In this Alternative Heating & Supplies post, you’ll find out what are the best tips & tricks on how to build your own wood boiler and the areas of concern that should be taken into consideration when building your own outdoor or indoor wood boiler.

Learn how to avoid the usual mistakes that are connected with the following boiler parts:

  • The water circulation inside the wood boiler
  • The quantity of water that is recommended to use
  • The burn chamber size
  • What materials to use for building your wood boiler
  • The chimney location


In this post, we're going to be talking about the following wood boilers parts: the water circulation system, the ports from the supply and return going in and out of the house, and circulation inside the indoor wood boiler. That is probably the biggest mistake that most guys designing their own wood boiler make. Most people forget that the circulation inside the wood boiler is critical. How much water is in the unit is another issue you’ll learn how to solve. A lot of companies who manufacture outdoor wood boilers claim that you need 400 or 500 gallons, while others claim they only need 100.

Burn chamber size is another issue, as some people that choose to build their own wood boiler units make chambers that are much too large and some make them much too small, so Alternative Heating & Supplies will help to give you an idea for sizing your indoor or outdoor wood boiler.

What kind of material will be used to build your unit? Most people use large propane tanks or oil tanks and all sorts of material that you can find (or already have) and modify. So that, stainless steel versus mild steel is not really an issue.

Chimney location is a personal preference, but further into this post Alternative Heating & Supplies experts share their recommendation.

Wood Boiler Building Plans

How is The Water Circulating Inside of a Wood Boiler


Wood Boiler Water Circulation System

Wood Boiler Water Circulation System

The Water Supply

The first thing you should consider when building a wood boiler is proper water circulation, and this means you should know where to put the ports for supply and return. Below is a drawing of a barrel and barrel style boiler and although some of you are going to opt for different styles, the concept is very similar. This example represents the most common design that people use when they are building their own wood boilers.

Wood Boiler Barrel in Barrel Plan

Wood Boiler Barrel in Barrel Plan

So we have the inside barrel, which is kind of your burn chamber, and the outside barrel is where it goes around and where your water supply is situated. So, inside the burn chamber, one of the things should be taken into consideration is the surface area that's going to get the heat from the inside chamber out to the water source. There are several ways of doing this.

One is a round barrel that has more surface area where it’s ramping up, compared to a square where the heat only hits at the top. Another option is the cross tubes across the burn chamber or even fins on the outside of the chamber, like an air-cooling system on an old motorcycle, if you remember fins.

In this instance, we have a design that we recommend because it is using the existing pump, that pumps the water back and forth to the house, for the circulation system and the inside of the furnace.

Where Should the Circulator Pump be Installed?

You don't want another pump circulating the boiler water. So we recommend using the existing circulator pump. The pump should go at the bottom, at the lowest port, because you want an atmospheric pressure on the back side of the pump.

Circulator Pumps are designed to have an equal pressure on the supply side, as well on the return side or from the back to the front. If the front generates 15 pounds of pressure, it should have 15 on the back, the way the pumps are designed to run.

Since we do not have that, we do not have a pressurized system. Most people’s systems are open to the atmosphere. So, all this water weight on the back of the circulator pump is going to generate some pressure on to the back of the pump. This is why we like the pump on the back.

Also, circulator pumps are air-cooled, so they're going to run more efficiently when they're cool, and that's usually outside where the boiler is because you're using it for heating where you have a cold environment.

Many people say that they want to put the pumps inside the house, but when you do that you usually mount it to a wall, and it hums. A circulator pump will be running 24/7 and creating humming effect on the inside of your house. That is why we recommend keeping it outside.

The Water Return

Since the supply is going to the house to be heated, and then comes back, we like to see the water returning to the front of the wood boiler.


The Water Return System

The Water Return System

Back to the water, if you just put it to here (see the picture above) the water is just going to dump back in and go right back out, dump again, and go back out. You want it to go all the way to the front, go left and to the right, you want it to go down the barrel because the heat inside the barrel is going up. So you want to drag the water down and around.

Now, many people comment, "Well, you should pull from the top because the hottest water is up there." Correct, but the water is now coming up and you're only pulling the hot water, so this water is never really heating up. Moreover, it's also not giving you a good water temperature rating.

If this water temperature is 115 and this temperature is 180 and is where you're pulling your water from, you have an opportunity to make a condensing boiler. This is a boiler that's not hot enough and it starts to sweat which makes your boiler rot, and rust, and everything else.

Thusly, you do not want to pull from the top, you want to pull from the bottom to make sure you're getting a good circulation of the water within the boiler, and this solves the circulation problem.

These are some instances that should be taken into consideration when designing your wood boiler.

How Much Water is in the Unit?

This is, again, a designer's choice, especially for those of you who are choosing leftover material - like large propane tanks or round cylinders - cutting them and cropping them to use them as a wood boiler barrel.

But a lot of the name brand companies out there have 400 or 500 gallon tanks. We will be covering that aspect further on along with why you do not need so much water with your air circulation.

Alternative Heating & Supplies experts recommend low amounts of water. The most efficient things on the planet are low-volume by nature. Domestic hot water tanks with fast recovery are usually 30 to 40 gallons or the most energy-efficient thing are tankless hot water heaters. No water.

The most energy-efficient boilers, in-house boilers, have 7 or 8 gallons. Water is not the magic trick. Recovery on the other hand, is the magic trick.

How hot and how fast you can recover from the loss of heat from heating your home is what you really want. Think of it like this, if you're heating 900 gallons of water and not heating anything, how much energy is it going to take to heat those 900 gallons? It will require a lot of energy to just keep that water hot. If you reframe the prior and say, “How much energy does it take to heat 100 gallons?", the answer is “A lot less if you're not heating anything”. So, keep the water count quantity nice and low while having the recovery. The only way you're going to know that is trial and error which, unfortunately, is it not so easy.

DIY Wood Boiler Kits and Plans

Here, at Alternative Heating & Supplies, we sell wood boiler kits that can help you with this process referring to doors, and solenoids, and aqua stats, to make the system work.


Wood Boiler Door Kit

Wood Boiler Door Kit

The doors that we sell are fan-induced, which will solve a lot of your water issues. If you get a couple hundred gallons, you should have no problem having a good recovery because the doors at Alternative Heating & Supplies will provide an excellent recovery rate.

The Burn Chamber Size

Burn chamber size depends on how large your house is, what the burn chamber size is for and how much surface area you need to transfer the heat from the chamber to the water.

People who have an average sized home, anywhere from 1,200 to 4,000 square feet, usually require 3 to 4 feet of burn chamber depth. If you have around 3 or 4 feet in diameter resorting to a circle, so, 3 or 4 feet deep - 38 to 48 inches in width - it should be plenty of surface area that allows you wood storage, that is what gets you the burn times. Burn times are what the point of storing wood for 12 to 24 burns inside the unit.

The Air Flow

The air flow inside the burn chamber is really important as well. We have door kits that will help you figure this all out, but if you want to design your own door, you need to be sure to have good airflow inside your unit.

Some people bring the airflow from the back, some people bring in from the front, some use a natural draft system, which is where you just open up a flapper and let the air flow in. We do not recommend it because it makes the unit smoke, and the fire really never gets up to temperature, you cannot burn green wood, you smoke out your neighbors, and it smells like you're burning garbage. The reason is because you are burning a cold fire. It smokes a lot like when you're starting a bonfire for the first time, and until that fire gets hot, you're not going to clear up the smoke or anything else.

This is where we cycle back to how much water is in the unit because if you have a natural draft system, you're going to require a lot more water as that fire does not get hot rapidly. So you're going to need more water to buffer that fluctuation of recovery.

However, if you're fan-induced like the doors kits that we sell, you're not going have to worry about it, because the fan is going to blow on that fire, making that fires heat rise quickly.

You will also need to get a gasification burn, and what a gasification burn is when you're burning over 1,600 degrees, basically, no smoke comes out of the chimney. That's what you're trying to get to as fast as humanly possible. Yes, we've heard, "Well, you're blowing the heat right out of the unit." That's not necessarily true if you design it properly. You're burning the gases and if your chamber is completely submerged in water, the energy is going into the water.

What Materials Should You Use for Building Your Wood Boiler Unit?

Most people are utilize their scraps or what is lying around their yard or shop.

The age old comparison is mild steel vs stainless steel. To be candid, we prefer mild steel. All the major manufacturers who are building boilers today, indoor boilers and outdoor wood boilers, are still building them out of mild steel.

Stainless steel is a great product to avoid rust and scale inside your water jackets. The back draw is that stainless steel is made out of two or more different metals so when there's fluctuation of rapid temperatures inside and outside of that burn chamber, they're going to go from 400, 500 degrees to 1,600, to 2,000 degrees. Ideally, we want the metal to be the same material, so it shrinks and expands equally. This is the primary reason why we recommend mild steel.

The Chimney Location

The Chimney Location

The Chimney Location

Some people prefer the chimney location coming out of the top. They will cut a hole here and drop the chimney down a little bit. This can block off a lot of your burn chamber area. We recommend it be located at the top and goes out. This concept works great with the doors that Alternative Heating & Supplies sells. It blows in so that it has to go all the way through the chamber and back around, and then back out to the chimney. This is represented in the picture below.

Top Chimney Design

Top Chimney Design

The fan is going to blow in, it will help the fire, and the only way the smoke can get out is all the way back to the front, go back and through the chamber, then up into this baffle design which is now going out to the chimney. This is the design that we recommend and prefer. It's simple, it's easy, and it's not hard to make.

Steel versus Stainless Steel

We prefer stainless steel for this aspect. Now, if it's coming out the top here, you just make a short six-inch piece here, put a stainless-steel Class A chimney right over it, drill a couple screws in, and that’s it.

The problem is when the smoke comes out, the crystal attaches to the cold surface. If it's just a steel chimney, the steel is outside, the crystal is going to attach right to it, and you do not want that because it will build up, drop in, and cause all sorts of problems. Also, when it rains and there's moisture in the wood, it actually drips out the side of the chimney, attaches to the crystal, and becomes an acid. It then drops down and drips on to the base of this, which rots off the chimney and off the top of your wood boiler.

However, if you put a stainless-steel Class A chimney over that, all the prior goes away. You will still want to keep this nice and sealed with silicone to make sure there’s no moisture, because the crystal still attaches. Remember, we do not want it to hit the base of the chimney.

These are the basics of how to build your own indoor and outdoor wood boiler. Check out DIY KitsDoor Kit and if you have any questions, call us and we’ll be glad to assist you!