A hydronic furnace is an unpressurized boiler and the E-Classic 1400 furnace controller maintains a water temperature of 175°F to 185°F .  As far as I know, all high-efficiency EPA outdoor wood furnaces are "gasifier" furnaces.  This means that the wood in the firebox is converted to charcoal as the volatiles (ie, wood gas) "boil" out of the wood (i.e., gasify) and exit through the coal bed in the bottom of the furnace. You would see the "volatiles" in a fireplace as smoke and the smoke ignites when the fireplace becomes hot enough.  Essentially, burning smoke is the flame you see coming off the wood.  The orange glow of a flame comes from the glowing carbon in the smoke before it reacts with oxygen to form CO2.

Smoke is the hot fumes of wood.  If smoke condenses upon the interior of a cold chimney, it forms creosote, which is tarry substance.  It can self-ignite and cause a chimney fire if the chimney becomes hot enough.  Smoke emanating from a chimney is wasted fuel so do NOT let your wood smolder!

Although an EPA-certified furnace is much more expensive than a non-certified furnace, we chose that option for 3 reasons:

  1. Our time is valuable and, by having a more fuel-efficient furnace, we would spend less time cutting wood.
  2. Because we don't have a large woodlot, we wanted to minimize our wood consumption.
  3. Although not required by local regulations, our neighbours would be happier if we kept smoke emissions to a minimum.

After having operated the furnace for several years, we've found that getting an EPA furnace was a wise decision.  Even though the furnace burns very cleanly when operating properly, it can be quite smoky when it's not.  Seeing as the prevailing winds often blow towards the neighbours, I'm sure that they would be quite unhappy to be downwind of a conventional wood furnace.