new to this. love your info. for our greenhouses, I'm considering 12" culverts for ducts. Is this problematic, or possible? I've considered maybe the diameter too large for heat to be efficient, the thickness in case heat cannot warm everything up enough, being corrugated shape for gases and their travel, being galvanized in case their is any bad chemical-type mixture, and finally, whether the ratio of sizes might throw the whole efficiency factor off, due to length or otherwise. I inherited these from a local construction job, and was thinking re-use could be good in this way. However I do not want to shoot myself in the foot before I even begin. If nothing else, I could scrap them for cash toward the proper material. Please help, I need to make fast decision, - in fact, today. Sorry... Absolutely love this place of yours, and have learned LOTS about EVERYTHING!!