I have a usstove hotblast wood-coal furnace. It was ran in paraell and it heated, but the house was a little harder to heat this way. The heat ducts from the coal furnace went into the furnace plenum. The problem was when the coal furnace blowers kicked on, and the gas furnace kicked on, the blowers would cancel out each other. So I redid the installation and did series. Now the only blowers that push out heat from either is the gas furnace. The wood-coal furnace acts as one big heat exchanger, its a firebox surrounded by a jacket. Now the house is 10 times easier to heat, and we use less wood, or coal. No air can go backwards, and everything stays cool on the gas furnace. The wood furnace has a limit control on the back of it, and when the wood furnace hits 150 degrees, the limit tells the furnace board to kick on the blowers only. When the air gets down to 90 degrees, it kicks the blowers off. Its a really slick system. Our house is 150+ years old, 10 foot ceilings, 2500 square feet, and the house averages around 76 all winter long. We used 100 gallons of propane, because our stove, furnace, and dryer are propane. I had surgery so not all of the time was wood, or coal. Also the wood-coal furnace has a full cold air system for the whole house, so there is better circulation.