I have been reading the forum for a while now and I am interested in putting the most efficient wood burning furnace I can find in my garage. Has anyone every seen a secondary heat exchanger on a wood burner like they have on a high efficiency gas furnace that condenses the water vapor out and the last bit of heat then is expels the cool burnt gas out a PVC pipe. they of coarse have a drain hose to ground or sewer drain. If you have an insulated secondary combustion chamber that is completely burning all the fuel what would be the harm?