Rocket Stove Bench

[TeeDee]

I've got a spot in the garden I want to create a half round semi circle seating area in - along with a fire pit for drinking rum in the winter whilst looking up at the dusky sky trying to figure out if Bats really do know how to fly or are they just winging it as they seem to be on the cusp of major pilot malfunction each time I see them. 9 Swallows - they are not.. )

Anyway - fire pit - and bench - I'm toying with creating a fire pit ( with closing door ) and then having the exhaust come back to the bench at the half way point ( split left , split right ) heat up the bench and then either contort through a 180° and exit.

I have a small Hoard of fire bricks I've collected and think with a little bit of jiggery - pokery and fire cement i could fashion something up.

My main question to any one whom has done this or similar is - regarding the location of the fire versus the length of the lateral chimney , is there a sweet spot or point of diminishing returns?

I assume as the hot air travels along the chimney - its imparts heat to the fire bricks before it is 'spent' and then exits accordingly.

But is there some clever equation for working all this out or is it build it and it may/maynot work ?

 

[Pattree]

I have no idea but I have owned a VW Beatle.

What’s that got to do with the price of kippers?
Well.
Early VW’s had a direct exhaust heating system. Exhaust gas ran forward through pipes in the passenger compartment and out the back again. This made the exhaust inefficient and despite German engineering, still managed to stink.

Later VW’s had a clean air system. A heat exchanger allowed a conventional exhaust flow while clean hot air heated the car. It was very effective!

This was my first thinking on reading your idea.

 

[Glow_worm]

I've built quite a number of mass heaters over the years as new ideas have been developed by enthusiasts. Masonry stoves have been around for centuries, but 30-odd years afo hippies developed an easy and cheap to build version with very clean emissions and low fuel consumption. These 'rocket mass heaters' are still popular- they have a vertical feed tube, horizontal burn tunnel and insulated heat riser inside an oil drum to act as a heat exchanger for quick radiant heat, then the exhaust runs through pipes in a bench for long lasting therma storage.

Gradually the design has changed and improved and morphed closer to traditional masonry heaters with 'bell' heat exchangers but with combustion cores far more efficient and cleaner burning than traditional designs. At some point I'm going to build one in my house and get it through regulations, but it's going to be a lot of effort.

Anyway, to answer your question, the exhaust from any contained fire is expanding and wants to head upwards. Restrict that and the fire will struggle, burn inefficiently and try and get out of places it shouldn't. Trying to make a stove which will shove the exhaust horizontally through a bench is a big ask, and you'll still need a chimney with good draw to exhaust from the bench.

There are tried and tested designs out there which can do it, but they have to be built precisely and if followed to the letter they will work reliably. Many hundreds of people have tried to deviate or invent their own and have given up because they have not hit the sweet spot in a complex thermodynamic dance. Would you try and improve your car's combustion chamber design because you think you know better? It's that kind of complex.

Reading this book is a good start, available free online. Follow instructions to the letter or don't attempt it, but it's a fascinating project and a huge amount of learning about fire and materials if you can find the time. A big section of permies.com is dedicated to mass heaters, and donkey32 is still just about hanging in there for the hardcore experimenters and enthusiasts.

 

[slaine_23]

fire pit - and bench - I'm toying with creating a fire pit ( with closing door ) and then having the exhaust come back to the bench at the half way point

I’ve been wondering the same thing recently, but was thinking to use pipes under/in the seats that ran through the fire but didn’t directly deal with the smoke/fumes - kinda like some of the hot tents set ups where the fire remains outside the tent to avoid suffocation and removes the fire dynamics from the equation.

 

[TeeDee]

I’ve been wondering the same thing recently, but was thinking to use pipes under/in the seats that ran through the fire but didn’t directly deal with the smoke/fumes - kinda like some of the hot tents set ups where the fire remains outside the tent to avoid suffocation and removes the fire dynamics from the equation.

That may work better to be honest .
Will muse upon it.

 

[slaine_23]

Hard to know, could be practically useless - what I’m thinking is for external use and plans in the linked book appear to cover the idea of a hot bench, but mainly inside (so far as have read).
There’s also this sand battery concept - https://www.howtogosolar.org/sand-battery/ - instead of clay etc etc use basic sand as the heat sink.

 

[Glow_worm]

With regard to benches, a rule of thumb often referenced is that heat takes an hour per inch to travel through the masonry of a hollow bell or piped bench. I can't see a heat exchanger transfering energy to a mass faster than it would lose heat, i.e. the bench is unlikly to get warm even after several hours.

 

[TLM]

There is no way one can prevent the bench from warming up if there is a warm channel inside it. The main problem could be the unevenness of temp on the bench.

 

[Glow_worm]

It depends entirely on the heat energy going into the bench vs. the heat energy lost from the bench due to surface area and outside temperature.

If the channel is simply 'warm' you are likely not going to be sitting on a warm bench even after several hours of firing.

And those are going to be smoky, inefficient hours of firing if you have not seperated the combustion process from the heat extraction phase, as in effective mass heaters designed as a result of clever people spending years experimenting.

 

[TLM]

Used to do heat balance calculations for satellites. Now I really would like to have an object that does not heat up when heated. With forced cooling cold rain, high wind snow the temp raise could be small but few people prefer to sit out in those conditions.

 

[Glow_worm]

Give it a whirl then. Spend a month or two of weekends building one, and post up the results from the IR gun.

 

[slaine_23]

The hours per inch calc is interesting, and is also designed to make sure the flue pipes don’t cause a fire hazard so you don’t burn your backside or anything else, which is less likely to happen with the secondary heating idea.

 

[slaine_23]

To take the extreme where a set of pipes sits on the fire and one sits upon the opposite end - the material, closeness to fire, fire size/heat output etc will of course be important or you’ll get an unspeakable blistering.

 

[Astrochicken]

It's interesting as I made a fire pit and bench circle before in a previous house, and even with a decent fire you just couldn't absorb enough heat from the fire to compensate for the cold elsewhere (behind you, the seat, any breeze), so the end result was that it wasn't used that much despite the effort to make it visually pleasant and not cramped. I should have put wind-breaks behind the benches to reflect some of the heat back, and to keep breezes away, but this would have also enclosed the space more.

I've been debating what to do with where I go next, as I'm just looking at moving house in the next 3-6 months, but it feels like a real challenge trying to create a heat-retentive seating space where you can enjoy being outside for more of the year. I'd wondered about some sort of heat exchange system with a semi-closed pipework system around an in-the-ground firepit area, but it's a total guess as to whether it would do anything like enough heat transfer through the pipe / fluid.

My bbq is a ceramic one, and I've got the inside ceramic bowl innards from it, as it split into 2 pieces and I got a warranty replacement, so I'd wondered about using this as a heat sink around a small fire pit. My thinking was that it would hold and radiate heat out from the fire pit itself, although this wouldn't address the heat behind you / under you, just potentially that it would help with the overall warmth of the fire itself (any thoughts on whether this would actually work?). If it won't make a jot of difference then I might as well use it as bulk in the bottom of my wife's garden plant pots.

 

[Glow_worm]

The rough inch per hour is for normal bricks, clay and other similar densities. High density firebrick and night storage heater bricks conduct faster.

Sand isn't a particularly good material for thermal storage- it's full of air pockets so the heat transfer isn't great. Used only because it is cheap and pourable/easy to work with, fine on enormous projects but a bad choice for scaling down.

To take the extreme where a set of pipes sits on the fire and one sits upon the opposite end - the material, closeness to fire, fire size/heat output etc will of course be important or you’ll get a unspeakable blistering.

You'd be surprised just how little heat energy will end up at the other end of the pipe. Stick one end of a scaffold pipe in a bonfire and leave it for a few hours and see how far the heat travels up the pipe.

Pipes or heat exchangers would be the equivalent of filling a bathtub with cold water and lighting a few candles underneath it. If you want a usefully warm bench, you'll need to run the exhaust gases of the fire through the bench, and the way to do that reliably and efficiently is to build a rocket mass heater which is a lot of time and effort, although great fun.

Alternatively, do something crude like a Roman hypocaust- find some thick sheet steel, support it on bricks and put a couple of council slabs on top. Light a small fire underneath and kick it out when the slabs get hot. If you only use it occasionally it might last a few years before the slabs crack/steel rusts away.

Any form of masonry bench outdoors will need to be kept dry, or it will keep you warm but damp every time you use it as the moisture turns to steam.

 

[slaine_23]

Sand isn't a particularly good material for thermal storage- it's full of air pockets so the heat transfer isn't great. Used only because it is cheap and pourable/easy to work with, fine on enormous projects but a bad choice for scaling down

Interesting, given the latest ideas around these thermal batteries, it’s the minimisation of air pockets that helps conductivity, the opposite of insulation.

Stick one end of a scaffold pipe in a bonfire and leave it for a few hours and see how far the heat travels up the pipe

Not something I tried admittedly but it does seem counter to the hot tents concepts have seen elsewhere. Might depend on how hot the air/heat flow needs to be.

The idea is for something in an outside space (covered from rain but not fully enclosed) rather than internal, as per Astrochickens post, in which case should be quick and doesn’t necessarily need to involve long term storage - the heating element is more important than the storage. Suppose one lights the fire, lets it run (for some amount of time), then one sits on it later. Perhaps around a small fire for atmosphere. How to make that efficient? The inch an hr eqn, how does that translate into actual temperature?

 

[Falstaff]

Glow worm is right on this. I was once asked if I would certify one of these for Bldg Regs and declined, would have been difficult if at all possible and on my personal & insurance liablity for evermore. There are mass heating stoves on the market, mainly in artic countries, but they are all for indoors. I doubt that you could generate and transfer enough heat into anything to make it work outside.
You need some form of enclosure, even if un-roofed, possibly even if you are sat on the firebox as suggested. Fire pits only generate radiant heat, anything else just goes straight up.

 

[Falstaff]

In the scaffold example it is narrow enough that the continued hot air input at the bottom will force the now cold air out the top but it'll never get hot.
On larger diameters the cooled air can block anything rising and even either, force it down or, create a warm air/cold air circulation inside the flue and never come out the top, much less get the top warm.

 

[slaine_23]

Okay the “scaffold pipe” idea may be bogus, but the outdoor use remains valid.
This video is light on the build details but from comments it appears the idea is basically a smaller burn barrel

Sourced here - https://permies.com/t/11556/Outdoor-rocket-mass-heater-concerns

 

[slaine_23]

Also - https://permies.com/t/224139/Rocket-Mass-Heater-Pebble-Style