RE: Tlud, I have built one of those and I have bought one commercially, called the Silver Fire. Here are my video results.
With paint cans:
The Silver Fire:
Hereâs my water heater. This is going strong, in august will be 6 years now. Iâve had to replace the grates once a year or so. Itâs plumbed right into the house and weâve used it day in day out since 2012. No welding, nothing special just an old gas water heater modified to burn wood.
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That is a very nice stove. How do you deal with the pressure issue? Do you know just how much wood to load in it?
I saw a guy do this with an old gas stove with a rocket stove, this is the first I have seen done with a conventional stove. ( unless you built that custom )
Pressure is dealt with no differently than if it was a gas water heater â nothing beyond the same safety valve that it came with, which relieves over pressure and over temperature. It is also quite fast, it heats 40 gallons in 45 minutes to an hour. I donât know how many gallons Martâs stock tank heater held.
This is a conventional water heater modified with longer âlegsâ and a wood-burning firebox hanging from the bottom. Iâm guessing it also has as good or better surface area, given the bottom end (which is 18" or so diameter) plus the 5+ foot flue pipe which is 3 inches diameter. Never tried frying bacon over it though.
And yes weâve gotten a sixth sense of how much wood to put in. Was easy to figure out and so almost never pop the safety valve. Basically when the waterâs hot just stop adding fuel and the fire goes out quickly enough.
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Mart, by the pressure issue do you mean how would we get the water in or out during an off-grid situation? We could probably just pour it in with a funnel at the top, heat it, then draw it out the drain cock. Or just run a generator to pressurize the well. Weâd need to run the well pump to get water anyways, so one way or another we need pressure because weâre not located by a stream. 
Over pressure was my concern, so yes you have had the safety valve open on it if you over do it got it. 6 years, that is a big savings on the electric bill.
Since weâre talking about water heaters. I had this series of ideas a while ago, based on heating from the top down instead of from the bottom up. And then had the notion that this is soooo simple. Someone must have tried it. So i did a search and yup there it is, filed away under âdiscontued productsâ. 
http://www.htproducts.com/phoenixdocuments.html
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Hi Rindert
what interest is there to heat up and down rather than down and up ?
Thierry
Rindert that does look super efficient during heating, to run the combustion gases back down over the outside of the tank. For storage I donât know how that would work, I think a lot of heat would get lost during âstorageâ time, i.e. when the water was up to temp, and no hot water was being demanded. With the tried and true domestic water heater design, there isnât a lot of heat escaping out the flue that needs to be captured, and you do need a little heat anyways to keep the fire drafting nice.
However, making steam is a whole different ball game. Seeing the term âsuperheatâ gives me the shivers. All weâre doing is heating water to a little over a hundred, or maybe 150 if someone got a bit zealous with firing. The safety valve opens at 160 which is still well below boiling point at atmospheric.
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I just saw a design for a rocket stove oven that was similar to this.
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Hi Thierry,
That is a concept worth knowing.
Hot water, and other fluids rise, and the tank stratifies (layers itself) with the hottest water at the top. So if you heat at the bottom the hottest combustion gasses give up their heat to the coldest water. You recover much more heat if you do it the other way around. In fact it is sometimes possible to have the exhaust gasses leave the unit at the same temperature as the incoming water. It might be worth while to study the concept of âcountercurrent exchangeâ. wikipedia has a decent explanation. Birds lungs work that way or they couldnât fly.
Dan,
It SHOULD give you the shivers. I wish terms like âsteamâ, âsuperheatâ, âsteam qualityâ, âdry pipeâ, âdry steamâ, ⌠gave everyone the shivers. Steam locomotives were retired from service in the 1950s largely because the were too expensive to insure. Read 200 tons boiling water moving at 70 miles per hour is just too bleepin dangerous.
A friend of mine makes steam whistles. Scares to livin out of me sometimes.
Rindert
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Iâm going to sleep a little less stupid tonight thanks to you. thank you Rindert:
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Please forgive me if someone already mentioned it, but the immersion heater idea has been refined well by the Snorkel stove folks (snorkel.com). They use aluminum for their immersed hot tub wood stoves. Aluminum is a great conductor of heat and canât melt under water.
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Thierry,
I donât think anyone who is willing to learn is stupid.
Rindert
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Yes, but in practice you have to watch out for exhaust condensation. The consept is used mostly in industrial scale combined with a stainless final heatexchanger, chimney and exhaust blower. Even plastic chimneys are used to be able to handle the corrosive condensation.
In small scale use the most efficient way is to let exhaust leave where the hottest water cooled surfaces are. That way you can squeeze the most heat out of the exhaust without risking condensation. Then let the stratifying happen in a separate storage tank where the heat is safe from escaping in the boilerâs radiator mode when the fire is out.
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Hi Jan,
You make a very good point. I recenly helped a friend build a rocket stove. I works very well. But when starting from cold it collects a little water in the bottom. Then as it heats up the water evaporates. Not a big problem in this case, but something you need to be aware of when you are desigingâŚ
Rindert
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JO beet me to it. A general rule is water surounding the heat exchafger must be 60c +. that way the acetic condensation is reduced.
Problem with heatexes runing too cold is allso that the creosote acumulates on the metal. This acts as a insulation reducing the efficiancy.
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I think he needs to have a cross sectional area of the inlet air to match the exhaust⌠Maybe even a little bigger since he is condensing so much of the exhaust with the water. A taller, skinnier tank with a taller exhaust would help too.
Heâd probably get a hotter fire if he had a better draw too. But then I guess you have to deal with better heat exchagnge.
Interesting Idea. And the bacon looks goodâŚ
Thanks for the link Bruce
I think it will work as envisioned, but I think it will suffer a lack of efficiency from the small surface area for heat exchange. Maybe the diffuser could be dispensed with, and a spiral copper tube used? Or an internal diffuser as envisioned, and copper tube spiral wrapped on the outside, like the heat recovery drain systems?