Rocket Stove Project need of advice

Hello,

I am going to build a rocket stove mass heater in a small summer house I plan to build.

I am a novice hobby builder so I would really appreciate it if someone could answer a few questions based on their experiences. I will naturally keep the thread updated with my build for anyone's future reference.

The area has a moderate risk of water accumulating as it is on a relatively low point and its a rainy UK area. For this reason I was thinking rubble then paving stones then celotex insulation and wooden floor. The wood frame would be mounted on a double but single brick height perimeter of bricks. Where the rocket stove would be (far corner towards the tree) no paving stones would be placed but rubble then brick as I have not so many. My question here is where to place the damp coarse for the mass heater and the floor in general?
Would you recommend concreting the area'
I would of course love earthen floors ect but I scared of humidity, insulation and stability and unlike about the rocket stove I haven't read much about the subject (great book Erica&Ernie).

Regarding the rocket stove I was thinking of using old night storage heater blocks and the fibre glass wool wrapped round the heat riser and wire mesh holding it?. Could also use an old insulated stainless steel flue pipe but have read its not so optimal ?

About the design of the thermal mass I was thinking that because the barrel is to be in the far corner, I would have the flue come allong the from for 3 feet then a T section with a clear out blank and then instead of rigid: pipe using chimney flue liner to curve back to the corner and then after going round the outside of the barrel base create a slight zig zag (as it is the only single pipe along the far side for about 5 feet and then up out of the cob with another T junction and eventually through the roof and then a 30º angle and another 3 feet or so of pipe so that it points away from the tree branches.
What is peoples experience with flexible chimney liner? How about with only one pipe going allog the far side thermal seat? Would a slight zigzag be worth considering or too much drag? Ahould I forget the additional 3 feet along the front and simply build a one way bench along the far side? (must be one way so that when the flue goes up it can be cleared from the tree with an angle. Would this angle be a problem for drag?

All in all Im excited and hoping I can find clay to build with in the garden.


I attach pictures of the foundations at work (havnt finished digging it up yet so some of the paving stones are there but will be lowered slightly so that floor is low enough that cabin doors open inside the extension and placed on rubble. Also attach a small diagram of the options for how the flue leaves the barrel in to the internal mass. Bear in mind the pc camera has switched it round so left is right and vice versa.

Final key question is whether to build the frame or the rocket & mass first.

Any suggestions that potentially could be time saving and of general improvement to build quality are very much appreciated.

Kind regards
MH

A few comments on your proposed structure, then I will concentrate on the heater design.

If your location is low and potentially subject to flooding of any kind, I think an earthen floor would be a disaster waiting to happen, regardless of constant humidity. A suspended wood floor sounds like a much better idea, though I would not lay celotex (exactly what sort of insulation is that? It is a brand name that could refer to more than one material here.) and wood flooring directly on the pavers. I would use the pavers and bricks to support something like 2x4 joists off the ground with a few inches at least of air space beneath, and ventilation so the space doesn't get saturated with humidity. I would put insulation between the joists and flooring on top of them. If that means you have to dig down farther, so be it. Alternatively, you could use the pavers as your floor surface with a good layer of gravel and a vapor barrier beneath them so the paver surface can dry out. A layer of water-impervious and strong insulation (maybe that is the variety of celotex you were speaking of) would isolate the pavers well enough for a summer place. You want a good thickness of gravel or insulating brick if you can get it between the RMH base and the vapor barrier below it.

For the heater, I understand that you plan to have the heater in the corner nearest the trees in the photo. You describe a possible short leg along the end wall farthest from the metal doors, then going along the back wall and up around the middle of the back wall. This puts the chimneyabout as close to the tree as you can get. A slanted chimney is a problem waiting to happen, as any wind from the wrong direction will try to blow the exhaust back down the chimney. The chimney needs to rise straight up. If you alter the duct in the mass to go along the back wall 5' or so, double back around the barrel in the corner, and rise near the middle of the end wall, you will have the chimney naturally about as far as you can get from the trees.

You don't mention your planned system size, but from context it seems your climate is mild and the space is small, so a 6" system sounds right. The recommended maximum horizontal length for these is about 35', minus 5' for each 90 degree elbow. So you would have 4' + 6' + 5' of duct, plus three 5' equivalent elbows, for a total of 30' equivalent - seeming quite reasonable.

Flexible corrugated ducting has significantly more drag than smooth ducting, so a serpentine duct through the mass would be a likely problem for draft. Straight lengths with elbows where necessary will work much better. Running the first leg of duct along the longer wall and doubling back will maximize your heat storage/extraction.

The dense bricks you mention for the heat riser will probably be durable (I haven't heard reports of using them in that location), though their mass will make it take a bit longer for the riser to come up to operating temperature. Fiberglass will melt at relatively modest temperatures and may not work well as the riser insulation, but mineral wool ("Roxul" or rock wool) has been reported to last in that application. The wire binder should work fine. The insulated stainless steel flue pipe would burn out in a relatively short time, and I would only use it as a stopgap if you really needed immediate function and couldn't get anything more durable.

On reflection, the possibility of water accumulation really makes me lean toward the idea of using the pavers as the floor surface, with a thick layer of compacted gravel (to below your local frost depth if any) to stabilize it. The amount of the footprint covered by RMH mass may mean that you have enough pavers to cover the remainder, as long as you have enough bricks to support all the walls.

On the thermodynamic side I can make two small contributions.

The angled riser AKA chimney: a chimney's draft is related to its average temperature (of that section) and its vertical height, even if it is on an angle. Your system consists of several sections with different drafts, at least one of which is negative (being a Rocket Mass Heater). There is a draft calculator in the library at bioenergylists.org (search for draft calculator nigel). You can use it to determine the sum of five or eight sections, can't recall which. At least five.


Second, the 'corrugated' flexible pipe sections which you can use for elbows: the flow 'restrictions' which these represent is not really because of the bumpiness. It is that they have a more effective heat transfer rate (higher surface area) than a straight smooth surface. In short, they drop the temperature more, reducing the heat available at the exit thus reducing the net draft on the system because it will be cooler at the end.

You have two main sources of draft: the initial internal chimney (use bricks) and the final exhaust. The sum of these is your total draft, pretty much. You should measure the temperatures involved at the end to prove you have 'some draft' at the late fire stage when things are the most dangerous. The heat stored in the bricks of the internal riser help greatly at that time.

Generally speaking feeding into the leg of a Tee is a bad idea, rather use a branch Y + 45 degree elbow on one side then a 90 degree elbow for the other direction, which doesn't have to go directly opposite because you can rotate it on its axis. The combination has less flow resistance that a tee heading off in two directions.

In that same draft calculator there is a section on the left which you can use to 'burn' your fuel and it will tell you the gas flow rate if you know the excess air level (separate discussion). The flow rate can be used to calculate the total flow resistance of the proposed pipe network using conventional HVAC methods. The bottom line is that below about 10 feet per second 'resistance' to flow is not significant, and is far outweighed by changes in total draft caused by small changes in temperature (as can quickly be seen using the draft calculator) and chimney height.  

There are municipal rules for the exit height of the pipe above the roofline for very good safety reasons (to do with wind).

It is common here in Central Asia to wrap the outside portion of the chimney in insulation to improve/conserve the draft at the end.

Good luck
Crispin in Bishkek

Miguel, What is the diameter of the barrel you plan to use?  My 55-gallon drum rocket stove, which is surrounding the burn chimney, gets to over 500 degf and I have actually measured temperatures greater than the range in my little infra red laser temperature sensor.  In such a small space you are designing, the size of the barrel and the heat output would drive you out of the space if allowed to get really hot.  So, heating such a small space would only require a very small rocket stove, usually not recommended in articles that I have read which say that 6-8 inch piping can be used.  Whatever you do, and whatever size of your rocket stove, make sure the walls in the corner are insulated with a metal "air wall" that has about 1 inch to 2 inches of clearance between the wall and the metal, also at the bottom of the air wall.  This allows air in to circulate by convection, thereby keeping the building wall cooler.  Also the wall behind the barrel should be fireproof.  If the system is designed properly the exit chimney temperature will be quite low in use, UNLESS THERE IS A PROBLEM, the normal 100-120 degf exit temperature could be compromised causing a fire if you do not install the chimney feed through properly with sufficient clearance and non flammable materials to handle such emergency conditions [like a chimney fire, for example].  My rocket mass heater is in a non insulated, translucent plastic, greenhouse that is 10 feet X 20 feet and 9.5 feet high at the apex.  The mass is a 17 foot X 2 foot high x 2 foot wide rock wall of sedimentary rock [bluestone] with two courses of 8 inch pipe and 2 tees for the return and clean outs.  The chimney exits and is about 12 feet tall.  Therefore the total pipe length is about 46 feet.  It works OK even in February when we are starting seedlings.  As further assurance to keep our plants warm, we use row cover and a 1KW electric heater at night with the thermostat turned to the lowest setting just above freezing.  I have an air wall over my barrel and do not experience any overheating problems even at 500 degf.

While the Y splitter configuration would indeed cause less flow restriction than a sharp tee, there is no reason to split the flow in a RMH flue anyway. It is likely to cause imbalanced flow and heating effects unless tweaked just right, and is recommended against by the most experienced practitioners. Not to mention, the OP was not planning on doing that anyway