I have FPX Hybrid large flush insert. Buddy of mine just got a new house and planning on installing FPX44 elite zc fireplace. I looked at the specs and ZC fireplace has 4.3 cu ft. but 76 000 btus per hour. My stove got 3 cu ft and 83 btu/hour. Also it has lower efficiency, 72% vs 80%. I'm wondering why. Do you have an answer? I