We would like to install a freestanding gas stove in a sunroom for supplemental heat in the winter. The room itself is 12x13 and it is open to the rest of the house. The primary heat source is a gas furnace. We don't want to overwhelm the space with too much heat but that room is cooler in the winter than the rest of the house due to the windows. since it is open to the rest of the house it would like it to help keep the furnace from running as much. What size BTU stove would you recommend?