Simple rocket stove heat exchange idea

Hi all I was hoping to run this idea by a few of you to see what you think.

I've attached a pic of the design, I thought of using a steel 200L barrel over a rocket stove where the flue passes through the barrel heating the air inside, which I was hoping the natural rise of heat would carry it into the house, otherwise a small fan pushing the air in could push it into the house. The rocket stove under the barrel would probably be made of mud or brick.

We need to heat our house this winter as the condensation from a small home even on double glazing will mould the window sills and affect our health. Because the air intake will be external air, would the air coming inside still contain humidity?

If so I don't think it would be too hard to pipe air from inside the house and through the barrel via a small fan.

What do we think of the design? I chose the idea as it would be very simple to build and my wife doesn't want any experimental fireplaces inside. The alternative is going to be a $4000 pellet heater! I really appreciate any help with this or direction towards a simple cheap solution, thanks!

Hi George,

Is the firebox with the barrel inside of the house?

Cristobal Cristo wrote:Hi George,

Is the firebox with the barrel inside of the house?

Hi Cristobal, the whole system would be outside, thanks.

It will not work. The gross of radiation comes from firebox. I read an opinion of a professional Kuznetsov heater builder in Europe and he said he tried such a build in a basement to heat the first floor and it was a failure. You would have to make entire channel from insulating firebricks and build oversized firebox and then maybe something would trickle to the house. Also the incoming air would be cold and it would be cooling down the combustion and the entire heater.

Hi George;
I'm afraid I have to agree with Christobal about this.
You will not gain the heat you are hoping to with your design.

You might be interested in a  Liberator stove  https://rocketheater.com/products/
They can be plumbed through a mass, you can burn wood or pellets, no electricity is required, and it is code-legal throughout North America.
Not cheap, but your wife nor your insurance man can complain about it.  Well, the cost is worth crying about but nothing good is free.
And a happy wife is worth a million dollars!

Many years ago I came across a post about  heating off grid cabins.
The author used barrel stoves, which he installed in dirt floored cellars beneath the cabins.
He surrounded the barrel stoves with tons of thermal mass in the form of dry stacked stones.
By firing the stove with hot, quick fires, he charged the mass with enough heat to keep the cabin warm while he was asleep or away, and it kept the cellar dry.

To appease your wife's legitimate concerns wife buy the cheapest legal woodstove you can find, surround it with bricks and other thermal mass, build hot and fast fires whenever you can.

If you want to use a homemade stove,  you can build a shed, greenhouse or hoophouse  just outside your home and put the stove in there.
Let the stove heat that space use vent pipe to move the heated air into a mass located inside the house.

Thanks for looking over it guys, I think I'll be looking at other options for now. The liberator stove looks nice but I'm in Aus and I reckon shipping would be a bit. Gonna keep an eye out for second hand pellet heaters.

William Bronson wrote: Many years ago I came across a post about  heating off grid cabins.
The author used barrel stoves, which he installed in dirt floored cellars beneath the cabins.
He surrounded the barrel stoves with tons of thermal mass in the form of dry stacked stones.
By firing the stove with hot, quick fires, he charged the mass with enough heat to keep the cabin warm while he was asleep or away, and it kept the cellar dry.

To appease your wife's legitimate concerns wife buy the cheapest legal woodstove you can find, surround it with bricks and other thermal mass, build hot and fast fires whenever you can.

If you want to use a homemade stove,  you can build a shed, greenhouse or hoophouse  just outside your home and put the stove in there.
Let the stove heat that space use vent pipe to move the heated air into a mass located inside the house.

I think we posted at the same time and I missed this! The thermal mass stove under the floor is a really cool idea. I also like the out-hot-house idea too, we will have a roofed area out the front door soon and eventually walled so we can do that in the future.

We do have a wood stove we got for $5 at a garage sale, its thick steel but the door is cracked, might see if we can get that fixed and just wrap it in bricks or cob.

Here is a link that original post:

https://www.backwoodshome.com/better-wood-heating/

This was the first time I remember learning that hot fire burn cleaner and the idea of thermal mass.
Researching these concepts eventually led me to rocket stoves and by extension, Permies.

heat exchange depends on three things: the heat transfer "resistance" between the hot side and the cold side, the temperature difference, and the area. I think you can achieve a pretty good temperature difference with a rocket stove exhaust stack (though you want to be able to minimize excess air flow, and just have enough to not produce smoke), but your diagram has a relatively low-surface-area solution. Ways to increase surface area would be to put fins or vanes of metal on the stack, or divide the exhaust into multiple smaller pipes. Thin-wall metal stove pipe has very low heat transfer resistance, but if the temperature drops and combustion is incomplete, there can be wood-tar condensation, "creosote" accumulation in the exhaust. That can really increase the resistance to heat transfer.

Heating cold outside air in a single pass would increase the risk of creosote, whereas recirculating indoor air with the addition of just a little outside air could give you better efficiency (the heat transfer rate loss when going from 400C stovepipe to 20C indoor air instead of -10C outdoor air is only 7% less).

I agree that the cheap wood stove surrounded by lots of masonry is going to work best. An outdoor rocket stove like you show will not have the efficiency of a proper RMH, so would not really benefit you. If you did need to build something like that, it would be imperative to take the barrel inlet from the heated space and not from outdoors. The more efficient your heat transfer from the exhaust stack, the less efficient and more smoky and dirty your fire will be. You really need an insulated heat riser for a few feet height before you extract heat from the exhaust gases.