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:

1. 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?
2. 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.
3. 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?
4. 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?
5. What material should the water heater pipes be... would copper be ok?

Would like to hear any other comments or suggestions

http://imageshack.us/photo/my-images...odheater3.jpg/
http://imageshack.us/photo/my-images...odheater1.jpg/
http://imageshack.us/photo/my-images...odheater2.jpg/

Hi,
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,
Rob

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.

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.

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?
Stuart

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:D
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.
Rob

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.

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?

Stuart

Maybe the pictures are just me, even if I get the imageshack url from the source it still doesn't come up.

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Originally Posted by Stustoys

Can anyone else see the pictures?

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Hi Stuart, I can't see them either? not sure why, other than that something in the network must be blocking them..

Gallegos, can you upload the pictures as attachments?

Regards
Ray

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:

p-godingrandrond-photo

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.

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

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.

Dean

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

These are the pictures for those who had troubles...

https://www.woodworkforums.com/attach...1&d=1309429603
https://www.woodworkforums.com/attach...1&d=1309429603
https://www.woodworkforums.com/attach...1&d=1309429603

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!!

As far as heating your hot water. If you really wanted to, I wonder if you could have a pumped (non pressurized) circuit to a heat exchanger closer to the hot water tank?
Stuart

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Originally Posted by Oldneweng

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.

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Those wall vents aren't sensible in all climates. I think such deaths in confined spaces is most likely due to carbon monoxide/dioxide buildup from poorly built fireplaces resulting in suffocation not from oxygen shortage due to huts being too well sealed and a partial vacuum forming. The fire does consume oxygen but the volume of air in the room stays constant so if air is escaping at considerable velocity up the chimney, as it does if you have a fire burning brightly, then cold air is drawn into the room from outside pretty quickly. Canberra is cold enough at night to be described in Australia as a fairly extreme climate but most buildings are so leaky that even if you went and blocked up all the holes (including wall vents) you could find and put seals on all the windows and doors there would be no risk of running out of oxygen. If you don't go around and seal those leaks (particulalry high up in the walls and ceiling) however, then you have to run a heater flat out to just to keep the temperature stable and comfortable because of the siphoning of heat out of the room and the entry of cold air. It does escape from conduction through materials such as glass as well of course. Possibly this is not a major concern in Bunbury as it probably isn't as cold so I agree an external air supply may not be a major design concern.



"That french heater looks about the right size but is a bit too ugly for me!!"

I was more pointing out the functional design principles than the colour scheme. :;
If you found out what the design of the flue at the back is for I would love to know. But it's the possibility of having a hot plate to heat a pot on that really appeals to me and if can't add in a water heating feature into the structure you can at least still heat water on a smaller scale.

just been poking about on google and i found a good one , being V8 though i dont know how many hours to the log you would get .

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Originally Posted by texx

just been poking about on google and i found a good one , being V8 though i dont know how many hours to the log you would get .

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Wonder how long the fan belt lasts?

I have seen a stove built out of a LARGE cast electric motor casing. It was about the size of a 44 gallon drum. It had great cooling (heating) fins and the door was made of the front casing with glass fitted into an enlarged opening. Was built by an engineer.

Dean

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Those wall vents aren't sensible in all climates. I think such deaths in confined spaces is most likely due to carbon monoxide/dioxide buildup from poorly built fireplaces resulting in suffocation not from oxygen shortage due to huts being too well sealed and a partial vacuum forming. The fire does consume oxygen but the volume of air in the room stays constant so if air is escaping at considerable velocity up the chimney, as it does if you have a fire burning brightly, then cold air is drawn into the room from outside pretty quickly.

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I have started to think about those vents a bit more. Spoke with a guy at work who has a nosy enquiring mind, about them. He said they were needed many years ago due to every house having open fires. My grandparents stone farm house near Minlaton SA had small cast grates in every room except for the ones with a large fireplace or slow combustion stove.

With modern heating they just let the heat out although most of the ones I have looked at are pretty blocked with dust etc. A down side of not having them in modern houses is that the fumes given off by most manufactured materials in the home are not vented as quick causing health problems.

The hut I mentioned was home made prefab aluminium and was very well sealed. Think ali windows / doors, not tin / slab hut. I read about it a long time ago but it was lack of oxygen in this case. That was what the article was about. There was no ventilation, the hut was small and cramped and too well built except for ventilation.

I might throw out some questions in the reno forum about vents and maybe save some heat energy.

Dean

I did a search in the reno forum and found that vents are no longer required in modern home building. Common sense in required of course and adequate ventilation is required especially in wet areas and cooking areas. I am going to block all mine up except for above.

Dean

Just thinking outside the square for using your fire box to heat water, what if you made the fire box into a water jacket still keep the 300 inside get some 385mm to go around it to form the jacket. Cut out 90° of both pipes and shape/weld flat plates to box it in, this 90° cut out will also act as a flange area for the door/glass which can be now flat glass :D
As for having a air inlet pipe from outside I think its a good idea, but the fire box will need to seal good for it too control properly. Theoretically it should extinguish the fire .

I had another look at those pictures and realised that there is a largish space at the top in the reducer for a water jacket. Think outside the square and opportunity presents itself. My understanding is that water jackets are often made of stainless steel plate. Working in the wine industry I lean towards SS for many things but then I have quite a bit of scrap collected over time.

We have a wood range in the kitchen that is all that currently provides hot water. I have often wondered about the design of these as it is impossible to get hot flame in the water jacket area. Maybe this is intentional but when you need to heat water in a hurry it is frustrating.

Air comes in thru the door and ash door vents which are vertically in the middle of the firebox. The air flow then rises to top of oven on right hand side, over the top of it, out the flue control flap then to back and up flue. The water jacket is on the far left of fire box.

I have often thought of having a water jacket at the top of the firebox where the most heat is. A clever person could build a convuluted water jacket or even a simple water jacket into the area of the reducer at the top of this stove as pictured. For example a disk shaped jacket with hole in middle for smoke etc to pass thru, or even two with offset holes. The other possibility is to incorporate the water jacket into the flue, or at least the lower part of the flue. This may only be cabable of preheating tho. Some ideas to ponder on in quiet times if nothing else.

Dean

My out-laws had one of these Godin designs and they work incredibly well. The hot plate on top is very handy for tea pots and soups and slow cooking and it opens to load in wood so you can put quite big long pieces in.

p-godingrandrond-photo

As you can see from the photos on subsequent pages there is dual outlet at the rear.

I think the two outlets were so the top one could be opened for starting and when loading in wood then shut down when it was hot.
I think Gallegos could make one of these from 300mm pipe and get five or ten good years use before it burned out.

Oldneweng, very few water jackets are made from SS, at work(lead smelter) only on furnace outlets/weirs would use SS or for corrosive metals like zinc.

Hi Harry,

Things vary from industry to industry, the food industry is always stainless, steam jackets, heat exchangers and so on, are always stainless, not as efficient for heat transfer, but takes longer to rust. :)

Regards
Ray

I think there would be enough room to fit a heat exchanger in the top of the heater comfortably but with the spot the heater will be in the house I couldn't get the piping to the storage tank the way it needs to be. This has to have a constant fall to the pot belly otherwise I guess the water doesn't naturally circulate and you get steam pockets and the like. I had a read up on materials anyway and the plumbing standard says no mild steel and that copper might be a problem if the wood is high sulphur... Guess that only leaves stainless.

Pumping water through the heater would work but the cost of the electricity to constantly run the pump would probably cost more than just using the gas hot water that I currently have set up.

I have been trying to find out more info on how quickly the steel will corrode if I don't have a refractory lining but haven't been able to find anything. Interestingly the standard for design and construction of wood heaters (AS3869) doesn't directly say it has to be refractory lined but does give maximum temperatures different materials are allowed to get too. I guess that this would force you to use a lining if the allowable temperature is low. For mild steel less than 6mm thick it say 460 Celcius max and thicker than this it is 100 degrees below the melting point, which would make it around 1300 celcius. To me this seems to say that if its thicker than 6mm it will be ok and it won't burn through... but obviously people have had this problem before.

Some of the links posted mention about having a secondary air inlet to ensure complete combustion and make it more efficient. The picture seem to show a small vent that comes in from the back and feeds air in the flue gases just above the fire. I am guessing you want to get this air into the flue gas just as it goes past the baffle so it goes up the flue rather than recirculating down to the fire. anyone seen this or know anything more about it how to make it work?

I like the idea of using an electric motor casing for the firebox but it would be a bit of a pain using cast iron... I am a big fan of welding things to death.

There are some very small wood stoves used in boats. Perhaps there is something to be learned from them.

Pete

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I think there would be enough room to fit a heat exchanger in the top of the heater comfortably but with the spot the heater will be in the house I couldn't get the piping to the storage tank the way it needs to be. This has to have a constant fall to the pot belly otherwise I guess the water doesn't naturally circulate and you get steam pockets and the like. I had a read up on materials anyway and the plumbing standard says no mild steel and that copper might be a problem if the wood is high sulphur... Guess that only leaves stainless.

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The only other choice is cast iron which is obviously out of the question. You are correct about the run of piping. Steam pockets are a no no.

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Pumping water through the heater would work but the cost of the electricity to constantly run the pump would probably cost more than just using the gas hot water that I currently have set up.

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I have a similar issue with a solar collector setup I am going to build. I thought about a solar powered pump but you need to turn the pump off when there is no heat input or it turns into a water cooler. The whole design starts to get complex which means more to go wrong. I decided to lift the hot water unit to the top of the roof instead of the ceiling to get around this.

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I have been trying to find out more info on how quickly the steel will corrode if I don't have a refractory lining but haven't been able to find anything. Interestingly the standard for design and construction of wood heaters (AS3869) doesn't directly say it has to be refractory lined but does give maximum temperatures different materials are allowed to get too. I guess that this would force you to use a lining if the allowable temperature is low. For mild steel less than 6mm thick it say 460 Celcius max and thicker than this it is 100 degrees below the melting point, which would make it around 1300 celcius. To me this seems to say that if its thicker than 6mm it will be ok and it won't burn through... but obviously people have had this problem before.

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6mm will burn thru, it just takes longer. Maybe the rigidity of the steel has an effect. If it is constantly flexing from heat changes it will burn thru quicker.

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Some of the links posted mention about having a secondary air inlet to ensure complete combustion and make it more efficient. The picture seem to show a small vent that comes in from the back and feeds air in the flue gases just above the fire. I am guessing you want to get this air into the flue gas just as it goes past the baffle so it goes up the flue rather than recirculating down to the fire. anyone seen this or know anything more about it how to make it work?

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I had an "Austwood" heater many years ago. It had a square tube sitting at the rear top edge of firebox under the baffle. It had about 3mm diam holes maybe 40mm apart facing the front of stove. One end was blocked, the other end aligned with a hole 18mm diam thru the side into the air heating chamber. The fan forced a small amount of air into this tube which could be seen at times with small (30mm) flames from each hole.

If the air is introduced as you say, the extra heat, which would be minimal, would go straight up the flue and be lost. May reduce greenhouse gases tho. With my heater the heat passed under the baffle then up over it. This heater was scientifically designed for max output and minimal pollution according to the blurb. It was big but apart from that I did not notice any noticable benefits from the fancy gear.

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I like the idea of using an electric motor casing for the firebox but it would be a bit of a pain using cast iron... I am a big fan of welding things to death.

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Should have pointed out that the motor case was in horizontal position. It was in a HUGE living room. The fire took up a lot of room. I also am a fan of good solid welding. I use a lot of recycled materials (euphemism for scrap) and this requires more welding than new materials. Thats my story anyway.

Dean

having built quite a few pot bellys/combustion heaters myself i have some advice that i think you may find useful, firstly if possible avoid running the flue driectly from the top rather do some searching around and get a heavy walled 100mm elbow. using an elbow will give you greater ability to regulate the heat flowing streight up you flue and into the atmosphere, unless you want to keep the birds outside warm use an elbow and include on the inside of the heater a baffle partialy covering the flue hole to prevent the fire shooting directly into the elbow and flue which again will give to greater control over the heat produced and will increase the life expectancy of the elbow and flue.
next think i would ask you to consider is building the fire box inside a seperate box. this way you have a cavity running nearly all the way around the fire box which you would either have cmpletly open at the base or fitted with vents. vents at the base will allow cool air to flow up into the cavity along the surface of the fire box and by the time it exits the vents you have installed at the top it pushing out hot air, you can even install a fan at the back or side at the base forcing more cool air in.

you could even run you pipes through the cavity hard up on the fire box, ive never done this so not sure about that,

ive got a drawing i did of one some time ago, just need to hunt it down and i will post it for you.

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Originally Posted by Harry72

Oldneweng, very few water jackets are made from SS, at work(lead smelter) only on furnace outlets/weirs would use SS or for corrosive metals like zinc.

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Stainless and cast iron. What else can be used? Mild steel is no good and copper is too easily damaged. What does that leave that is readily available and affordable and workable. After all you can weld stainless with an arc welder. How much easier can you get?

When I mentioned stainless water jackets I was talking about after market jackets for wood stoves which are stocked by dealers and can be ordered to suit.

Dean

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Originally Posted by gallegos

I
I like the idea of using an electric motor casing for the firebox but it would be a bit of a pain using cast iron... I am a big fan of welding things to death.

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Actually cast iron is very good material for a fire-box, and I would expect that for similar thickness of material, cast iron would outlast steel plate, You would just have to bolt up to it or perhaps braze if the part didn't get too hot. One thought I did have, which may have legs for you would be to make the opening for re-fueling in the top, with a removable hot plate like some pot belly heaters have. You could also incorporate a removable funnel like duct below the hot-plate, a bit like the ones in the old bathroom chip heaters. That might work well with the need to keep it to around 320 mm dia., and still be able to handle longer pieces of wood. There have been lots of good ideas here, from all contributors, I hope you have every success in sorting the different ideas, and coming up with your working solution. Keep us posted,
Rob.

Why cant mild steel be used?
We use it in a blast furnace and many other furnaces, there is refectory lining to protect it from the molten metal, but were talking 1000°c+.
In the refinery the molten lead is held in pans from 450~660°c, the setting/chamber they sit in are gas fired the cutout is set at 800°... all mild steel!

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Originally Posted by Harry72

Why cant mild steel be used?
We use it in a blast furnace and many other furnaces, there is refectory lining to protect it from the molten metal, but were talking 1000°c+.
In the refinery the molten lead is held in pans from 450~660°c, the setting/chamber they sit in are gas fired the cutout is set at 800°... all mild steel!

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Good question. I was thinking of rust in hot water system. Combination of water and heating. It may be feasible but would not last as long. I keep thinking about steel fittings and welsh plugs in car cooling systems and wishing that brass or copper was used instead.

It wasn't the heat I was concerned about. You can melt lead in an aluminiun saucepan. I was more concerned about corrosion.

What do others think? Is mild steel feasible as a hot water jacket?

Dean

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Originally Posted by Harry72

Why cant mild steel be used?
We use it in a blast furnace and many other furnaces, there is refectory lining to protect it from the molten metal, but were talking 1000°c+.
In the refinery the molten lead is held in pans from 450~660°c, the setting/chamber they sit in are gas fired the cutout is set at 800°... all mild steel!

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It is not so much that you can't use mild steel, but more that Cast Iron will last longer I am led to believe. C.I. won't last forever either, but it is what they use for replaceable fire grates in many stoves or heaters, and stoves such as Warmray, Aga, Rayburn were all constructed of C.I., and they had the hot spots refractory lined as well, except for the grates. These days plenty of wood fired heaters are fabricated from steel plate, and most use linings as well, and for home made ones, steel will generally be easier, as not many of us have foundry skills and equipment, whereas metal cutting and welding is well within the capabilities of most forum members I would think.
If the steel is protected in the hotter areas, it can last for years, - witness the various form of barrel heaters which use either a whole or a part of a 44 gallon drum as the firebox. they usually are part lined AFIK, and as long as you keep the rainwater out, they can have a long life.
On the other hand, using mild steel to make a heat exchanger is likely to cause you grief I would think, unless you used an automotive coolant or similar anti-corrosive fluid to transfer the heat to another heat exchanger where the heat was transferred to water or air or whatever you wanted to heat. Stainless steel is probably the easiest material to make a box or you could use stainless tube, or even copper tube as long as you protected it behind refractory or even behind a thickish steel plate. If covering with refractory, it may be an idea to wrap it first in insulating wool so that the refractory wasn't stressing directly against it with the heating and cooling cycles. My two cents worth anyway,
Regards,
Rob.