Home Built External Wood Furnace

Around 2003 I became interested in getting a wood furnace for supplemental heat to take advantage of lot of free wood from down trees on my property.    I had looked at many of the commercial furnaces on the market first with the thought of purchasing one and later from the aspect of designing and building one.

The  furnace needed to be located out side my home and be water coupled to my home heating system.   This was dictated by there being no easy access to a chimney in my basement nor good access for bringing in or storing wood.

My research covered at a wide variety of designs and customer reviews of different furnaces.  The majority of the furnaces on the market were of the box in box design.  Basically a steel inner firebox jacketed by water contained in an outer box.   There  were also furnaces with a firebox surrounded by a large solid thermal mass, typically stone and sand with water or coolant passages built into the thermal  mass.   A minority of the furnaces were of a high efficiency the “down draft” design with firebrick lined firebox and “gasifier” combustion system.

I also spent time looking at old books and articles, mostly from the early 1900’s that addressed wood boiler designs.  Here too, a variety of designs appeared.  Fire tube vs water tube boilers, updraft and downdraft and horizontal vs vertical tube boilers.  Guidelines were found in the old literature for sizing the air inlet, the flue area and the tube areas of the boiler.

Ultimately my design approach had to do with materials I could both find and or purchase at reasonable costs.  The payback on a purchased new furnace was what kept me from considering one as well as reliability concerns about corrosion and leaks in the firebox that could ultimately result in costly repairs.

There were several things I decided I wanted in the design.

  • Firebrick lined firebox to maximize temperatures to reduce smoke and improve efficiency of the burning
  • Larger thermal storage capacity so the furnace could run at full capacity for significant periods of time and provide heat of long periods of times with the furnace unattended.
  • Water coupling to my normal furnace, without antifreeze.
  • Large firebox and door for loading
  • An ash box below the firebox for easy ash removal
  • A fire tube design, to allow easy cleaning of the tubes (water outside – combustion products inside)
  • Ability to burn waste oil
  • A serviceable heat exchange – long term corrosion concern
  • A tall chimney for good draft

I purchased a used 500 gallon fuel tank to use as my thermal reservoir and used 2.25 inch ID water sprinkler pipe for the fire tubes, the balance of the furnace construction was new or salvage steel and new fire bricks.  Used temperature controllers were purchased on e-bay to control a new flue control motor and a new electric water circulation pump. I was also able to find used insulated stainless steel flue pipe sections to construct an approximately 19 foot chimney.

The furnace  construction  is much like a old fire tube locomotive boiler but with a fully firebrick lined firebox and a bridgewall.  24 tubes run through the 500 gallon water tank as a heat exchanger.  At the exit end of the fire tubes, there is plenum and access panel for inspection and cleaning of the fire tubes.  The tubes are sloped slightly towards the exit plenum and a drain is provided in the plenum bottom for the draining of condensation.

The water tank is insulated by about 12 inches of fiberglass insulation surrounded  by a steel enclosure.  The firebox is located in a separate steel enclosure.  A variable speed duct fan provides forced combustion air to the firebox.  The air inlet is near the top of the firebox enclosure so air is pre-heated by the heat from the outside walls of the firebox on its way to the air inlet at the bottom of the firebox above the ash pan/clean-out.

Furnace control is provided by shutting or opening the flue as a function of the water temperature.  Mechanical stops can be set to control either maximum or minimum flue area.  I generally set the upper limit water temperature in the 180 to 190 degree F region and shut the flue about 95% leaving a continuous slow burn.  The water temperature may continue to drift up limited by draws of hot water to heat the house or by boiling if there is insufficient heat draw from the house.

Control of heat to the house is made by the on or off control of hot water.  A temperature controller, in the return air duct of the forced air heating system, turns on the flow of hot water to a heat exchanger in the furnace plenum when house temperature drops below a preset value.  The furnace blower is set on continuous circulation, so the return air temperature gives a good indication of my average house temperature.

Below is a picture story of the construction with additional notes.  The furnace has been in use now for about 14 years!  I have measured the the approximate boiler horsepower of the furnace using the water capacity and it’s rate of temperature rise under a full burn – it is approximately 100 boiler horsepower or about 254 btu/hr.

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Welded firebox moved to concrete foundation pad.  Pad has 2 inch insulation around it.  Wires for power and control brought up through the bottom along with Pex tubing for water circulation to the house.  Firebox has 2 bolt on panels to service fire brick if need someday.
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Water tank and heat exchanger coupled to the firebox.  Only 2 of the 4 chimney  sections attached.  High temperature silicon sealant and mechanical slip joint seals heat exchanger header plate to the firebox.
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Middle legs of firebox are bolted to the concrete pad, outer legs have rollers to facilitate thermal expansion.
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Inside of water tank is accessible through a cover on the top.  Inside of tank is coated with an epoxy coating to prevent rust.  23 tubes are welded on and  run between two bolted on header plates.  Entire heat exchanger can be taken out of the tank if repair is needed someday.
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View through access panel of the tubes.
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Fire brick lining in firebox as viewed through door.  Opening at bottom for ash  drop-dow nand air inlet.  Corrugated cardboard was used to space bricks from walls during construction to provide a gap for expansion to avoid stressing the structure.
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Top  view of vaulted ceiling of firebox.  Welded steel I-beams help support the ceiling.  Bridge wall in back of firebox forces burning  gases up and then down into the tubes.  This provides more time for combustion before the gases contact the tubes and minimizes ash transfer into the tubes.  I later added a steel top plate to the top of the bridge wall as reinforcement and a 4 x 4 inch air tube to bring supplemental air to  the top of the bridge wall for improved combustion of exiting gases.
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Completed Furnace, the old propane tank is being used for an oil drip to burn waste oil.
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A view of the furnace 14 years later.  There’s snow on the furnace as it is not currently being used due to limited wood supply and the very cold weather.  The 19 ft chimney and support are visible in the photo.  I also now use a home built peristaltic pump to meter in waste oil when I have some to burn.

The above video was shoot one evening of a typical wood fire in firebox, with flames and burning gases dancing over the bridge wall. Originally the ceramic material, cast a few inches thick in the furnace door, was smooth,  but today numbers appear on it.  Thus, my “Hell Fire Furnace”!

There are a few things I would change from my experience with the furnace.

  • I built in an ash shaker rod in the bottom of the firebox.  This was both unneeded as ashes fell quite freely and the rod distorted from heat so it would not turn freely.
  • I would have added a side access to clean ash build up behind the bridge wall.  I have only needed cleaned this area once.
  • I would make the bridge wall 8 inches thick (2 -brick layers) versus the 4 inch I used and design in  reinforcement.  I needed to be very careful not to damage the wall trowing in logs.  A welded steel cap on top of the wall was my fix to the present design.
  • The access panel for cleaning the tubes has a bolt on cover.  I would hinge and seal this like a furnace door to make cleaning quicker.
  • I would fully  enclosed the area around the ash box to allow better control of the air supply and to provide the ability to fully seal the intake when the furnace is shut down or highly throttled.