Paul, the best thing you can do is to stop right there. You obviously need to do more research and homework. 16 Ga.!?! are you kiddin' me!? Thats exhaust tubing for christ sake!! DD's aren't very efficient! That design is the standard for the best efficiency. I will even admit that and I'm a huge garn fan. Have you seen a DD or a garn or a seton/greenwood in person in action? My guess is no. Because if you did you wouldn't be typing. Grainger a manufacturer? To the best of my knowledge they are a industrial supply company. You are correct about the poorly insulated underground piping but that is about it! I got a ? for ya'. Which is more efficient? A chunk wood heater burning full out or a chunk wood heater cycling? I did my homework on wood boilers and could of built anyone I wanted to but chose the garn design. Here is the link to my home built unit so you can see that I'm not blowing smoke and do know what I'm talking about. https://www.hearth.com/econtent/index.php/forums/viewthread/11262/ I'm sure in your mind your unit is the best unit for the price, but you don't bring a knife to a gun fight.