I had a 750 HD installed this May. I was able to source 4 cords of seasoned wood along with 6 cords of greener wood, stacked in my woodshed for the 6 months since then. I hoped to get through the winter with only 5 cords total used, so I started out burning each load with 80% from the seasoned stack and 20% from the rest. I am very satisfied with the performance of the 750 on this mix. Then I noticed that my burn rate was probably going to have me run out of seasoned wood well before the end of winter and I switched over to about 2/3rds from the seasoned pile with the rest greener wood. Burn performance has gone down quite a bit based on peak reaction chamber temperatures (I was getting above 1500*F regularly, now occasionally getting above 1200*F). I think it would be nearly unusable if I were burning 100% from the greener stack. I will be splitting my wood a bit smaller to see if I can get back up to my 4:1 performance on this 2:1 mix. I would be very concerned about creosote buildup and needing far more wood than otherwise, if stuck with only unseasoned wood. Perhaps if split very small and managed carefully one can get away with greener wood, but I would agree with the dealer here.