Mines in series, the only air that goes through the woodfurnace if from the lp furnace. The limit from the woodfurnace is tied into the lp furnace. We have at least doubled the heat output doing the installation in series. Its just like having the furnace running. The forced draft lowered our burn times, but doubled our heat output. Keeps the house at a more stable temp, and burns very clean. I could have gotten away without sweeping my chimney last year. We burn soft lump coal sometimes, and when its very cold, be will burn hard coal. It works pretty good, but will test your patience at first. Takes at least 2 hours to get a good coal fire going. We are very happy with ours, we heat our house which is 2400 square feet, 10 foot ceilings up and down with ours at 76 to 78 all winter long. With the forced draft you can get more heat from less wood, better combustion. In 20 years I fixed the warped baffle, and replaced the firebrick, not so bad. We set our on temp at 140 on the limit, and the off around 80 so the air coming out will be cooler at times, but will run almost all of the time.