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Alternative Fuels boiler system. (How to build one.)

Where do you fit in on Alternative Fuel heating systems.

  • I own factory made and it works good.

    Votes: 0 0.0%
  • I own factory made but it doesn't live up to it expectations.

    Votes: 0 0.0%
  • I built my own and it works good.

    Votes: 2 66.7%
  • I built my own and it's rather poor, but I want to make it better.

    Votes: 0 0.0%
  • I want to build or improve on one, but am not sure about the details.

    Votes: 0 0.0%
  • Could care less, I am just bored and will read anything Tcmtech posts.

    Votes: 1 33.3%

  • Total voters
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Most Helpful Member
This weeks bigger home heating personal project (new heat exchanger for the old house system) got me back into thinking about how I and many others use non conventional, or at least now less than conventional standard, methods (Natural gas, Propane, Fuel Oil, Electric) for heating our homes and workplaces for any number of reasons. Most often, cost of fuel being the primary one, even when we won't admit it because we want those 'ever so important pat on the back feel goods' for being able to claim we are environmentally friendly or whatever. Not me, I used to burn old tires and now burn used oil and am not afraid to admit it!

But really, just how effective are these alternative fuel heating methods and to what end does one pursue them in terms of personal time, effort and money plus inconvenience, exhaustion, frustration, injury, anger, rage, etc. we never admit to dealing with, way too often for it to not all also count for something just the same, before they become less than desirable in the long run? For me it's nearing 17 winters of it on my own as my primary heating method and very much in all of every point of contention listed and probably more.

BUT, and it's a big but! What is there gained for it all that make any daft fool ever want to try and use any form of Alternative Fuel (AF from here forward) heating method for any reason? Well, for me it always come down just a few simple things.

1. Cost avoidance.
When I started on my journey of AF heating it was as a kid growing up with cheap parents. We had a fireplace that burned wood and coal. Two of them actually.
The first one was a cheap POS that burned about half a cord of wood a week to heat a ~1200 Sq Ft doublewide house margionaly well and it was primarily my job to keep that wood supply in order. We typically burned wood that came from our family land plus any neighboring land along with free scrap lumber from tearing down old buildings people wanted to get rid of plus hauling in wood pallets and whatever scrap wood stuffs from town too. And like any right and proper cheap family we traded irrational almost shameful amounts of time, effort, and those other things mentioned earlier (a lot/huge amount of them), to save a few dollars on heating costs.

Starting out at the begining that first dismally efficient fireplace stayed with us for some years until a better one came around. Fortunately, the second better built and designed one burned 1/2 of the wood to heat the place better but it was still a load do of work anyway and, beyond the mass collecting and hauling home of the rough cut bulk of the wood it burned, keeping its fuel supply processed to size and up to stock was my job.
A job I came to hate simply for the utter lack of efficiency in it all. Burnable wood is pretty much free around here if you are willing to put any degree of effort into going and getting it and I always liked that aspect of AF heating. The rest of the physical work that came from the shear all encompassing inefficiency of it all from start to finish, not so much.

2. Labor.
Well for me, when I set out to build my AF system to heat my old house, once I had decided to put down permanent roots where I am now way back around 1999, That physical work part was going to get beat down into the most practical and efficient way I could find even if it killed me even though I didn't have a clue as to how to do it, yet.

To start back around 2001, I landed a job for a while at a small business that worked on commercial boilers and that where I learned how they worked for both the good and the bad and where I came to learn the secret to making AF more practical, HOT WATER BASED THERMAL MASS STORAGE!
The thing is, a normal conventional fireplace only puts out heat as long as it has a fire and it's never even or efficient. It starts out too hot then gradually tapers off until dead unless you constantly feed and fiddle with it.

Functional but inconvenient and I hate that premise to the core of my lazyest cell in my body. (and boy do I have a lot of lazy cells when it comes to burning wood as a source of primary heat for myself!) Plus, unlike my parents, being I have no kids to pass off all that labor and inconvenience off to, I have no choice but to deal with it on my own.

So that's where the boiler part of this comes in. Burn the wood at its most efficient, and workload wise, effective and convenient rate and method and store its thermal energy in water and from there draw that heat that off as needed over a far longer period of time. Now, when done properly, that single change in the main set of heat energy conversion and reuse processes reduces the overall time and workload, and even a substantial operating/material processing cost, investments by a factor of 10 or more and if done big, a lot more.

The reason being, as anyone who has ever owned a fireplace knows, A typical in house fireplace has to have all it's fuel made into small convenient sized pieces of which take the majority of the physical effort and time in processing any form of bulk raw wood product down into something useable. With the old fireplaces I grew up with every piece of wood had to be under 20" long and less than ~ 3" dia and turning a 60+ foot tree into pieces that are all under 20" long and ~3" dia is huge amount of work.

However, with a large AF boiler not so much. With a boiler if built right the upper end size requirement is pretty much whatever you can physically handle by whatever means you have available to you. For me as a 6' 3" 250# muscular farm boy, that upper size limit was any single piece that weighed under #100's in the case of my original first boiler design, 50" long by~12" dia! Not a lot of work to turn a tree or any type of source wood into pieces that fall under that size requirement! Huge 10X+ labor saver right there!

3. Convenience.
Burning solid fuels makes solid byproducts as in ash and unburnable secondary materials, like nails and fasteners that come with old lumber and pallets, which in a home fireplace that stuff is messy and a nuisance when it plugs up a fireplaces ash grating with half melted metal bits. Plus there is a limit to how much ash you can accumulate before it needs dealing with in some other equally messy marginally time consuming inconvenient way.

With a boiler not so much, being a boiler can be physically much bigger than a common home fireplace and located either outside or in some dedicated location where such messes can be easily dealt with in both larger volumes and far more convenient time frames. Dumping out a gallon bucket or two of ash and other stuff every day from a home fireplace and transporting it through your house is a messy nuisance. Whereas dumping a 3 - 5 cubic foot ash chamber into a dedicated ash handling system once a week is not and the whole mess is someplace where nobody cares!

4. Efficiency.
Now this one's a bit counter intuitive in some aspects being the efficiency is relative to a number of factors. By my views potentially giving up a bit of thermal energy transfer efficiency from the theoretically perfect combustion of whatever AF being used to whatever end point heat is gotten out of the system is a weighted measurement that can not be simply reduced to a theoretical percentage value of say, X units of energy was in a specific volume/mass of fuel and X - y percentage of it was lost before any productive heat came out of said system. And here's why.

Think about it for a bit. With an outdoor boiler you have reduced the physical workloads of both the wood gathering and processing plus system upkeep by a magnitude of order, if not far more. What's that worth in time and convenience plus other factors Vs the costs of said near free fuel? Is it worth more than say the heat value of the fuels being burned and if so how much? Is burning 5 - 15% more fuel that now takes you 10 - 20x less effort and work to process a real issue anymore and if so why? For me it's a no brainer.

And BTW, a properly designed AF boiler system can be considerably more energy efficient than a common home fireplace. I've built several for people now and they all say they cut their annual wood consumption rates down to 2/3 to 1/2 or less depending what old worn out heating systems they had been using while at the same time having reduced their overall workloads by a factor of 10+ and had whole houses and shops with comfortable uniform and stable heat throughout just like what their conventional heating systems in them produced.

5. Nerd factor.
I can't say that's not a high one on my list even if its the last major point! The thing is, given the advancements in technology we have now even a home built design can be made smart and largely self maintaining in its primary functions and operations, if a person wants to do so. And by adding some smart(ish) tech to a AF boiler in the form of basic digital logic and process loop controls plus using said control systems to fully integrate its operation in with any existing heating system to the point it can render the conventional fuel system to being a secondary or even tertiary backup is well worth it in the end.

I mean, hey. If you can make burning anything so simple that it can be reduced to -light a fire, push a button and walk away- why not? And when I say burn anything I do literally mean that with a properly built physical design of the boiler combustion system plus the use of a smart control system, you don't just have to burn the best of the best woods or coal or anything. I didn't! I burned everything in my old boiler. Good hardwoods, half rotten and low grade wood, old construction lumber, coal, railroad ties, garbage, plastics, tire chunks and eventually used oil of widely varying mystery mixes.

If it can hold a flame on its own it can be burned and not just burned, burned with reasonable cleanliness and efficiency at that!

So with that I plan to give a fairly in depth write up on how to build a AF boiler system based on my now 15+ years of personal hands on experimentation and development, that started out with burning old wood that was just a normal byproduct of my rural life, leading up to my now completely automated used oil fired mini boiler that heats my old house plus a 14' x 20' work shed on ~ 1000 - 1200 gallons of mystery mix used oils I collect for free from the local area.

I won't be building any actual boilers as this thread progresses but rather simply discussing the work that went into what I have done over the years and using what pictorial references to what I have designed and built, and rebuilt as I go.

I don't know how often I will update this thread but I will put time into it as I can, which being it's still the heart of winter for me here right now, I have a good deal of free time to play with.
Now to get into the basic meat of what such an endeavour as this involves just in the sheer upfront planning alone before the first tools beyond paper, pencil and a calculator gets picked up.

Something like this in not a small project so don't kid yourself on thinking it will be because I made it look and sound almost simple. It's what I call a 95/5 project. Yes, 95% of it is basic simple-ish concepts and fabrication work plus some hard labor (looks challenging but in a fun and interesting way and rather is) but that last 5% is what make or breaks it on the overall practicality and function of the whole concept.

That, and under the 95/5 rule, ~50% of your invested time, effort and money will complete ~95% of the project than the other 50% will get burned up in the last critical ~5%. So if you are considering building one of these think of it as a long term investment project where, if done right, it will last you 20+ years (even if you neglect and abuse it a bit) while at the same time carrying the potential for you to save 10's of thousands of dollars on heating fuel costs.

My point is, don't be cheap and lazy on building something that could save you the cost of buying a bunch of your new favorite toys over 20 years to save yourself some time and money at the begining. Believe me, every dollar and hour you save on being cheap and lazy up front will come back to cost you far more than that later and it will always do it at the worst possible time to.

Been there, done that and hated myself every second of it. FWIW, by the rough numbers of heating what I have, to the comfort levels l like for as long as I have, I have saved myself well over $30K on heating costs so far so that gives you some idea of the potential financial numbers involved on the long term view if you live in a climate like I do where heating of some kind is necessary for 5+ months a year.

Now with that, here's what you actually do need to put some serious thought into regarding the idea of building your own system and if it's even worth it and if you are even remotely capable of doing it to begin with.

1. How much do you spend on heating everything you do now or would plan to heat if you had a AF boiler? What does it add up to for say 5, 10, 15 or 20+ years? Thousands or 10's of thousands? If its the latter end this might be worth doing. If its former. Maybe not.

2. Do you have a known source of fuel that will likely to be accessible for foreseeable long term future? If so, good! If not, then rethink things again.

3. Can you add such a system to your existing property to begin with? These things take up space and make smoke and there is no hiding either plus the install is very much going to involve tearing up something major someplace on your property so do you have the ability to fix what you have to break to put the system in?

4. What's your metalworking skills? Got any? If not then surprisingly you're in luck (if you are needing an excuse to learn welding and baic metal fab work) being pretty much all of the metal work is very basic level welding and cutting stuff. You're going to need a welder, a good grinder, and preferably a plasma cutter or at least oxy/gas torch for the main work which can be rented if you don't want to buy.
The rest is basic layout stuff like tape measures, carpenters squares, markers and drill bits of assorted sizes, cardboard and tape. Really cardboard and tape because you are best off mocking the whole thing up in cardboard to check the shape and fitments of every single sheet of metal before you cut out the first one.

5. What sort of electronics skills do you have? You will need some to wire up the control system and the various sensors and power input and outputs it uses. None of it is high level stuff but there is a lot of detail work involving a good deal of critical wiring. Plus beyond that, the control units are digital so you will need to know some programing or be willing to learn some programing. I use Teco PLR units because they are cheap, rugged reliable and their software is free.

6. What's your skills with plumbing and fluid flow related mathematics? There's some basic math and calculating that will need to be done for the pump flow rate curves you will have to figure out for the given sizes and runs of pipe you will use and a good deal of other similar calculations that will be needing attention too. That is if you even know how where and how and why you will run your lines to begin with.

7. Do you know anything about basic thermal thermodynamics and heat load calculations theory? You're going to need to to do the necessary calculations to determine how big your boiler will need to be to given the run times you want between loadings for the given fuels you may use in the working environments and load demands you are going to be dealing with. It's not terribly hard math but it's absolutely critical to have it figured out well in advance of starting the build.

8. Source of materials. Got some and got a way to haul them? This type of project is going to take a fair amount of metal in the form of sheets and larger diameter pipes, so do you have access to them for reasonable prices plus a way to get them home and move them around once the boiler is built? A good sized solid fuel fired boiler that can heat a 1200+ square foot house for ~ 12 hours between reloading is likely going to weigh over 1000 pounds dry and empty. Same with fittings, pumps, lines, wiring and hardware of various forms. You will need to have a reliable source for all of that.

9. Got anyone reliable to help you? Although I have built and installed all of my boilers pretty much single handed on my own having a reliable buddy, or several, with some skills is going to be a huge help in things. Especially if they have skills and even tool and or material resources in an area required here you may not, yet.

10. Where you putting this thing? Rather important to think that one out in every imaginable detail you can. It's big, hot, dirty, makes noise, eats a lot, breath smoke and poops ash for its efforts and always will for the whole of its life, so it had better be someplace where none of that and everything even indirectly relatable to those effects will ever matter.
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Now if you haven't been chased off by all of that here's where things start.

How much heat do you need at any time now or for the foreseeable future in terms of base load?

What does that mean? Well. What does your present heating system put out in terms of BTU's and for how many hours in a 24 hour period does it run in both the normal and worse case scenarios?

Say for instance you are heating a house and you have a forced air furnace system that delivers 80,000 BTU output and in long term averages runs 6 hours a day and up to 9 hours a day in worst case known cold weather events.

80,000 x 6 = 480,000 BTU per typical day and 720,000 a day in worse conditions. (Say 500K and 750K for simplicity sake from here out.) That's your daily base BTU load you need to be aim for in the design.

How often to you want to reload your boiler using both the estimated typical type of fuel you will be using and with the likely worse fuel you may ever use for any length of time and how does that break down by the numbers?

(Typical BTU content of different woods found here, .)

Oak has the highest BTU per cord of any common wood at ~36.6 MBTU while Eastern Red Cedar is the lowest at 12.3 MBTU per cord.

Looks obvious enough that Oak would be the best choice by BTU content per volume right? But what about BTU content by mass?

36,600,000 / 4840 = 7438 BTU per pound. Vs 12,200,000 / 1913 = 6777 BTU's per pound, or a not so big 661 BTUs or roughly a 10% best to worse case difference based on weight.

So why does that matter by mass rather than volume? Well think about it for a bit? What's the easiest method of measurement you have for an unknown amount of wood? What a given piece weighs, right? (Weight of different woods.

Now that you have that rough number that most any mixed volume of wood is going to have a typical BTU content of around 7000 BTU's per pound you can start making realistic estimates to how much wood you need to be able to put in your boiler for any given runtime based on your anticipated typical heat load demands.

So say you want a typical run time of around 12 hours between loading events given your normal 500K BTU per 24 hour usage rate. No problem, sort of and by sort of is that even though you are looking at your mystery wood based on its rough weight in the end you still have to go go back to a final volume anyway.

So given you want a run time of roughly 12 hours between loading events with your mystery mix wood.

250,000 / 7000 - ~36 pounds of wood. Not bad at all! (in theory anyway.) In reality there is a bunch of other factors that come into play that will push that number up considerably. Overall system efficiency, idle time, wood dampness, is it rotten, what else is mixed in with it and so on which unfortunately will make burning any common undefined largely raw unconditioned woody material require 3 - 4x that weight per 12 hour cycle.

Now what does that mean in reasitic volume terms since how much wood you stuff in the boiler during each loading cycle is what matters in the long run? Well, depending on the wood and the fact you can't stuff it into a boiler burner box as a perfectly solid brick , you need to adjust those numbers for realistic free air space requirements which in realistic typical conditions may mean a as loaded density of maybe 1/4 to 1/3 of a cubic foot of wood per usable cubic foot of burner box volume.

36 #'s of wood may only take up 1 - 2 cubic feet of space but you need 2 - 3 times that for worst case efficiency losses making it now 3 - 6 cubic feet of which at best you can get 1/3 - 1/2 that as filled density multiplying those numbers again by another 2 - 3 times leading to a final volume of around 6 - 18 cubic feet of rough processed wood per loading event.

Which to be honest is about what I saw with my old boiler burning raw rough cut minimally processed mixed woods of various species, age and condition. Some high density hard woods stacked in well burned for near 24 hour run times while low grade rough stuff barely made it 8 hours and the worst of the worst less than that given the working dimension of my old boilers burner box was ~ 24" x 24" x 50" or about 17 cubic feet.

So given the theoretical base loads of 500K nominal to 750K peak BTU per 24 hour period ,using best and worst grades of wood and typical and worst case heat loading demands, you would see the need to load it at best once a day or at worse every ~ 6 hours, if it was built around a burner box with a ~ 17 cubic foot working volume as mine was.

For pictorial reference this was my old boiler when it was in the process of being installed way back in 2001!

The burner box with some pallets in it. On a typical day of burning mixed pallets I would go through about 10 - 15 of them given their widely varying wood types and construction designs. Shipping pallets are mostly made of widely mixed varieties of woods and their 'fluffiness' (large volume low mass) makes them less favorable to large tree stock, utility poles and railroad ties. They look big but with the majority of them there is less than a half a cubic foot of wood in one and a lot of nails, staples and cleats to deal with on top of it.

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Where the thermal mass concepts comes into play.

Now that you have your theoretical fuel volumes/masses to anticipated run times figured out here comes the next big, but faily simple to understand once broken down into its component parts, part that make its all work smoothly.

Thermal mass based energy storage, as in making the boiler work as a big liquid based rechargeable heat battery to take the peaks and lows plus extend dead fire systems run time of the fires thermal output and make it into a more stable and far slower changing input/output value set in the overall grand set of equations.

The problem is, when you burn a large volume of solid fuel it puts out a lot of heat energy while burning but, when its not needed at that rate it needs to be shut down yet not snuffed out and killed just the same.

It's rather a process of an full on 100% burn rate followed by as little burn rate as possible (but making sure you don't kill the fire either) output you are working with that needs to be smoothed out into a more stable even range of temperatures in the fluid that the actual heat transport and transfer components in the rest of the system can work with, within reason.

Also beyond being the temperature output stabilizer, it's also the thermal battery that stores usable heat energy well past when the fire has gone out which in practical operation is a sort of -more the better- up to what ever practical limit you can work with in the actual physical construction of the boiler.

My rough rule was to try and have at least 3 - 4 hours of usable heat energy stored in the boiler from it's maximum set temperature (I prefer to run mine at ~ 190 - 200 F top end) to it's minimal cut off point which is around 100 F and where most liquid to air heat exchangers transfer efficiency drops below being practical to work with.

So given our theoretical house uses 500K BTU per 24 hours its average thermal load is about 21,000 BTU per hour, plus the inconvenient reality that hot water based heating systems have a in inherent nominal heat loss of their own associated with them,themselves from such things as that even when the fire is out they still have some moderate thermal losses associated with the idle air draft moving through them plus whatever heat is leaking out through anywhere else.

Given those losses I typically added an extra 10 KBTU per hour loss rate for the rest of the system into the equations. If insulated well, yours won't be anywhere near that high, but if not, then you could easily see 3 - 4X or more which illustrates the necessity for doing your build correctly the first time.

Now that's our baseline thermal mass number of ~ 31,000 BTU per hour for say 4 hours or about 125,000* BTU hours of thermal energy storage required.*(Rounded up for convenience in the numbers.)

So, how much water is that going to take? Well it's more than just water that's hot. It's also the mass of the steel the boiler is made from too and if you built it like I would, it likely has near 1000 pounds of hot metal in play beyond whatever volume of water it has inside it.

So here's how those numbers break down.

Water has a specific heat value of 1 and steel has a specific heat value of ~.1, or 1/10th that of water. It matters but not a huge amount, yet it does if you are running an antifreeze mix of say 35 - 40% in your system, being it will balance out the reduced specific heat value of the water/antifreeze mix that the anti-freeze takes out.

For simplicity I am going to do the numbers as if clean tap water is being used and the steel is not being factored as well which falls close enough to those of what a 35 - 40% mix of water and antifreeze plus the steel also happen to workout to.

**broken link removed**

Now for the actual numbers and why.

A 1 degree Fahrenheit change of one pound of water is one BTU and you need to come up with a mass/volume that holds at least 125,000 of them in a temperature swing of 90 - 100F.

125,000 / 90 = ~1400 pounds of water. A gallon of water weighs 8.34 pound and a cubic foot weighs 62.4 pounds, so at minimum your boilers water jacket fluid capacity needs to hold at least,

1400/8.34 (62.4) = ~168 gallons (~22.5 cubic feet) of water

and, as I stated earlier, a bit more is never a bad thing, so when you get to dimensioning your boiler you're going to be shooting for a usable internal volume that is someplace over those numbers.
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Hi TCM woww thats a very comprehensive setup and figures to back it up, I seem to remember reading about your original. Well sad to say I have no metal bashing facilities at all and no source of stock so I have to make do with what I have plus my electronic skills. I do have a small woodburner (grate 1ft x 1ft) that has no back-boiler :( but in any case I run it evenings only as it's situated in the lounge. Again wood here was easy to get but is becoming harder as landowners see value in it as many people are now installing burners! Unlike you I have a small woodless garden so most what I burn is roadkill and scrap.
My serious whole house heating is done by an ancient oil furnace, we use water heated radiators in the uk rather than warm air, that does me first thing in the morning that can be as early as 5am due to shifts, nobody wants to light a fire then unless you can bank it up overnight.
Solar makes a major contribution by using electric panels (1Kwe) to heat the water in the hot-water storage tank fitted to my house via its immersion heater, this means I do not have to burn oil at all in the summer months and saves some oil in the winter by reducing what the furnace has to do. Often the tank get's as hot as is safe (around 65degC/150degF) and so the power diverts to a grid tie inverter to power the house. To save wear and tear on my equipment I do not export more than a few watts of power to the grid, a control system matching production to in-house consumption.
So very different to your set-up but what I have been able to acheive alternative energy wise given the resources available to me. I am also very cost concious, in my view there is no point building anything that cannot pay for itself in a few years, hence no heat pumps or other capitally expensive stuff with dubious returns around here.
Now if I lived in a place like yours instead of this small overcrowded country (uk) wowwww.................

My intent here is to walk people through the reality of using the now less conventional fuels along with reviewing the necessary technical details of making them more practical and user friendly to where anyone who is willing to put forth the efforts could conceivably integrate their use into their lives to where using such fuels in day to day normal operation could be done with minimal effort and maximum user comfort and convenience.

That, and to show the realistic costs plus the theoretical long term savings they could get for it as well, if they live in area like I do where home and related heating is very much a unavoidable higher cost of living outlay in life year in and year out.

I can't count how many people I have talked with who bad mouth burning largely free alternative/renewable fuels simply because they only see the worst case scenario effort and cost aspects of them. Sure, anyone can make anything unnecessarily time, labor and cost intensive if they so choose to, but that does not set the standard and reality of the whole of the concept in how it can work when done right.

Then there's the other aspect of those who would possibly be willing to give it a go, but have no clue what needs to be taken into consideration and to what options or ways they could go about doing it to begin with, which is what I plan to break down into the various smaller details so that most anyone can work up some sort of educated idea of what they would be needing to do to see if it's conceivably viable for them, or not.

I've always been a proponent of the DIY movement and felt that anything anyone else can do I can learn to do as well, and if I so see it justifiable, learn to do it better, faster and cheaper than they do it for too!

I'm not sure how long of write up this thread will end up being but this is only the first part of it.
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Things to consider in fabricating your boiler.

So now that you're half tempted to build your own AF boiler what are you going to need for materials?


Yep. really, cardboard and tape because you're going to want to mock the whole thing up before you ever touch a piece of metal and the reason being is, you need to know exactly how you're going to assemble everything so that you can weld it together and not hit any dead ends with your welds on either side of every seam. This thing needs to hold water for many years to come so doing double sided welds is one simple way to accomplish that.

So how do you know you're assembling it correctly? Well, if you can butt two joints up and tag each side with a marker all the way along and not have a clearance or interference issues with the fat marker along the way you're doing good.
You have to run that welder stinger or MIG gun along those joints later so if you can't get at them with the fat marker you can't get at them with welder either and if not, then you need to change your assembly approach, and if needed, break the overall design down into individual subsections that will go together as larger assemblies rather than piece by piece.

How you approach and finalize your assembly puzzle is up to you but I always start with my layouts on the bottom piece and work my way up and out from the firebox ending with the top.

This is my assembly order preference, Plumbing Fittings on sheets first --> Base plate + Front + Firebox + Flues + Sides + Internal side gussets and bracing + Back plate + Internal back gussets and bracing + Top + External door and cleanout extensions + Doors and cleanout covers that go over them. If you do your assembly right the top sheet or sheets should be the only parts that will not have full welds on both sides.

Obviously, your cardboard boiler doesn't need the plumbing fittings added to it but it does need to have every place one goes marked out and double checked for clarence's inside and out so that any and all external components like valves and pumps have adequate clearance for installation and service in the future.
Few things are more inconvenient than getting the whole boiler built only to find out you have two fittings too close together, or too close to something else, for whatever valve and or pumps attach to them to correctly attach and line up with whatever they need to. That or on setup find a critical one or more totally missing!

Sourcing the components.

This parts surprisingly simple being the vast majority of the components are going to be easily sourced from most home building supply centers and any local steel yard, metal fab shop or commercial manufacturing center. The only key is if you have never went looking for such items and where you will need to go to get them.

I for one get all of my steel sheeting from a buddy of mine that has a large scrap metal yard that has salvage contracts with various manufacturing facilities. Granted what I get is leftovers and pieces that were damaged in some way during the manufacturing places fabrication processes, but they are still new clean steel, just odd sized.
Also being scrap it's as cheap as it comes. I trade labor on fixing his various scrap yard equipment for materials but as a walk in and buy sort of deal anyone could get what I get for around 35 - 40 cents a pound or about 1/4 to 1/5 or less what new off the shelf would cost me locally.

Given the main component of the boiler is the steel sheet cut up into various pieces you're going to need to find a source and preferably one that's cheap and easy to work with.

Most good steel suppliers and metal fab shops can sheer all your sheet for you, for a price, if you give them a list of what you need. Granted, it will likely cost you a hour or two of shop time (typically $75 - $125 and hour) to get all the pieces cut to spec but in return everything will have neat clean straight and square edges which will greatly reduce your assembly time.
That also emphasizes the need to do the full mockup in cardboard ahead of time so that you know exactly what pieces you will need in order to best utilize every square inch of sheet you do have to purchase.

Material selection wise is up to you but my personal experience is that most any common steel will work just fine and last for many years even with less than ideal treatment. My very first boiler was made of 1/8" mild steel sheet and only ever ran untreated tap water yet it made it ~14 years before it started to rust through on the bottom, but once it did it went rather fast.

Now for new boilers I use at minimum 3/16" steel sheet (I really recommend 1/4" though or at least for the base and firebox) and add the primer paint or epoxy coating to things as it's being prepped for assembly.

Downside is that new steel is always greasy and gritty so it needs a good scrubbing and prep job first. It's not hard but it's time consuming given you need to wash and scrub it real good to get the oils and grease off.

Being I am cheap and I have done metal working for years I have always used the old school poor man's cleaning method as listed here but you can use whatever method you prefer just so long as you get it clean enough for the paint/epoxy to bond to.

Cleaning and prep work.

Now that you have your steel sheets all cut into the shapes you need and at the place you plan to build your boiler at, it's time to start preparing them for assembly which starts with cleaning everything given you're going to want to paint/seal it for long term durability (internal rust and corrosion protection) and to plug up any pinholes in the welds you may have.

Several basic but important steps are involved.

Grind and debur any sharp edges. If you don't you'll regret it when you go to scrub everything because those edges always have sharp burrs and slivers just waiting to draw blood and I can assure you that the oily residues on new steel can be quite an irritant in a cut as well.

2. Wash everything you will need to weld and or paint with soapy water. I typically use common dish soap being it's dirt cheap and does a great job with most any oily stuff. What I do is put it on raw and go at everything with a stiff bristle scrub brush until everything has been worked up fairly well then give it a good warm water rinse while scrubbing it down more.

If you want to know if your metal is clean just watch how the water flows over the sheets being it's pretty easy to tell what's clean and what not just by how the water beads up or flows over things. If your not sure what you're looking for just clean one half of one side of a sheet and not the other and have a look at how the water behaves on each side.

3. Wipe down everything with rag soaked in Carburetor Cleaner or a similar solvent type cleaner and see how well your soap and water job did. If they come off with minimal residue the soapy water treatment did what it needed to do. If not repeat step 2.

4. Primer or epoxy the steel. I used to use Rustoleum Engine Primer for this sort of work but unfortunately they changed their formula and now the last boiler I used it on had some issues with chemical compatibility between it and the antifreeze solution I run in it.
Instead of doing what its supposed to/used do and make nice solid protective layer, it turned into a paint jelly of sorts that eventually turned my antifreeze primer brown. (Thanks enviro-nazi tree huggers, you ruined one more thing that was good.)
Because of that I now recommend using two part epoxy paints even if they are going cost you 2 - 3x as much and be more labor intensive on their prep work and application.

5. Apply your paint/Epoxy to every sheet but leave a 1 - 2" space along the edges and anywhere else you need to weld something to. Once the welding is done then clean those bare areas and the welds with the soap and water treatment again while watching out for weld splatter. Some of those tiny splatter beads can be razor sharp so take the time to grind or scrape as much of that stuff off as you can before you get back to scrubbing and painting.

If you did the layout and prepping stages correctly you should now have all your steel sheets, and any other parts, all prepped and ready to start welding them together next.
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Welding and assembly.

What can I say here about welding that isn't likely covered better by countless other sources of information anyone can find online? Not much really, so given that I will say that if you don't already know how to weld with some degree of basic proficiency have a good look around at what there is out there that is already published on the subject.

That said, I can however give you some tips on how I prefer to assemble things being I do tend to try and find the fastest simplest easiest way possible that will still give me a solid long lasting result!

So given that, here is a basic list of the common types of welds and the correct terminology that goes with understand what is what.


The vast majority of the welds you will be doing will be the Corner Joint types with an internal side secondary weld or the Seam welds. Now as some of you who are good at welding may say, the inner second weld is not really needed and I would agree since a boiler of this design does not need to hold pressure or require any high degree of structural reinforcement on the welds themselves.

However as some of you may know when it comes to welds being exposed to water for a long period of time they tend to be the point wants to rust through first due to slightly different metallurgical properties of the welding material alloys involved creating weak but very persistent galvanic corrosion degradation issues at them.

I've done loads of repair work welding in my life and weld rust out in water tanks is a common thing I have seen so by my views that second inside weld is just buying you more time in the future. I didn't do the inner side welds or paint liner on my first boiler that ran raw water all it's life and that's where it rusted through. The primary sheet steel is still good but the welds all around the base are what started to corrode out of it.

That, and unless you do professional welding work for a living the odds are you wont run every weld perfectly and have zero pinholes in anything either. ( I always have pinholes)
Granted the inner primer /epoxy paint coating will fix small pinholes that would just seap until they eventually rusted themselves shut but for larger ones the paint/epoxy is not a reliable plug in my views.
Especially if you are going to run antifreeze in your system and need to buy 100+ gallons of it at $8+ a gallon.

Assembly tips.

The bulk of the boiler body design is pretty simple so there is not a huge amount of difficult technical structural concerns to deal with other than trying to balance out the pressure stresses on the larger sheet surfaces so that nothing has too much surface area without some sort of reinforcement that counters its natural want to bow and flex and thats where the various small gussets and bracing that goes on the inside come in.

The main points of stress are the two large side sheets and the three sides of the burner box that are exposed to water. Although this design is an open system that can not build pressure it still has to deal with the raw mass of the water it holds.

At ~.43 PSI per foot of liquid depth in the boiler those large sheets are under a surprisingly large amount of stress. Especially the closer to the bottom you get!

For a larger boiler like the one I built for my brother a few years ago that had side sheets that are ~ 48" x 60" that water pressure has a cumulative force of ~ 2500+ pounds pushing out on them at all times in an uneven force distribution from top to bottom. Same with the burner box that has similar combined forces pushing in on it.

Because of the need to balance out those forces several bracing gussets need to be put between them so that the forces on each cancel each other out and reduce the overall forces being placed on anyone area of any larger sheet. For that I just use leftover cuttings form the sheets to make bridges between the two affected points and set them in to have one for about every 2 - 3 square feet for the bottom half of each sheet and one for every 4 - 5 square feet near the top.
How many you use is ultimately up to you but they are one of those small details were a few extra really won't make anything worse.

here's one example of how I laid the bracing out for one early boiler design of mine.

The grid of dots on a ~ 12" spacing are the bracing rods that tie the inner firebox and flue assembly to the outside body sheet so that their shared forces are better distributed. The design worked well but it was a waste of time given all the holes I had to drill because I forgot to add the gussets while the top was still off. (A large reason for that plan ahead and mock everything up with cardboard before assembling step I mentioned.)

Had I not overlooked that critical part I could have easily added larger internal gussets that tied in better to key points like the top sheet of the fire box and flues as to where the top two rows should line up with but don't. (Close enough to work but not perfect in my view.)

Beyond those outer tie ins the top of the fire box and the top and bottom sheets of the flues need some sort of cross bracing to help keep them from flexing inward as well. For that I use the same leftover sheet material cut into strips that go across them in 4 - 5 places on the top of the firebox and 3 - 4 on the top and bottom of the lower flue passage and 2 - 3 on the top one. Optionally you can also run them down the sides and back of the fire box as well. Dimensionally wise 1.5" high is likely more than sufficient.

Overall layout wise you can go any number of ways you wish and either have a flat top design with an external surge tank like my old boiler and mini boilers use or a pitched one like this one has that keeps all of its water self contained. The main thing is to factor in where you are going to be placing it.

Pipe fitting ports.

Next is figuring out what all points of line connections you will need and you are going to need a bunch.

Every pump circuit should have its own supply and return ports plus you will want to add multiple large diameter (2" preferably) clean out ports at bottom of both the front and back in the corners for the inevitable day that you will end up draining the systems to either change out the antifreeze mix or do some sort of other work to it. With those you have the ability to get into it with a hose and flush all the crust out of the bottom.

Beyond that you will want a smaller drain port you can attach too plus you will need ports for the temperature and water level sensors as well. And I also highly recommend a front top center port where you can install a mechanical temperature gauge as well. And above all you will need ports on the top that either connect it to your surge tank or that will be the vent port so it can breath or vent off if it ever gets run so hot it starts to boil. For those I recommend larger diameters (1.5" - 2") be used as well.



I forgot to add the clean out ports and front top center temperature gauge ports to this one before the pictures were taken as well. (Seriously, mock yours up in cardboard first and then stare at it for hours while thinking about what all you need or may want to add to or have do to it someday later before you start assembling it.)

The flues.

Simple is better designs here too.

Just come up from the back of the fire box and then forward then up and then back and then out.

You can make them from larger diameter pipe (3" - 5") or as I have found and really recommend , make them an open box type simply for the ease of cleaning them out.
The open box type wont drastically reduce your overall heat transfer efficiency but it will make the necessary clean out loads easier since you can just reach in with a garden hoe, rake or flat shovel and scrape them clean. Pipes are far harder to clean out.

Size wise for the box type flues I do not recommend making them any smaller than 3 inches high (by the width of the firebox) simply due to the necessity to be able to reach in and clean them. For a larger boiler I would recommend going up to 5" - 6" but not really any bigger than that.

Same with the smoke stack. No less than 6" diameter for a small boiler (~75 - 150K BTU) and at least 8" for anything larger. (~150 - 450K BTU)
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Base, floor draft system and ash cleanouts.

Simple yet if done wrong will make your whole systems function and serviceability annoyingly difficult to work with.

Being you are burning solid fuels you will have solid materials in the form of ash and noncombustible other stuffs that accumulate in the system at various points that will need regular removal. If you burn salvaged scrap lumber and other such materials you will have nails, screws, staples, bolts cleats and whatnot that will show up on some quantity. For years I burned wooden shipping pallets and scrap lumber materials as a primary fuel which came with all of that unwanted material and more of every imaginable size, shape and design. So much so that in a good year it was fairly easy to accumulate around a ton, 2 - 3 55 gallon drums worth!

Given those challenges I redesigned the concepts several times to adjust for better handling of said work. The biggest issue is simply having an ash collection and removal method that as simple and easy as you can make it which, depending on what design you use, breaks down into three primary variations in the bases design.

1. Is the simplest. The base assembly is a solid flat floor made of either refractory cement (most recommended for best service life and durability), common concrete (good but will break down and wear over time due to the high heat and thermal cycling effects) and brick of either a common construction or preferably refractory service rating (least cost and easiest to replace individual bricks as they wear out).

With this concept there is no base draft to bring air in from under the fire which with wood buring it not really a big deal beyond having slightly poorer combustion efficiency and more unburned wood that will be left when the fire dies out.

It's the simplest and would require all ash and other material to be scooped out or scraped out the front door. It's also poor for burning coal and other such fuels that need a good deal of air flow coming from some direction other than their exposed faces.

The overall design is very simple. Just make it out of the same steel sheeting as the rest of the boiler to a height that is comfortable for you work with and dimensionally a little bit smaller than the footprint of the boiler itself, About 8 - 12" is a pretty good height to aim for and ~ 1/2" - 1" narrower and shorter in width and length.

Beyond that simply fill it with reasonably clean sand and or gravel up to what ever thickness the floor will be. For the refractory cement or poured concrete verizon I recommend making it at least 3" thick and with a lot of rebar in it. Rebar on 3" - 4" centers is my preference.

With this design it uses a fan forced air flow which comes down and in from the right hand side through a 2" pipe and the ashes were scraped forward into the trough and the pulled sideways out the front side.

The ash trough system worked good for normal ash but when I went to pallets and scrap woods it proved to be too small to be easy to clean things out with plus it was too thin of metal and heat warped after the first season. If you were to use such a design I really recommend making it at least 5 inches wide and of a similar depth plus of heavy steel like the center draft and grating is.

2. Is basically the same but with the addition of a floor draft system which can be done as either a central grate, like is shown in the picture, that covers approximately 1/3 the width and 1/2 length of the fireboxes inside dimensions.

For that design I highly recommend going with heavy steel and not the lighter stuff the boiler body or anything else is made from due to the high heat it will be exposed to . The design in the picture is made from 3/4" steel with multiple cross braces between the sides plus the top plate is also the same 3/4" steel with a grid of 1/4"- 5/16" holes drilled on 1" centers.

This design works well and is easy to work with except for two things. Every now and then that grating has to be pulled out out and all the fine ash that falls through those holes has to be cleaned out plus those holes need to be cleaned out while your at it.

The fist part is easy. Just scoop it out with a hand shovel or suck it out with a shop vac but the second part is a bit more time consuming given that with air coming in from under the fire the heat being generated above it can be so high it will actually melt metal and even the ash itself which over time will plug the holes with slag that can too often only be drilled out with a good masonry bit and hammer drill.

A sub variant of that is to put 1 1/2" - 2" diameter pipe in 3 - 5 runs in the floor itself, with a string of holes in the top of each that come up through the floor, that extends all the way out the front and back so that when they need cleaning you can just push the ash through or suck the ash out. Easier to clean but then you do have to actually crawl inside the fire box to redrill the plugged holes.

3. Is a fully integrated ash catcher system where the grating is large enough to let the majority of the ash and other things fall though and down into a dedicated large capacity pan or tray that is easy to pull out and dump. I recommend that design the most but it also takes a fair more effort and work to design and make.

For that design the base may need to be taller by several inches to accommodate the added clearances for whatever size catch system you come up with. For one boiler I made I made the catch system ~12" -14" wide by ~ 30" long and 6" - 8" high.

Great capacity and easy to work with as well. For the grating I used old commercial storm drain sections I picked up cheap from a scrap yard that gets a good deal of old and damaged ones from the local highway department and city roadway department. The are a good grade of cast iron or cast steel and last a long time.

If you cant get any from your local roadway matiance department or such public works places have a look around your local home building supply centers in their landscaping sections. They carry a lighter version them that will work but they wont last as long and likely will cost you a bit more.

As seen in the picture the top half of the floor grating system is made from the heavy steel and the bottom half form the lighter sheet.


The front door to it was simple top hinged concept that both flipped up to pull the ash pan out and was operated via a push rod to let air in when the control system called for heat. Very simple and passive draft based design that works very well with minimal parts.

Large capacity ash pan as it sat in the base.

Draft control actuator unit mounted to the back of the base.

Completed base installed on site.
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