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 you should take 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 boiler
- The quantity of water that is recommended to use
- The burn chamber size
- What materials to use for building your unit
- The chimney location
In this post, we're gonna 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 boiler. That is probably the biggest mistake that most guys designing their own boiler make, namely, they forget that the circulation inside the 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 units make chambers that are much too large and some make them much too small, so I'm gonna give you an idea where to go on that.
What kind of material to build your unit out of? Most of you are using old large propane tanks or oil tanks and all sorts of material that you 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 you’ll find out in this blog post what Alternative Heating & Supplies experts recommend.
Wood Boiler Building Plans
How is The Water Circulating Inside of a Wood Boiler
The Water Supply
The first thing you should consider when building a wood boiler is a good water circulation, and this means you should know where to put the ports for supply and return. I made a drawing of a barrel and barrel style boiler and although some of you are gonna opt for different styles, you should know that the concept is very similar. This example represents the most common design that people use when they are building their own boilers.
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 that you need taken into consideration is the surface area that's gonna get the heat from the inside chamber out to the water source. There are several ways of doing that.
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 very 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 case, we have a design that I 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 Pump be Installed?
You don't want to want another pump circulating the boiler water. So what we do is we're gonna use the existing pump. The pump should go at the bottom, at the lowest port, because you want an atmospheric pressure on the back side of that pump.
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. So if the front it generates 15 pounds of pressure, it should have 15 on the back. That's the way the pumps are designed to run.
With us, we don't have that, we're not a pressurized system. We're open to the atmosphere, most of us. So, all this water weight on the back of the pump is gonna generate some pressure on to the back of the pump. That's why I like the pump on the back.
Also, pumps are air-cooled, so they're gonna run more efficiently when they're cool, and that's usually outside where the boiler is because you're usually using it for heating where you have a cold environment.
I hear a lot of people 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. It's a pump that's running 24/7 and it hums. That is why I recommend keeping it outside.
The Water Return
So the supply is going to the house to heat it, and then it comes back, and then what I like to see is the water returning to the front of the wood boiler.
Back to the water, if you just put it to here (see the picture above) the water is just gonna dump back in and go right back out, dump again, 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 wanna drag the water down around.
Now, people are saying that "Well, you should pull from the top because the hottest water is up there." Absolutely correct, but the water is now coming up and you're only pulling the hot water, so this water is never really heating up. It's also not giving you a good water temperature rating.
So if this water is 115 and this is 180 and is where you're pulling your water from, you have an opportunity now to make a condensing boiler, which is a boiler that's not hot enough, equally, and it starts to sweat which makes your boiler rot, and rust, and everything else.
Thus, you don't wanna pull from the top, you wanna 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.
And this are the things that you should be taking into consideration when designing your 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 companies out there, they have 400 or 500 gallons, which is ridiculous. I'll be covering that aspect further on, together with why you don't need so much water with your air circulation. So that'll make sense a little bit further into this.
I personally like and recommend low amounts of water. The most efficient things on the planet are low-volume. 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. So water is not the magic trick, recovery 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. Because if you think about it, if you're heating 900 gallons of water and not heating anything, how much energy is it gonna take to heat those 900 gallons? A lot of energy to just keep that water hot. But if you turn that 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, as little as possible, while having the recovery. And the only way you're gonna 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 kits that can help you with this process referring to doors, and solenoids, and aqua stats, to make the system work.
The doors that we sell are fan-induced, which will solve a lot of your water issues. So if you get a couple hundred gallons, you should have no problem having a good recovery because the doors that I sell will provide a good recovery rate.
The Burn Chamber Size
Burn chamber size depends on how large your house really 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.
So, people who have an average sized home, anywhere from 1,200 to 4,000 square feet, they usually need is 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, too. We have door kits that'll help you figure this all out, but if you wanna design your own door, you've gotta make sure you 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. I do not recommend it because it makes the unit smoke, and the fire really never gets up to temperature, you can't burn green wood, and you smoke out your neighbors, and it smells like you're burning garbage. It's because it's 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 gonna clear up the smoke or anything else.
So this is where I go back to how much water is in the unit because if you have a natural draft system, you're gonna need a lot more water as that fire doesn't get hot rapidly. So you're gonna need more water to buffer that fluctuation of recovery.
But if you're a fan-induced, like the doors kits that we sell, you're not gonna have to worry about that, because the fan is gonna blow in on that fire, making that fire hot fast.
You're also gonna 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, I'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 as much as humanly possible.
What Materials Should You Use for Building Your Unit?
Most people are using scraps or what they have lying around their yard or shop or whatever.
Mild steel vs stainless steel. I kind of sum it up real quick. I like mild steel. All the major manufacturers who are building boilers today, indoor boilers, are still building them out of mild steel and I like that.
Stainless steel is a great product to avoid rust and scale inside your water jackets. I get it, I understand it, but 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 gonna go from 400, 500 degrees to 1,600, to 2,000 degrees. I want the metal to be the same material so it shrinks and expands equally. That's why I like mild steel.
The Chimney Location
Some people like it coming out the top. They'll literally cut a hole here, drop the chimney down a little bit. I don't like that, personally, because that blocks off a lot of your burn chamber area.
I recommend it to be located at the top and goes out, but the concept works with the doors that I sell, that 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.
So the fan is gonna blow in, it's gonna 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, and then it goes out. This is the design that I recommend and like, personally. It's simple, it's easy, and it's not hard to make.
Steel versus Stainless Steel
I like stainless steel on 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 gonna attach right to it, and you don't 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 off the top of your boiler.
But if you put a stainless steel Class A chimney over that, that all goes away. You still wanna keep this nice and sealed with silicone to make sure there’s no moisture, because the crystal still attaches, but is a lot less, and remember, we don't want it to hit the base of the chimney.