Thread: pot belly wood heater design

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

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.

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 .

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 .

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

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.

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
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

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.

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.

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 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.

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.

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.

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 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.

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.

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.