How to build an Outdoor Wood Boiler

If you want to heat your house with wood but don't want the mess of wood in the house the outside wood boiler is the way to go. This works with your furnace just like an air conditioner does. Instead of having a cold exchanger inside the plenum you have a heat exchanger. Now we will add a second thermostat in the house to control the temperature of the wood heat and leave the first thermostat to control the temperature of the main heat source, albeit gas or oil. You don't want your main source of heat (gas or oil) to heat the plenum, so that thermostat is set lower than the wood thermostat.

Now let’s start the process of building our boiler. The boiler fire box is the first thing to determine the size of. A good size is 2.5’ square by 4.5” deep. I like the door to be full size for throwing wood in. The steel for the box should be .250 thick, stainless is best but if you want to save money regular steel is ok. Now the reason this is called a wood boiler is because we are heating water, which is what runs through the heat exchanger inside your furnace. To heat water we must have a water jacket around the wood box. This jacket should hold approximately 150 gallons of water, which is enough to keep a nice reserve of hot water to heat the house even on the coldest of days. With my boiler I can even heat my 20 x 24’ workshop. You can have a water jacket on 3 sides of the boiler which makes for easier fabrication, the top and 2 sides. If you want to get fancy you can also put it on the back side; however, this makes it harder to build because you chimney is coming out of the back. The steel thickness should also be .250” thick; I built mine with .125” thick water jacket and had problems with leaks due to rust after 5 years. The chimney should be approximately 8” in diameter and come out of the fire box about three quarters of the way up in the back of the box. This way the heat doesn’t escape out the chimney without utilizing more of the heat created by the fire. If you had your chimney at the top all of the heat goes right out and doesn’t heat the water more efficiently.

Next article we will continue with the plumbing aspects.