It was the winter before last, the cold long one. That December I put the PH in (avatar) as in October I knew Oslo wasn't going to cut it. So, when all said and done, that Oslo was in that hearth for 5 years and consistently burned 5 cords per year. PH at the end of season, a little over 2. No supplemental heat and the house was warmer.
Impressive, especially if the wood species was the same and there were no other heat loss improvements done to the house.