When you burn anything, the products of combustion are produced. This includes soot, carbon dioxide and water. The exhaust stack on an indirect fired heater carries these out of the system so these undesirable substances do not enter your space. When you put a fired heater directly into your building and vent the products of combustion into your space, however, you are introducing that soot and carbon dioxide—as well as large amounts of water—into your space. Welcome to a dripping ceiling.