Personally I'm not a fan of forced draft myself, been there done that. I don't know how Hy-C tested their furnaces, but something tells me the draft blower flap was probably open for testing. To close it and rivet it, if it wasn't tested that way would make it illegal I would think (for the company). I have a natural draft furnace, but there's provisions in the manual to adjust for extra air if needed. In the case of firecheif, by riveting the flap shut, they're assuming that all the drafts, wood load, moisture etc. remain equal across users, when its not true. If you load the furnace and it's at draft requirements and it's operated per manual, it should not be puffing smoke. I see that as an issue. Hopefully the company figures it out. As far as cracking, hearing from the face temps of the furnace and possibly glowing there's no doubt it will happen. The firebox should be fully lined and insulated so the furnace can come up to operating temps. With good seasoned wood, within 10 to 15 on a full load, our furnace becomes an inferno and it can be closed down. If the wood is unseasoned, it could take 45 minutes to an hour with little to no heat output.