My DIY Outdoor Wood Furnace
October,  2003

Hello Garden Grapevine,

I researched all kinds of different boilers, and found that they cost a lot of money !  So I built an outdoor wood boiler this past summer, and I am having a really great time with it.  It heats my house so easy, and it will last about 12 to 14 hours between loadings.  I am a welder by trade and will tell you what I have learned.

The boiler stands 54" tall, 24" wide, and 43" from front to the back. The firebox inside is 35" tall, 17" wide, and 40" deep.  The firebox bottom is 6" from the actual bottom of the boiler. The water Jacket is 1/8” thick Mild Steel, and the firebox is 3/16” mild steel plate, which costs about 30 cents a pound up here in Maine. The base is made from 2 pieces of 6” I-Beam to act as skids.  I notched the ends, bent the bottom plate up, and re-welded it in place. 

I built this in about three weeks of spare time. I insulated it with Fiberglass; 6 inches thick around the sides, and 10-12 inches thick on top.  Then I built a frame around the outside of it, and put metal roofing all around it to make a small house-like looking thing. 

It is about 145 gallon capacity, and is not pressurized. I chose to go with not pressurized for the simple fact that if anything happens, it is a potential bomb.  Here is a diagram:

It heats unbelievably well !
I have almost no heat recovery time; it is almost instant.
However, I did have to buy an 80,000 BTU heat exchanger. It cost $155.00 and is 13”X18”x4 ½” thick. I have had nights drop below freezing, and I have to open a window now and then. I use a line thermostat to control the blower motor that blows air through the heat exchanger. It is the same kind out of a regular Miller furnace.  I also use a furnace gun, “Blower only,” to act as forced draft.  I have not had any issues with it overheating any metal inside the boiler;  It can’t get over 220 degrees.  The system is not pressurized, and the water keeps the temp down so it doesn’t warp.

A strap-on Aquastat controls the forced draft to the boiler.  It is set for 175 degrees, and kicks off at 180 degrees.  This maintains the water temperature inside the boiler.  It sometimes steams off a little when the temp outside gets warm, and the inside blower that heats the house isn’t cooling off the heat exchanger because it doesn’t need to run (to heat the warm house).  On days like this, it is a good idea to have some kind of a dump zone to expel the excess heat.

As far as an anti-freeze for the boiler; don’t bother.  Just use water.  It is cheaper, and easier to get up to temperature.  It is also not recommended by some of the brand name boilers on the market.  If there is a chance that it should freeze, then use the RV Antifreeze.  It is not toxic and available at any Marine or Boating place.  It usually costs around $2.50 to $4.00 a gallon.

I am heating about 1200 square feet, and it works better that I ever expected.  The key is the water jacket.  You have to have the heat transfer with no air in between to draw the heat away from what you are trying to heat.  The Radiators would work well as a heat Exchanger to get the heat from, but not to heat the water with.  They can’t get enough heat off any flat surface like the side of a stove. I also tried to use base board as a heat exchanger.  It doesn’t work well.  I had to buy the real deal and It was worth it.

I run my Taco 007 Circulating pump non-stop.  It uses only about as much electricity as an 80 watt light bulb. It also keeps the temperature down, and helps the boiler not to overheat.  Don’t build too big of a firebox, and don’t have too much water. You need a happy medium, or it will overheat, or not heat up quick enough.  Make the firebox large enough to last for 12 hours or longer.  Check out some of the other boilers on the internet.  One suggestion though, stay away from the barrel style.  They are over rated, and do not work as well as they say they do.

The above-ground  1/2" copper supply and return lines are about 60' long. I covered them with about 12" of sawdust,  and covered the sawdust with a sheet of 4mm plastic.  Then I covered the plastic with 12" of soil and loam.  The sawdust acts as an awesome insulator, as well as a cusion for the soil. The pipes are packed about 2 inches away from each other. The reason I chose sawdust is because they used to pack ice in it, and it would stay frozen all summer. 
Also about the smoke stack, I found out something recently; if you use a tall stack, you will burn a lot more wood, and get a much hotter fire. I had to shorten the stack because of it overheating the water, and causing it to boil. I had a 10 foot chimney. I now have a 3 foot chimney, and it works a lot better. The wood lasts for over 12 hours. The temperature is about 180 degrees.  The only downside is that the smoke is worse. If you do not have any issues with the smoke, then I would recommend the shorter chimney to try to conserve on the amount of wood you burn. [We at GardenGrapevine, assume that this all relates to draft production. A taller chimney tends to create more draft, which makes a hotter fire, which burns the combustion particles that produce visible "smoke". A Draft Indicator will measure the chimney "vacuum" in units of 1/10  inch of water column.]

You need the length of the chimney to get the draft. Every 90 degree elbow in the chimney creates a 5 foot loss of draft. What that means is if I have a chimney that is 7 feet tall, and one elbow. I actually have a 2 foot draft.  I loose 5 feet due to the elbow. Keep the chimney straight, and it will burn more efficiently. 

I also have some other information that you may find useful. If you want something to heat a hot tub, Central Boiler has a water-to-water Heat Exchanger for sale.  Take a look at it, and see if maybe you could build one.  I did it in about 2 hours.  If you have any questions, feel free to e-mail me.  I welcome your comments.

Thurlow Harper - email
Maine, USA