Thread: pot belly wood heater design
29th Jun 2011, 09:58 PM #1
pot belly wood heater design
I have a tiny house that needs a wood heater and as all the regular commercial ones don't really fit I have been working on a design for one I can make myself. I have been thinking about it for the last few months and as I work in a refinery and do a bit of piping design I am thinking that making something out of piping would be a good option. Also hopefully I can get the pieces that I need cheap from the local scrap merchants who get deal in the sites scrap.
So anyway after seeing the recent thread about the flue damper butterfly valve there seems to be quite a bit of knowledge on the subject so I was hoping I could get some advice. I have a basic concept as per the photos which is based on using 300mm pipe for the main body with a buttweld endcap on the bottom and eccentric 150 reducer on the top. the door opening would be an oval based on 250 pipe with a 100mm section added and the door another piece of 300 pipe. The stand is currently 200 pipe and a flange but I am tossing up making this 150 instead as it looks a bit out of proportion.
It will definitely have a flue damper and will also have perforated mesh guard around it to keep the kiddies safe... working on the detail of both still. Am also considering including a heating coil to heat our hot water.
Anyway there are a few other thing i was hoping people could help with:
- Air supply: I have seen some info (mostly from really cold places) that recommend running a pipe in from outside for the air supply. apparently this will reduce draughts in the hose due to the fire drawing air. Has anyone done this here? is it worth the effort?
- Does it matter where the inlet air vent is located? most commercial wood heater seem to be in the door of above it at the front but I guess its just a convenient access thing.
- I was thinking of an ash tray type arrangement perhaps under a grate that the wood and coals can sit on so the ash drops through. Would this work?
- I wouldn't mind a glass door but I'm not really prepared to compromise on it being curved which I'm guessing would be pretty expensive. Anyone done this or know?
- What material should the water heater pipes be... would copper be ok?
Would like to hear any other comments or suggestions
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30th Jun 2011, 12:09 AM #2
I'll have a go here, but first up I'll say that I have never built a heater, although I have maintained a few and we have a ducted wood fired furnace which I currently maintain.
Your initial design has me wondering whether it is large enough in diameter, because I would be lining the fire-box, at least where the fire sits, and where burning wood will rest against the inside. You could cast fireproof cement into shaped tiles to fit the interior. In my heater they are around 25 mm thick, but they last 3-4 years before requiring replacement. If you have access to 3/4" steel plate and heavy plate rolls, you could also make them in steel. This will not leave that much room for your fire, using 300 mm tube for the body of the heater, and you will need to use small pieces of wood for your fuel. I would make the base of lighter material, maybe a tubular frame to minimise heat conduction to the floor. and to allow heat to be more effectively transferred to the air.
I would not bother with bringing in outside air for your heater, - that would be more applicable to an open fireplace in my opinion, they do cause drafts with the large amount of air drawn up the chimney, besides the air used in the combustion process - this is less of a problem with an enclose heater.
I don't think that ti matters where your air inlet is, I think that they are often placed above the door when there is glass in the door. The idea seems to be that the air circulating down past the glass, to the fire will burn off any carbon deposits on the glass, keeping it clean. It may work that way sometimes, but not much in my experience.
I would not bother with a grate and ash tray because it would mean a lot more maintenance I think, and a deep bed of ash protects the steel from the fire - it acts like more fire-brick. Just make your heater tall enough to allow the ash to accumulate for some time before needing to be emptied.
You could do a glass door, they usually have narrow strips of glass which in your heater would give you a faceted curved window. You would need to seal it with glass fibre cord or tape either side of the glass, or maybe the red high temperature sealastic would do the job, but you will need to fabricate a metal frame to hold the glass in the door either way, - doable but fiddly I would say.
Heavy guage copper will be ok for your heating coil, but you would need to keep it out of the direct flame front, and protected from mechanical damage from heavy handed loading of the firebox. placing it behind steel plate or firebrick/ fireproof cement would be my recommendation.
You may not be legally able to fabricate your own heater, because they now need to conform to an AS as far as I know, (for air quality regulations amongst other considerations) and this could also have insurance ramifications in the event of fire damage. In any case the flue will most definitely have to conform to a standard which as far as I know requires 3 concentric tubes, and there are minimum distances it must be from any flammable structural materials, and there would be requirements for the base under the heater as well. I hope that this is some help and I,m sure that others will chime in too, especially if I'm off the mark anywhere,
30th Jun 2011, 06:38 AM #3
point one is correct
as for copper tubing..it would be short term...for long term use, use boiler tubing ( heavy walled pipe)..once outside the firebox you get then connect to copper...copper erodes (if thats the correct terminology) when in contact very hot heat/flames.
30th Jun 2011, 08:36 AM #4
The guy we bought our house from was a bit of a hero - thought he could do anything. He was some kind of fitter and a crash-hot welder. He built a combustion heater. I guess it didn't work because it was removed and now gathers rust outside. He built another one, which we still use occasionally. But it doesn't work well. With everything wide open it burns weakly, unless you feed it lots of softwood. I can't for the life of me see what he's done wrong.
My point is, it's not as simple as it looks. It's quite possible to spend a lot of time and effort and get a poor result. So I would be doing lots of homework before I cut anything. Sorry if that's preaching to the converted - you are here asking questions after all.
Here's one link I found that briefly describes current design principles. Slow combustion wood heaters and how they work by Abbey Fireplaces.
30th Jun 2011, 08:38 AM #5
Can anyone else see the pictures?
300mm sounds like a tiny fire if you plan on burning wood.
Where is your hot water tank and what sort is it?
30th Jun 2011, 09:58 AM #6
Stuart, with an iMac, I go control - left click, on those icons, and I get an option to open the picture in a new tab, or in a new window. I'm not sure what the Windows action is to display them, but it would be similar I think.
I agree Brian, you could do a lot of work for a poor result, but in a previous life I worked for a mining company on the west coast of Tasmania, and lots of heaters were fabricated by the boys in the workshop with good results. Chipping hammers which wee stock items in the warehouse store, made great handles for firebox doors with their wound spring handles
I agree with eskimo on the need to keep any copper out of contact with flame. It should be behind steel or fireproof cement, as should any other tubing you might use.
30th Jun 2011, 01:39 PM #7
yeah it is quite small but the space I have available is very limited so its small or nothing I'm afraid. 300mm pipe is actually about 320mm OD and I need to make it less than 400mm including the guard and heat sheild. I have a big square wood heater I bought a few years ago but as it won't fit its now headed for shed duties.
I hadn't thought about checking australian standards but had a quick look and this has answered some questions straight away. I'm now embarssed I didn't check this earlier. I will try and fix the pitures and post some more detail when I get home tonite.
30th Jun 2011, 01:59 PM #8
I'm just wondering how big the door will be as far as chopping up wood to fit through it goes. But if nothing else will fit it will just have to do.
I guess briquettes cost big $ these days?
Maybe the pictures are just me, even if I get the imageshack url from the source it still doesn't come up.
30th Jun 2011, 03:04 PM #9
30th Jun 2011, 03:48 PM #10
I saw a small wood heater in a friend's house many years ago which always stuck out a bit different to me. I was told it was a French design and if I remember rightly it was like a 300mm diameter long pipe coming off the floor with a flat top you could heat a kettle on and the flue came out the back some way from the top. I was also told the principle was you got some sort of secondary combustion in the top portion of the tube but I have no idea how this was physically achieved. I have never seen another one like it again. It was small size though and you could feed the fire through the top with small diameter but longish timber or feed it through the bottom door when getting it started.
A bit of web search and presto. A website that sells old ones. This is probably something like it:
As you can see from the photos on subsequent pages there is dual outlet at the rear. Not sure of the reason for the two placements but it may be to do with what I mentioned above.
The external air supply makes good engineering sense to me and would probably greatly affect the efficiency of the ability of the fire to heat. Stopping drafts is one the major improvements you can make to energy efficiency of a house. If you have a constant stream of hot air and smoke going up the chimney and no air supply feeding the fire from outside you will automatically be drawing cold air into the house from every gap in the structure around doors, windows, ceiling vents, skirting boards etc which will reduce the temperature indoors considerably and sucking a lot of warm air straight into the sky up the chimney.
30th Jun 2011, 06:44 PM #11
I'd be interested in knowing if anyone has had any experience with the "new" design of quad burning wood combustion heaters. I am looking to build a "next generation" type of unit. Something that complies with AS 4013 and is really efficient. Actually this AS is only for the method for the determination of flue gas emissions.
I found this basic schematic at Premium Firebox Design - Heat Exchanger and Quad Burn Systems for Cleaner, Longer Burns
30th Jun 2011, 07:14 PM #12
All buildings need ventilation. Thats what vents at top of the wall are for. If the fire uses some of this ventilation it should be no major problem. I have heard of cases where people have died in huts built in alpine regions without proper vents and the wood burning heater sucked all the air out of the hut, well maybeb not all of it.
I have not heard of external air source used for combustion stove in Oz. Probably something for climate extremes.
I agree with protecting the steel from direct heat. If not you will have to replace it when it burns out. May last a long time still but I hope you are around when it burns through.
I have plans to build an outside stove from old tractor rims. About 3ft diameter at max. I saved the door from an old slow combustion stove and plan on fitting it between the rims in a section of curved plate steel. 3 ft diameter hot plate on top with flue out top as well. Beats a 44 gall anyday.
30th Jun 2011, 07:15 PM #13
i have seen a plan for a pot belly stove in a mag years ago i think it was the "grass roots "mag .
the stove has since been built by quite a few people who said it worked very well .
the body of the stove was made from 2 large truck brake drums ( thick cast iron ) you can get some where one will sit inside the other i had a pair a while ago but never used them .
.one with the axle hole downover and the other with the axle hole up .
the top axle hole is where you bolt a flange for the flue , you need to cut the door with an angle grinder and fit some legs .
maybe a google search on brake drum stove or some thing could turn up the plans .
johno'If the enemy is in range, so are you.'
30th Jun 2011, 07:29 PM #14
These are the pictures for those who had troubles...
30th Jun 2011, 08:01 PM #15
the door opening will be about 250mm wide buy about 350mm high. I was planning on just cutting the wood to suit as although its not very wide the fire box is relatively tall. If I have to add refractory lining that might bugger things up though. Of the two heaters I have at the moment one has bricks only on the floor and the other doesn't have any at all. I was sort of hoping that because the pipe will be about 10mm thick it would be ok...
I would like to make it as efficient as possible but I will be out on a couple of hundred acres with a plentiful supply of wood so its not critical. Also the house is tiny donga (about 60m2) and well insulated because it made of 100mm thick freeza panels so even a small heater should do the trick.
I like the idea in one of the links posted of preheating the air so I might try adding air intake heat exchanger pipes that come in through the back of the heater at the top to do this. I looks like i wont be able to heat water after looking at the aus standard for plumbing wood fired water heaters. the pipes need to have a constant fall from the storage tank to the heater without any high spots and that just wouldn't work in this case.
That french heater looks about the right size but is a bit too ugly for me!!
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