Sites like http://www.naturescomfortllc.org offer the best advice and the best suppliers of outdoor wood burning furnaces.  This article is a collection of tips from them to help you with your furnace purchase.

When buying an outdoor wood burning furnace, there are at least 7 crucial things you should examine. These are

Chimney length

If the furnace has a manual or electric damper, pipe length is crucial. In such furnaces, a long chimney is required to draw air over the fire if the fire is to burn continuously. Furnaces with forced air draft are a liability due to the extra chimney length that causes creosote.

Location of chimney

Far more creosote is present in furnaces with external chimneys than chimneys that go out the top. When unburned gases exit into the chimney, they cool off, then condense and finally form creosote. Outside chimneys have massive creosote buildup because they run much cooler.

Chimney caps

Some manufacturers do not supply chimney caps. These items are important as they keep out the rain when it is off season. In season, they are good for preventing wayward sparks from exiting. Caps such as the clover leaf designs perform all these duties while also preventing downdraft. Downdraft causes creosote and makes for a slower fire.

Furnace door insulation

The furnace door has to be well insulated to a minimum of 4-inches. Some furnaces only have 2-inches of insulation while others a very thin outer door with an inner water jacketed door. Since both these doors allow severe heat loss, it means burning more firewood to get the heat needed. Watch out for hollow doors that have little insulation and opt for thick doors with about 6-inches insulation.

Door anti-blowback catch

Does the door prevent flash burns by use of an anti-blowback safety catch? The catch slowly releases the door allowing gases to escape and in turn prevents flash burns. Furnaces without such a feature should be avoided.

Door gaskets type

The best doors are made from mild or cast steel. They also have a recessed lip around their outer edge. Inside the recess should be a silicone seal covered door gasket. The seal has a memory that forms an airtight seal every time the door is closed. Doors not covered by silicone seals become hard creosote and break down after some time. Leaky doors cause runaway fires and creosote.

Door construction

Down, up, out and in are the four directions that doors need to be adjustable. This allows them to compensate for any warpage. Take note that a door recess that does not have a silicone seal will not be able to stop air leaks even when the door is adjusted. Water jacketed doors have hoses that can leak and are more prone to corrosion. Double doors have a similar problem unless both doors are properly insulated. The best option is a single, lockable, well –insulated door with an anti-blowback safety catch.

Door size

The right door is big and should run from 20-inches to 30-inches in every direction. Lifting heavy logs into a firebox is challenging and low, small doors add to the problem. In case you still find it challenging to choose the right outdoor wood burning furnace, consult with professionals in the market.