rocket stove mass heater without a chimney, is it possible?

I was looking at the
Rocket Stove Mass Heater page
that is part of the Permaculture Articles by Paul Wheaton

http://www.richsoil.com/rocket-stove-mass-heater.jsp

one thing that struck me is that in the illustrations it shows the exhaust from the stove to be at the same level or Below the intake!

without some heat rising up a chimney, how does it flow? (drawing in fresh air for the fire)

thank you

The "stack effect" or chimney effect is accomplished within the heat riser. The main "force" behind the rocket effect is the difference in temperature between the inside of the burn chamber and the outside air. Even though the heat riser is only approximately four feet tall, it is doing most of the work. That and what I assume to be a siphon effect (haven't figured out the equation yet) of the air being cooled at the barrel is adding some force to the flow.

So to answer your question, you must have a "chimney" or heat riser to accomplish the air movement. Or simply put, no. It will not work without a "chimney".

i read the OP as a question to where the exhaust exits.

nope you dont need a vertical exhaust. you do however need a way to block the wind from pushing air back through the system.

Wow, that is significant.
I understand the concept where hot gasses rise in the center of the heat riser, and then fall as they cool in contact with the outside barrel.
but did not think that would be able to pull enough fresh air in for the fire.

In all liquid siphons, the outlet needs to be lower than the input so that the fluid runs downhill
I think this is because the downhill side weighs more than the uphill side.

I think all other types of stoves require a chimney.

If you add a chimney does it flow too fast and waste heat in the exhaust?

thanks

I'm hoping to have my rocket stove burning in a week or two here--first I will be test firing it with hardly any mass and the exhaust going outside just a few inches from ground level. If it works well I may leave it. If I have draw problems I'll add a 7-8' flue and see how that goes.

http://www.youtube.com/watch?v=3I1BxdqUHBE

13 minute rundown of the design and flow of the rocket mass. The vid uploader tells you how the exhaust travels, simple but cool explanation.

well the depends if you leave enough heat to actually make it draft. if you dont have a higher heat than out side all the exhaust does is cool and block the duct. which in turn makes your stove smoke back into the room. to combat this you simple make the ducting run a bit shorter and have exhaust gasses at least 100 degrees F this gives you lift and for the most part has little effect on the burn.

I am hoping to warm up a draughty stone croft on the west coast of Scotland with a rocket mass heater, ideal as very little wood!, just getting the materials together and have a couple of questions, is the exhaust flue pipe diameter critical to 'draw' I am looking at 6" or 8"?
In the UK old oil drums only come in 45gal, so how critical is the gap between the heat riser a barrel, I am looking at 4" or 6" down the sides with a gap of 1 1/2"at the top? Any help on this welcome There is no store to pop into on the island, best if I get it nearly right at the start.

Mike (crofter!)

Mike Hyatt wrote:I am hoping to warm up a draughty stone croft on the west coast of Scotland with a rocket mass heater, ideal as very little wood!, just getting the materials together and have a couple of questions, is the exhaust flue pipe diameter critical to 'draw' I am looking at 6" or 8"?
In the UK old oil drums only come in 45gal, so how critical is the gap between the heat riser a barrel, I am looking at 4" or 6" down the sides with a gap of 1 1/2"at the top? Any help on this welcome There is no store to pop into on the island, best if I get it nearly right at the start.

Mike (crofter!)

For the 8" riser size(and 8" ducting), you need 2-3" at the top(doing the math for square inches is about 2", but a little wiggle room since the gasses are hitting a flat wall would seem appropriate to me, so I'm planning on 2.5"), and doing the math you only need just under 1" down the sides, but again, a little extra wiggle room there would be fitting. My theory states that if the gasses slow a bit as they are falling it will give them more time to dump more heat into the drum(releasing it into the house), and cool the gas which would make it easier to push through all the ducting, and could move slower through the ducting(more dense).

Thanks, less room needed than I thought.

Mike

One more though--make sure you have enough room for the gas to get into the horizontal portion of the pipe. If you only have 1" and a 8" tube attached straight sideways, that's only HALF of the square inches you need(a 1" gap x the circumference of the 8" pipe(~25" circumference). For the 8" system, you need ~50 square inches throughout the system, with extra at the turns being good.

Sounds good I get the idea, I did also read that in the exhaust tube 'T' type junctions are better than 90deg bends, which also ties in with what you are suggesting.

yep gary is pretty correct ; you actually only have 1/2 inch to play with at the gap and the area around can be way larger but no smaller than an inch. I would say an 8 inch system in your Croft since its a pretty damp place can get a tad chilly. your going to want a dry heat that will heat the mass in the walls. if you are near a peat mine you can use peat as fuel it will need to be dry and your going to have to watch it for the first little bit till the trick gets figured out. the best supply of wood in scotland is the coppices in the hedge rows. lots of fuel for a rocket not much for a box stove. make the heat riser from brick and insulate the hell out of it the moors are no place to need to fiddle with a stove.

Thanks Ernie, does that mean a 6” exhaust duct system would heat up the thermal mass more than the 8”? We are planning to use Paul’s RMC in a box system so when we re-roof the cottage we can move the heater out into a shed for a while.

All peat was used up some time ago on the island but we are busy planting shelter belts with plenty of willow and alder so that we can start coppicing in a few years time and feed the heater, there is enough storm damaged trees at the moment to keep us going in the mean time.
We are hoping that our RMH will generate some interest on the island as most people burn coal at the moment and I am sure we could all be much cosier on self sustaining local wood, but that does mean a lot of tree planting is needed too!

Well your out on Lismoor thats a bit different, I hadnt noticed before.
You can burn Coal in a rocket and its going to burn cleaner than most stoves you currently have. I believe you will want an 8 inch system. Paul's can be made as an 8 inch with a little modification to the box size. I would keep to that. I remember my very short time in Scotland and it was drizzle and windy and cooollddd; having spent years in the arctic it wasn't too bad but i could see where it could get pretty miserable with little or no heat.
What is the Croft made of? how old is it? What kind of heat does it currently have?

Its traditional thick stone walls and probably thatched in the past now corrugated iron, small and damp just now but we will be insulating it over time. At the moment there are 2 open fires at each end which are eating through our meagre wood supplies, I'll try and upload a couple of photos to give you a picture of the place.

that was the picture i had in my head. As i understand things cold and damp is pretty much the norn till you get the place deeply warmed up (having never lived in one I dont know) I would say that you need to get the thing heated good and warm so the stone begins doing its job. You Croft is built to keep fire out of the house cause it was all folks had and it was always going. so two ways to do this. The first way is to build a Rumford fireplace inside the fireplaces. The Rumford is going to cut your fuel cost down because its going to reflect heat out of the fireplace and into the room. its fast and easy to build and works a treat. that would take care of your immediate need of heat that works for you. Second way is of course the go buy a bullet heater and light it off in the house for several hours a day. Moving on: Pauls rocket may work well for you but if this is a short term solution you might look at a pocket rocket in as central of space as you can put it. I dont know your situation but i do tend to go with the cheapest methods first. Rumford takes a little work with brick or stone to make a simple one that will do the job, pocket rocket takes scrap and can be built in a day by a suburban princess. (I know I have taught them to build pocket rockets). the pocket rocket is happy burning scrap and it will burn Pithy/ rotten wood. the rumford will burn anything you can stuff into a normal fireplace and it will reflect the heat back into the room. prepare to adjust it a little so there is enough heat in the smoke to rise up the chimney.

there are lots of places to get wood scrap if you look around. scrounging wood is a good thing in the cities. so take a truck when you go and fill it with a little scrounge wood. I have always been amazed at how much wood you can collect in any modern city. Any ways that is my comments.
Its a lovely little house and hope your relationship with it is a long one.

Ernie, Thanks for the great advice, I will definitely give the suggestions a try and yes simple and low cost is definitely the way I want to start, and the enjoyment is going to come trying these ideas out. As you say if we can scrounge supplies and burn efficiently then we should be ok. We will keep you informed of progress and of course if any Permies are heading in this direction they would be welcome to visit although much like camping at the moment!

Talking to people about flues and general durability of the steel parts of the rocket stove, is there any information/advice on life span of the different elements and if i was going to invest in better quality or thicker quage metal where is the best place to put that money. I am thinking about pipes that might be difficult to get at the future if parts are built in.

I have a friend who could make up a heat riser out of solid 5mm guage steel, could this be better than 1mm or 0.6mm flue liner? Would it need the same double skin with insulation or would that effect the downward cooling of the gasses?

The best place i can see to spend money is in the insulation and brick. better quality insulation allows you to have more space in the stove and cuts down the foot print of the stove core. the second area is in the through ceiling and chimney this is where you will be spending lots of cash. If you want to spend money on pipe and duct it should be SS for the bench duct. If you are wanting to do a good job of it make the heat riser out of fire brick (half bricks are real nice) and use rock-wool insulation to wrap the heat riser and burn tunnel to the manifold. This puts the best materials in the most critical areas.

Ernie, thanks again for the advice.