Usually the BTU rating is the input. Multiply the input BTU by the % efficiency to get output. 85,000 X .85 (85%efficiency)= 72,250 net BTU output. The 85,000 BTU stoker PROBABLY would be enough. Cannot tell precisely without an accurate heat loss calculation. Without an accurate heat loss, making a recommendation is nothing more than and educated guess. I'd still go with the double stoker. It, almost without question, could heat the house. If the single stoker works better for you, give it a try. If it doesn't have enough snot in the really cold weather, you can always supplement with the pro pain heat.