tabs and bend them upwards so you have something to join your chimney too, I used tap cons as the area is small and their is no real pressure + High Temp Silicon. I have since added 5 inch duct pipe around the outside of the chimney pipe and I have a duct hole on the bottom of this pipe ontop of the BBQ cover and a intake vent at the top on the inside of my hut near the roof. I use a 12volt computer fan connected to a solar panel to take the hot air that sits at the top of my hut, push it into the 5 pipe covering the 3 inch chimney, the chimney gets hot, so this re-heats the existing air and pushes it over the top of the lid of the HOT BBQ spreading a nice warm air flow around the hut. Any questions email me as I have no idea how to post pics. Total Cost New mini BBQ $20.00 or used $5.00 I had 3 extra's Chimney Home Hardware $5.49 per section (36") I needed 3. High Temp Silicon $12.00 Bolts I had, Tap Cons I had, scrap tin from Duct work. I can heat my 10' X 8' X 6.5" high hut in about 8 mins.
The good thing too, these BBQ's have Low, Med, High settings.... Oh I used the same legs and bolted them to the floor, I ran out of washers so I used Beer caps to screw the legs down in place.
Cheers!