Hi Manny,
The exhaust is a 5' high, 7" piece of HVAC duct. Not super high, but higher than the grill itself. I've built a few cores (all have been pure cook stoves where the pot is placed at the top of the heat riser and used for maple syruping), but this is the first stove I've made with the riser enclosed by a barrel and this introduces variables with which I have no first hand experience, hence my questions for those of you that have some.
The core itself is made from the mix I've used successfully for several stoves and is mostly a mix of clay and ash and the dimensions are mostly good, although I did have some sagging of the riser as it was drying out.
I think most of the problems lie in the issues I've laid out above, and I'll work through them one at a time.
Finally, don't misunderstand, I'd love to have a heater in my home, but there are others that need to be convinced before it would happen. I haven't given up, but am also trying to be realistic, so the BBQ version is a way for me to use it more frequently than the 3 weeks of sap boilin' season.
Cheers!