DIY Outdoor Wood Furnace: Guide to Build One Yourself

Winter is on its way. But this time you don’t want to go for a bank-breaking gas furnace to enjoy the warmth.

Well..well, it’s an outdoor wood furnace that some people suggest. But is it warm (and safe) enough? 

The truth is, if you can keep the smoke and fire danger away, this is a smart way to stay warm without breaking the bank.

There are factory-made furnaces that come for a few thousand bucks. Moving into the more cost-effective zone, you can make your DIY outdoor wood furnace yourself. 

Either way, it might sound adventurous, but we all know making that happen it’s not as easy as it sounds. If you are not that determined you can hire an expensive handyman to do it for you. 

But if you want to learn how to do it yourself, well, my friend, we’ve crafted this entire post exactly for people like you.  This covers EVERY single thing you need to know and learn before you go for your own outdoor wood furnace this winter. 

Warning- it’s a 4000+ words post. And it’s full of technical details. So take enough time to read it thoroughly, or bookmark it so you can access it later. 

Let’s go- 

DIY Outdoor Furnace: The Basic Knowhow

First, an outdoor wood furnace is different from an outdoor wood boiler or an outdoor wood stove. Even Google itself provides confusing results among these three things. 

For the record, we are going to deal with a forced-air outdoor wood furnace only, not a boiler, not a stove.   

How Does An Outdoor Wood Furnace Work?

Outdoor wood furnace is driven by a method called ‘forced-air’ heat transfer. Air that’s heated by the heat of a wood-burnt fire, is forced through the ducting into your home’s ducting system. 

Based on the temperature level, this process keeps taking place in a repetitive manner. For example- it runs for 20 mins and shuts off for another 20 mins until the temperature is cold enough over again. 

To keep it running, you’ve to ensure a regular supply of firewood- the fuel. As an obvious consequence, there will be smoke, ashes, fumes, etc.  

The 5 major parts of this outdoor furnace are- 

1. Firebox. 
2. Hot air chamber. 
3. Chimney. 
4. Ducting.
5. Ducting fan.  

The firebox is made of tough steel, and it’s made leak-proof by flat slabs(welded). And the same goes for the rest of the ducting. Both supply air(hot) and return air(cool) are required not to be exposed to the environment by any means. 

Benefits of Using This Type of Outdoor Furnace

Simply put- there are a handful of perks of outdoor wood over indoor gas furnaces. We’ve put together a list of them all-  

• It runs on wood- the most practical form of renewable energy. 
• Consumes way less electricity compared to ones that burn oil/gas. 
• The overall(installation+fuel) cost is way cheaper and more available.
• Doesn’t bother your home with smoke, ash, dirt, and the overall fire risk(yes, it exists!). 
• Easier to maintain and troubleshoot than complex gas furnaces.  

Where to Install an Outdoor Wood Furnace

One of the basic questions happens to be regarding the place, position, and distance of the outdoor wood furnace. At this moment, we’ll be answering it right away. 

The best distancer to install an outdoor furnace is not more than 10 feet. As air will be directly traveling through the duct(instead of some underground, insulated path), the less distance it has to travel, the warmer it will be. 

But keeping the furnace close can bother you with the smoke drawn out through the chimney. Therefore, we’d advise making the chimney as high as it doesn’t get into your or your neighbor’s households anymore. 

Lastly, it’s about the alignment with the ducting. No matter if it’s through an underground garage window, or a ceiling port, make sure the path from the furnace opening to the house ducting is as short and straight as possible. 

What You Need To Know Before Making An Outdoor Furnace

What on earth does it mean? Well, by ‘preparing’ yourself for a DIY outdoor wood furnace, we mean to set the expectations right and ensure the required skill sets. 

 We are breaking them down below- 

Settings the Expectations Right 

• It’s not going to be that much energy efficient. At best you can touch the bar of 50-80% energy efficiency. That goes for ready-made outdoor wood furnaces as well. 
• Unlike indoor/gas furnaces, there will be smokes and fumes. Unless you don’t manage to route them well, it’s going to give your neighbors some tough time. 

Skills You Would Need

As long as you’re not willing to hire a professional to install your furnace, it will require a number of handy skills. In case you want to build your own wooden furnace, it’s even a bigger nut to crack. 

Keeping both kinds of works in mind, here is a list of the skill sets you’ll need through the process- 

• Welding Skill
• Electric Wiring Skill
• Metal Sheet Cutting Skill
• Trench cutting skills. 
• Ducting installation and insulating skill.
• Firewood cutting skill.   

Installing An Outdoor Wood Furnace: Both DIY One and Factory-made One

Furnace Type 1: Installing A Diy Outdoor Wood Furnace

Making your own furnace instead of purchasing a ready-made one might be easier than you think. But, starting from easy to hard, there are a number of ways of building a DIY outdoor wood furnace. 

For now, we’ll take you through one of the simplest processes of doing that. All it will look like is a steel box inside another. But thankfully, it serves the basic purposes of a furnace anyway. 

Let’s start with what you’d need for it- 

Things You Will Need

• ½ inches of steel sheet roll. 
• Sheet metal cutter. 
• Stainless steel Hinges.
• MIG/TIG welding machine. 
• Ducting(8”) and ducting accessories(collar, clamp etc).  
• 8” Duct Fan.

Steps of Making A Simple DIY Outdoor Furnace

Step 1: Cut the Boxes

We will need boxes to be prepared for structuring the furnace. We’ll talk about one after another. 

The outsider box

We would need three pieces of ½” steel for the bottom of the box. On the side, the bottom should be 1-2” bigger. The same goes for the back of the base. This extension will help you to make the seams. 

In case you’re not one, take help from a welder to get done with the seams. 

The size of the box will depend on your firebox. Whatever size the firebox has got, should be ******

The Insider Box(Firebox)

You’re right, this is where all the burning will happen. 

It can be a round box or a rectangular one, where both of them come with merits and demerits. Regardless of whichever you choose, you need to make sure that there is a space of 8-12 ” between the top of the outer box and the insider one because that’s where the air will be heated and pushed towards the ducting. 

About the size of the firebox, it’s recommended to be 20-24” in both length and width. In case it’s a round one, the same will go for the dimension of the diameter. 

Step 2: Make The Door

Based on whether the firebox is circular or square, the shape of the door will be determined. While you’re cutting the door hole, be careful enough to make it a smooth one. 

The cut-out portion from the door hole should be added up with a ½” steel sheet with at least 1” extension than the size of the door-cut sheet. 

As the door will be installed on hinges, you need to cut out two steel blocks for that one too. If you want, you can install the hinges on the door as well.  

Step 3: Create Holes on The Tops

On both of the insider and outsider boxes, you need to create holes to pass the air away.

The insider box will contain one home of 8” diameter, and the outsider box will contain two holes of 8” diameter each. 

Make sure the insider 8” hole is in perfect alignment with one on the outer box. This will drive the fumes and smokes away from the firebox. 

Step 4: Add the Legs On The Bottom

To support the furnace on the ground, it’s better to stand it on 4 metal legs. They can be added on each corner of the bottom, but with a fair distance of 1-3” from the edge. 

Once you’ve got the legs prepared, weld them up to the bottom by the help of an experienced welder(if you’re not one). 

Step 5: Weld Up The Seams

This is the step where you’ll need serious welding skills. The most common form of welding done in such cases is MIG welding or gas metal arc welding. You can also apply TIG(tungsten inert gas) welding here. 

No matter which way you go, make sure the seams are all welded up perfectly. 

Step 6: Do The Ductworks

Remember we created one 8” hole on the insider box and 2 similar holes on the outer ones? If you do, you might also recall that 1 hole from each of the boxes are in alignment.

Well, this line-up will take the fire smoke and fumes away. Add an 8” ducting from the insider box through the hole on the outer box. Make sure it’s leak-proof. 

Next on, add another 8” ducting on the second hole of the outer box. This will take the hot air to your home duct. To make it move the air better, add an in-line duct fan of 8” diameter to put the hole when needed. 

Step 7: Build the Wood Rack

To let the wood burn well inside the firebox, you need to build a rack for them. The rack will be 2” tall from the bottom of the firebox, and a little bit shorter than the width of it in order to make it a perfect fit. 

The 2” gap between the rack and the firebox will contain mostly the ashes from burnt wood. 

Furnace Type 2: Installing A Ready-made Outdoor Wood Furnace

These kinds of outdoor furnaces are easier, more efficient, healthy and practical. Although it would cost you significantly more than building a furnace from scratch. But it’s more reliable indeed. 

On the other side of the coin, they’d cost almost 3-10 times more than what you’d need to build your own wood furnace. 

Cutting the clutter, here is a good quality outdoor wood furnace that you might consider buying- 

• FIRE CHIEF FC1900 OUTDOOR WOOD BURNING FURNACE

Installing A Outdoor Wood Furnace

No matter whichever furnace we’ve got in hand, the process of installation is nearly similar. To keep things simple, we’ve generalized the basic steps of installing an outdoor wood furnace. 

Here we go- 

Things You Will Need

Almost most of the ready-made outdoor furnaces come up with all the settings accessories required. But there are a few more things that you have to arrange. 

Here is a full list of what the furnace comes with, and what else will you need- 

Making sure that you have the right model of the outdoor wood furnace along with all other accessories, here goes the installation process- 

Step 1: Find The Right Base and Distance

The outdoor furnace must be placed on a non-combustible base of about 4 feet x 8 feet in size. Preferably, it should be a concrete base instead of plain soil. 

Theoretically, the base should not be longer than 30 feet from the home. Nonetheless, it’s best if kept as close as 10 feet from the home, so that you can enjoy the maximum efficiency of the furnace. 

In case you’re going to install it further than 10 feet, you need to have a 6-8 inches Class A HT 2100 chimney pipe. 

Here are a few recommended ones- 

• Dura-tech 6” x 36” Stainless Steel Chimney Pipe
• Shasta Vent 6″ x 36″ Class A Chimney Pipe.
• Shasta Vent 8′ x 36″ Class A Chimney Pipe

Also, make sure that it’s connected to an electric circuit to power up the backup generator. 

Step 2: Install The Chimney

With the furnace, you should be receiving an anchor plate to host the chimney. Before anything else, make sure that the chimney you have purchased fits the anchor plate. 

Once you’re certain about the fit, use silicone to install the anchor plate on the top of the furnace. Make it a leak-proof joint. 

Now, we get to attach the chimney pipe to the anchor plate. For that, get a set of heavy gauge stainless steel wire and a guy wire bracket. Here are a few recommended ones for you- 

Once the essential tools are done collecting, install the chimney on the anchor of the furnace. However, it’s advised to use a Roof Guy Kit while you’re installing it. This will provide protection against wind and snowfall. 

Step 3: Dig A Trench for The Power Lines & Ducting

Now, we have to dig a trench from the furnace to the house which will contain the power lines and the ducting in it. 

The trench has to be below the frost line as it will be preventing heat loss of the pipe in this way. Mostly, the trench should be about 24 inches deep and 6-12 inches wide. 

To dig the trench, the most appropriate way is to use a towable backhoe. Because, if the distance is long enough, doing it manually with a shovel will be one hell of a task. 

Now, you need to put the power cables into the bottom part of the trench and cover it up with soil. While picking up the wire, make sure it’s rated for underground use. Usually, 14-3/14-2/12-2/ cables for ideal for this purpose. 

Here are some recommended underground wires- 

• Southwire 55213243 14/2 Cable
• Southwire 55213443 12/2 Cable
• Southwire 13056722 10/2 Wire

Once you’ve separated the power lines with a layer of soil, keep the rest of the trench to house the ducting in it. 

Step 4: Check for Local Regulations

As we’re done with placing the furnace up along with the wiring, it’s time to connect the dots. 

But before that, take some time to check whether the installation and positioning of the furnace had met the local requirements. In case you’re not certain, ask a local building inspector for codes related to the installation. 

Step 5: Power The Furnace Up

Now is the time to power the furnace up. The grounded power wires should be connected to a grounded 110V power outlet at your home. Done so, connect the thermostat and furnace power cord into the outlet on the other side of the wiring. 

You will have an ‘Electrical Control Kit’ shipped with the outdoor furnace itself, and there will be detailed guidelines on what to connect with which kind of power outlet. Stick to that for avoiding power manipulation. 

Step 6: Lit The First Fire Up

Once you’re done with the power works, build a small fire before connecting it to your home ducting. While making the first fire, don’t load the wood burner into its full capacity. 

Due to the protective paint or oil into your new furnace, the first fire will be smoky and will contain odor. Once it’s burnt off, the odors and initial smoke will escape. 

The initial firing also ensures a perfect cure to the metal. Once completed, you’re done to connect the furnace to the ducting. 

Step 7: Connect It To Your Ducting

If you’ve made it so far, it’s time to put the furnace in some real action. By that, we mean to attach the hot air and cold air duct to the furnace. 

And this is easier than you might think. 

Here are the substeps of the process- 

• Direct the openings of the ductwork to the place where it’ll connect the furnace. Preferably, it can be a small window in the basement. 
• Measure the distance from the furnace to the indoor duct, and get galvanized ducting of the same length. 
• Proper ducting is mostly about proper insulation. So, insulate your duct with a firm layer of duct tape. For further insulation, wrap the ducting with a ⅛” mylar roll.
• Now, lay down the ducting into the top part of the trench that we’ve dug in Step 3. Cover it up with soil. 
• Connect the ducts on both ends in a leak-proof manner. Tools like band fasteners and screws can be used. 
• Once the connections are done, seal the window through which the duct enters the house. You can use PVC-coated plywood for this purpose. Make sure the seal is completely weatherproof. 

Once the entire workload is taken care of, connect the starter collars of the furnace to the heat supply vent and the cold air return vent. 

Step 8: Get, Set, Go

The final step is about revisiting everything we’ve done before. By now, we hope that you’re done with the thermostat wiring and installing. Done so, check for any leak, crack or similar mistakes throughout the entire process. 

Once done, open the air registers, fuel up the furnace and turn it on! 

Steps of Building The Wood Store/Shed

No matter if you have installed a ready-to-install wood furnace or built one yourself, one thing you’d need in common is the firewood storage. 

Depending on how bad the winter is, the amount of firewood might vary from year to year. But usually, it’s a smart idea to store one year(winter) worth of firewood for your outdoor furnace. 

Now, where and how would you install it for maximum usability? Here goes the step- 

Step 1: Collect the Materials and Tools

First thing first, we’ve to collect the wood and other materials. 

For different positions, the size has to be different. But the kind that is best is “treated pine wood”. 

Here are the sizes of treated pine wood- 

• For the floor frame: 2”x6”x8’ treated pine. 
• For the floor, back and side slats: 1”x4”x”8’treatedd pine.
• For the floor frame, roof rafters and joists: 2”x4”x”8’ treated pine.
• For the posts: 4”x4”x”12’ treated pine.

Among other materials, these are compulsory- 

• Roof: OSB plywood of 4’x8’x1/2”.
• Screws: ¼”x5” hex screws, 3” deck screws, 1-½” deck screws.
• Liquid Roofing: 1 gallon.
• Deck bolts: 12” concrete deck screws 

Apart from them all, you’ll also need some tools and gear. Here goes the list- 

• Ladder. 
• Saw. 
• Level. 
• Measuring tape. 
• Triangle square. 
• Drill machine with ¼” driver. 
• Brush. 
• Protective gears. 

Wow, that’s a long list, right? But having them all is quite compulsory. Once you’re done, proceed to the next step- 

Step 2: Cut and Stain The Wood

We’ve provided sizes for a wood store that can contain about 1 cord of wood. In case you want to customize it, you’re welcome to do it anyway. 

As these woods will dry eventually, if not coated with protection, they might rot as well. Therefore, we’ll apply a stain on them. 

You can get along with oil-based stains available in the market. In the absence of those, you can use a natural stain as well. 

Here are a couple of the oil-based stains for woods- 

• Rust-Oleum Wolman F&P 14396
• DeckWise Semi-Transparent Wood Natural Finish

On the other hand, using a ball of steel wool soaked in apple cider vinegar will provide a rich, brown stain on the wood. 

Step 3: Build the Side Panels

The side panels are needed to be supported with cross supports that connect through pilot holes to the posts. On top of them, the slats are installed and built as the side supports. 

Connect the supports and side panels well with screws, adhesive, etc. 

Step 4: Set the Footings for The Walls and Base

The base is built on the footings/piers. Without installing these, building a strong structure is quite hard. 

Therefore, set the four corners footings and make sure the distances go well with the lengths of your pinewood slats. 

Once the piers are set, raise the walls on them. Also, set the floor right on the set of footings.  

Step 5: Add the Beams, Posts, and Joists

It’s time to strengthen the structure with the supports of joists, posts, and beams. We believe you have them ready to install by now. 

To fasten them, you’ve to use joist hangers, nails(16d), and deck screws. 

Step 6: Install The Roof

For this project, we’ve taken the roof as a plywood-made one. To provide them further strength, you can use rafters and connect them to the beams. 

Use screws along with a rubber gasket to strengthen the structure of the roof, as it’ll be the major layer of protection to the wood inside. If you want, you can also add a couple of diagonal braces to the posts.

Are Outdoor Wood Furnaces Worth the Money?

We’re sure you’ve thought of this question a few times before you actually decided to build an outdoor furnace. 

So, if you ask us again- the answer would be YES! It’s a sure shot regarding inexpensive ways to heat your home during winter. 

Don’t believe us? Well, here goes the proof- 

Cost of Outdoor Wood Furnace vs Indoor Gas Furnace

The costs of an outdoor wood furnace come in three parts- 

1. The constructional cost.
2. The fuel(wood) cost.

The Constructional Costs

The construction cost widely depends on how big of a furnace you’re building. If you’d like to stay within budget, it’d take about $700-1200 to set the whole thing up. In case you’d like to move towards a higher end of built quality, it can go as high as $1500-2000. 

If the furnace is ready-made, the construction cost will be a bit less because several items will be in the package. Other stuff might cost about $300-400 at best. 

However, there is another construction cost, and that’s for the woodshed. Based on the instructions we’ve provided before, the cost of that can be around $200-300.

The Fuel Cost

The fuel cost is, however, the only ongoing cost in this list. And it is quite dependent on the weather and the size of the home space you’re heating. On an average point of view, it takes about $1300-1500 of firewood to feed the furnace for an entire winter. 

Total Outdoor Wood Furnace Cost-

• Average cost of DIY Furnace Construction(w/ setup): $700-2000
• Average cost of Ready-made Furnace(w/ setup): $1500-2000
• Average cost of firewood store: $200-300
• Average Fuel Cost: $1300-1500/year

Let’s look at the other side of the coin now- 

Installing a good quality indoor furnace would cost around $3,000 – $8,000. This usually includes the setup costs, as the setup is usually done on behalf of the manufacturing. 

What about fuel? 

A house that is heated by a gas(propane) furnace would cost about $500-700 of propane in a month. 

• Average cost of indoor gas furnace(w/ setup): $3000-8000
• Average Fuel Cost: $2000-3000/year
• Average Utility Bill: $300-500/year

The verdict? 

An outdoor wood furnace is a clear winner regarding cost-effectiveness. 

Outdoor Wood Furnace vs Indoor Gas Furnace- What Matters?

As we have been through the steps on DIY outdoor wood furnaces, we are missing some of the perks that modern gas furnaces would bring in. 

In this section, we want to acknowledge the differences between these two and get prepared for what it 

Bottom Line

If you’ve made it this far, we know how stuffed you are with information. So, making the verdict no longer, we’d advise you to consume the whole guide section-by-section. 

And don’t forget the local building inspector to provide the essential green signal. 

Stay warm, within a budget!

Tim David

HVAC tech with over 30 years of experience. Retired and doing repair work on the side around Madison County, AL.