Four Rocket Mass Heater Questions

Dear All,

Greetings from the North Cascades.

I have three questions regarding people's rocket mass heaters:

Have anyone designed one that uses outside combustion air?

Is 6 inches is a good minimum diameter for your exhaust stack?

Is 2 inches clearance between the top of the riser and the barrel covering it a good dimension?

Has anyone built a hybrid that works with a water jacket?

Thanks and Happy Winter,

Ruwenzori

Ruwenzori wrote:
Dear All,

Greetings from the North Cascades.

I have three questions regarding people's rocket mass heaters:

Have anyone designed one that uses outside combustion air?

All of them use "outside" combustion air   if it comes from cracks in the wall, open windows or a pipe just for that purpose. Using a pipe just for that purpose can be a safety hazard. The pipe should exit into the living space close to the heat not directly into the heater. This ensures the living space is not negative pressure to any part of the heater sucking flue gas into the living space... I know the idea is not ever to have a leak... and I build and test for this in my design... I just want to be sure...

<soapbox>
My personal opinion is not to use outside air from a pipe as we keep some windows open at least some all year around. I am grateful that my RMH can help pull fresh air into my dwelling for me to breath. In my opinion, it is more important for me to have good air to breath than not having cool drafts. Just a quick note on building codes as they relate... I have read that those who use lab rats make sure their rats have two or three times as many air exchanges per hour as the people taking care of the rats. They want healthy rats... I guess the people don't matter. If the rats need so much fresh air to be healthy, maybe I need a lot more than the building code suggests. Instead of using air to keep the house warm it is better (in a leaky but healthy house) to use mass.
</soapbox>

Is 6 inches is a good minimum diameter for your exhaust stack?

6 inches is a good minimum internal CSA for a RMH. The air intake should actually be 1/4 to 1/3 of that and the exhaust flue can be as low as half the system CSA in my tests.

Is 2 inches clearance between the top of the riser and the barrel covering it a good dimension?

Remember, constant CSA. The 6 inch system has a CSA of 28sqin.  So:

28/(3.14x6) = about 1.5 inch gap. In practice, just a bit more seems to work well. Paul's was 1.75 inch and that has worked well for me too. 2inch (or just over) would work for an 8 inch system with a 50sqin CSA.

Has anyone built a hybrid that works with a water jacket?

Yes... I have seen it in a video. I am not sure how well it works though, as I have never experimented with it. The normal warning applies... hot water expands... when it becomes steam it expands very quick and has lots of pressure. There have been a number of explosions from this (not talking RMH here) and hot water/steam hurts... think hospital trip. There are also systems that have run safely for many years, mostly open to the air, but some with pressure too.

Has anyone tried using an old vehicle radiator as a water jacket?

Lolly K wrote:Has anyone tried using an old vehicle radiator as a water jacket?

I have not heard of anyone trying that. My concern would be that within seconds of any part of the radiator having no water in it... it would leak. The older ones are soldered together so if it ever went dry even for a short while, the solder would melt. The newer ones have plastic tanks. Most vehicles have a coolant run temp around 180F. Even a wood stove exhaust temp is higher than that.

However, once the water is hot, they do work well for using that heat to warm the room up.

Ruwenzori McCoy wrote:

Has anyone built a hybrid that works with a water jacket?

This video clip from YouTube shows Geoff Lawton and a rocket mass stove water heater.
Flashtoons is a very good YouTube channel, everyone should subscribe to it.

I tried to put the url in I don't know why it didn't show.
http://www.youtube.com/watch?v=YTnr8ua54Uw&feature=relmfu

I had an idea for using a couple car radiators to heat a space, but I haven't had a chance to try it yet. The basic concept was to have a 40 gal or larger water heater tank, preferably with side taps, and run a heat exchange coil from an RMH or a solar panel, then run additional lines to the radiator(s). This would be run entirely through thermosiphoning, so the water tank would need to be above the heating coil, and the radiator would need to be above the tank. Hook up a small PV solar panel and a 12v battery to the radiators fan and you get forced air. The tank has a TPR valve, so overpressure shouldn't be an issue, and pop the radiator cap to add more water or burp the system. I would be inclined to add a small pump or 2 just to give peace of mind and also not need to be too concerned with placement of each component.

I'm not 100% sure about the whole thermosiphon thing. I know going from the heat exchanger / solar panel the water will circulate if the tank is above the heat sorce as the cold water from the bottom of the tank has more density than the hot water from the panel and pushes itself through the system. I would think going to the radiator would be similar as long as there were no air bubbles in the system. It works in my head, but I'd still like to see it in practice.

Dear Len,

Thank you for your input. Have you used a 4 inch diameter chimney?

Does CSA stand for "cross-sectional area?"

Am I correct in understanding the the CSA of the vertical wood feed should be 1/3 to 1/4 of the CSA of the insulated chimney, and that the exhaust pipe leaving the riser can be 1/2 of the insulated chimney?

Finally do you have guidance on ratios of the diameter of the chimney to the diameter of the riser?

Thanks for your time and attention.

Best,

Ruwenzori

Ruwenzori McCoy wrote:Dear Len,

Thank you for your input. Have you used a 4 inch diameter chimney?

I intend to. I have tested blocking half of a six inch exhaust.

Does CSA stand for "cross-sectional area?"

depends on the context, but in the case of a RMH sizes discussion yes it means cross-sectional area, not Canadian standards association(like UL).

Am I correct in understanding the the CSA of the vertical wood feed should be 1/3 to 1/4 of the CSA of the insulated chimney, and that the exhaust pipe leaving the riser can be 1/2 of the insulated chimney?

No. a vertical feed should be same CSA as the riser. the wood is expected to block 2/3 to 3/4 of the air intake area. I am using a sealed wood feed and separate air intake... so things is not normal. I have lots of mass around the barrel so the flue gas is cooler exiting the barrel. At 1/2 riser CSA it still draws. I have heard the chimney should be able to be 1/2 riser CSA after the mass. The exit from the riser itself into the barrel should be at least riser CSA and just a bit more seems to work better. On a 6 inch riser the riser exit would be 1.5 inch gap, but most people find 1.75 works better.

Finally do you have guidance on ratios of the diameter of the chimney to the diameter of the riser?

Most systems that I know of use the same CSA as the riser for all plumbing past the riser including the chimney. Mine will be one of the first test cases for something smaller that I am aware of. However, my knowledge is a fairly small subset of what is known. Some people have built 100s. The common guidance is to first build it outside (sans cob) to make sure everything flows well. then install inside. I have one more test outside on mine with a bench mass and a short (10ft or so) 4inch CSA chimney. Then it will get tried inside. A CO alarm is always recommended for any fuel burning indoor appliance.

Dear Len,

Thank you again,

More q's should you have the patience:

Are you calculating the riser CSA by subtracting the CSA of the insulated chimney from the CSA computed with the internal diameter of the riser?

Are you exhausting your RMH inside or outside of the house?

Are you aware of any materials that would serve as a good thermal mass, but are lighter and more appropriate for a mobile RMH.

Thanks Again!

Ruwenzori

Ruwenzori McCoy wrote:
Are you calculating the riser CSA by subtracting the CSA of the insulated chimney from the CSA computed with the internal diameter of the riser?

Riser csa = area of the circle or the flue gas path or inner pipe. CSA at any point is the area the flue gas has to pass through at that point.

Are you exhausting your RMH inside or outside of the house?

Outside. All pictures so far show the unit running outside so the exhaust doesn't go to a chimney. I do not want to fill up my house with even the products of clean combustion (CO2 and H2O) let alone the other by-products.

Are you aware of any materials that would serve as a good thermal mass, but are lighter and more appropriate for a mobile RMH.

Water.... and all its problems Or something the phase changes at the right temp.

Kia Tikaboo : you can look into Glaubers salts, (try Wikipedia ) its a steep learning curve but the stuff is so cheap they use it as filler in laundry soap. properly set up it could be a remove-able part of your thermal mass. lift a self contained amount of it out of your bench and take it with you to be a personal heater at a separate location !
Pyro ---Allen

allen lumley wrote: Kia Tikaboo : you can look into Glaubers salts, (try Wikipedia ) its a steep learning curve but the stuff is so cheap they use it as filler in laundry soap. properly set up it could be a remove-able part of your thermal mass. lift a self contained amount of it out of your bench and take it with you to be a personal heater at a separate location !
Pyro ---Allen

I would actually prefer something with a higher phase change temperature. Sort of double-ish. In the range of 150F to 180F or so. Glaubers salts phase change at 90F which is good for solar projects as even on less sunny days 100F is often achievable and 140F is the max most are designed for. However, in a wood burning situation, the normal max temp should be able to be much higher, at least in some parts of the system. The temperature of the phase change is where the mass will sit longest and the higher that is the more useful it can be... and the longer it can last. I would happily use tin at 183C (361F) and put an insulting layer around it to keep the outside temperature at 140F or so to heat the room.... Or to use without insulation to cook on or heat an oven.

I am not sure how much heat per pound could be stored at what temperatures though.

Len : if you know a good source for cheap tin,go for it, your system would need to be fume-proof but other wise close to ideal You may already be aware of the use of molten tin in the manufacture of float glass where they keep the tin bath in a nitrogen atmosphere under positive pressure to deal with Tin Oxides ! I can not see a reason why a thermo-syphoning, system would be impossible, and I don't think there is much change in volume with phase change ! My answer was for Kia Tikaboo who will never get an ideal material for use in a thermal mass that will make a portable Rocket Mass Heater, but a R.S.M.Heater, can make use of a phase changing material for portable heat ! Some where i have basic BTU per # info, it will take me a while to find ! Pyro-Al

I had this passed on to me printed up into a loose leaf binder and slip cover- but it was never in it's cover when life got messy, someday i will have to have a new copy !

www.engineeringtoolbox.com/thermal-conductivity-d 429.html

www.engineeringtoolbox.com/specific-heat-solids-d 154.html

allen lumley wrote:Len : if you know a good source for cheap tin,go for it, your system would need to be fume-proof but other wise close to ideal You may already be aware of the use of molten tin in the manufacture of float glass where they keep the tin bath in a nitrogen atmosphere under positive pressure to deal with Tin Oxides ! I can not see a reason why a thermo-syphoning, system would be impossible, and I don't think there is much change in volume with phase change ! My answer was for Kia Tikaboo who will never get an ideal material for use in a thermal mass that will make a portable Rocket Mass Heater, but a R.S.M.Heater, can make use of a phase changing material for portable heat ! Some where i have basic BTU per # info, it will take me a while to find ! Pyro-Al

I figured on sealing the tin chamber, not just for fumes but for safety too. Molten tin is hot enough to cause really bad burns if it spills and maybe hot enough for some things to reach flash point too. Volume change at phase change seems not to be a problem. Although the volume changes as much as 27% in the change from white tin to gray, the formation of gray tin seems to only happen on the surface and so a sealed container would stop that. I have seen a sealed container used as heat storage for a solar cooking setup that uses a large parabolic reflector (3meter) and allows cooking after the sun has set. In their case they use a lot of insulation because they what to keep "all" of the heat till needed. They have an insulated cover for the cooking surface as well.

For a heater... Lets call this a "Rocket Phase Change Heater" (RPCH) I have been thinking about this for over a year actually and have mentioned before. The idea is that this would be a heater for a smaller area than a RMH, say 100sqft or less such as a small cabin or boat. Places where it needs to be small. The flue CSA would be small as well and so a long flue path to heat the PCM (phase change material) would not be practical. This would in fact be based of the rocket cook stoves where the flue path is mostly a simple L shape. I am thinking a 2 or 3 inch CSA would do the job depending on the heat required, so the whole thing might be 12 to 18 inches in diameter and maybe 3 feet tall. It might look like a pot belly stove in shape. An insulating plug on top could be removed to cook on with pots that fit exactly in the hole. The tin container in the center could be removed and the inside would make an oven with the tin container on top again. The tin container could be put in a portable insulator and the heat used elsewhere. In fact a second tin container could then be heated for other use. I have (obviously) a picture in my mind I should make it into a picture so others can see. There are actually two tin containers (before I started typing this message there was only one) the removable one and one that is not removable. The operator would be responsible for sealing the flue path after the burn so as not to lose heat through the flue... this means the fire has to be completely out. Perhaps a removable ash-pit that can be set out side would be the safest idea.

Anyway, when I have time (and money) I will try building RPCH

Len Ovens : - thinking off of the top of my head there all many types of insulation that protected from abrasion can be effective in thin layers, With a small rocket stove co-located with the boats propulsive system for safety, weight distribution C.of G. common intake/ separate exhaust, your thermal battery would be an almost ideal material for a weighted keel !?!!

As an exersize, or flight of fancy this might appeal to Ernie Wisner who is a Steward in these pages ! Pyro-Al

allen lumley wrote: Len Ovens : - thinking off of the top of my head there all many types of insulation that protected from abrasion can be effective in thin layers, With a small rocket stove co-located with the boats propulsive system for safety, weight distribution C.of G. common intake/ separate exhaust, your thermal battery would be an almost ideal material for a weighted keel !?!!

As an exersize, or flight of fancy this might appeal to Ernie Wisner who is a Steward in these pages ! Pyro-Al

The insulation doesn't have to be thick, I think, as I do want heat to pass through. It is amazing what a few layers of wrinkled foil will do as a heat shield (400F to 300F in the test I did). An insulating layer of more than one kind of material may be more effective. Outside the insulation should be sheet metal for even radiation of the little bit of heat that does make it that far. The needed R value could probably be calculated knowing the output heat and temperature and the room temperature and the expected outside surface temperature... But in my case it will probably be figured out by experimentation (read trial and error).

While the idea of putting the unit in the engine room has some pluses... personally, I don't want to sit in the engine room to keep warm My feeling is that this would be much safer than a pot bellied stove or diesel heater already commonly used in boat cabins. There are two reasons for this:

1) the unit would be running less time so there would be less likelihood of starting it and forgetting it while it burns unattended.
2) The surface temperature is designed to be lower than the flash point the cabin walls etc. It would also be less of a burn hazard to the occupants of a small space... possibly crowded. (I haven't seen a boat cabin that is not somewhat tight)

Using the keel to store heat is probably a pipe dream... Lots of boats use this area for cooling because the ocean makes a great heat sink. When an engine is running, the very best source of heat is the coolant. Just as for a car cabin. The heat is free or wasted if not used. So a wood (or diesel or propane) heater is only of use when the engine is not being used. In boats that normally don't use power... there is no engine room anyway. Often an outboard is used for getting in and out of the marina (in places like Vancouver where it is law that sail power is not allowed as power in the harbour). The idea of using a lower mass phase change heat store is to keep things lighter... the mass could probably be offset by the house batteries, which although heavy should not be in the bilge where they might be flooded by a wave breaking over the stern and down the hatch.

Just some thoughts...

Len Ovens : I didn't describe my thoughts clearly, I was thinking that putting the weight down low as in a weighted keel, it only needed protection from water temps, i was thinking that the lightness of the insulation would be more than off-set by the weight of the weighted keel , co-locating all heat sources seemed a plus but again this is over my head, my most nautical happenstance is often sleeping in a hammock ! pyro-maticly yours A.L.

Maybe you could contact these people to find out what kind of phase change material they use.
http://www.joulies.com/
Closer to the same temperature, glad I remembered the kickstarter project.

hi all
got hold of a 23" by 28" L shaped metal tube, that i intend to work up as a heat riser. i know it is unusable in this shape, but i would like to keep the elbow section, if possible.
http://img145.imageshack.us/img145/4669/riserpipe.jpg
can somebody tell, how much of the burn channel i can replace with metal tube? i know the vertical section between feed hole and heat riser needs to be with the smallest cross section of all. so i intend to fill up the vertical channel to 1/3 with clay, flush with the brick section of the burning channel. of course the entire burning channel+riser will be embedded into perlite-clay.
i also think on an oil dripping appliance, that would let fall spent oil by rate of 1-2 drops/min into the throat of the heat riser.
for that i want to make a tapered socket at the right position into the top of the 55 gal barrel.

Kia Tikaboo : -if it seems like your thread has been captured by pyro-heads and ran off with , yah, it happens all the time, just repost your thread with more questions and updates, as much as we want to see our profound thoughts in print we All want to here from you ! Pyro-maticly yours , Allen l.

Richard Gurry wrote:Maybe you could contact these people to find out what kind of phase change material they use.
http://www.joulies.com/
Closer to the same temperature, glad I remembered the kickstarter project.

They claim that the phase change material is something that qualifies as food. It phase changes at 140F which would mean it could be used uninsulated.... but, one would have to be careful where in the system it was used. The idea would be to keep it at a lower temperature than 212F as I would guess it contains water (remember it is expected to have boiling water poured over it). Just my guess, but certainly once food gets much above boiling the word they use for it is "burnt".

The other reason to keep it at a lower temperature is that it heats more efficiently. If the heat source is hotter then the differential from the heat source to outside the building increases and the travel of heat across the wall insulation increases too. This is one of the reasons a mass style heater is best. The outside temperature of a mass heater is generally under 200F instead of the 450F of a steal wood stove even run at a smolder. I think this PCM (at 140F) would be good for a mid size heater where there was some separation from the burn area to the PCM itself. Brick could be used closer to the flame.

The whole thing is a balancing act. Size, temperature, materials, etc. to get the best use of the heat generated. In all cases we want the fastest burn possible and yet let as little of the heat get out the flue and not over heat the room by storing some of the heat for later when the flame goes out. For safety we want to be able to store heat for at least 8 hours (more is better and possible on a larger system). That way the burn can happen when someone is awake/close by. It should not be necessary to leave it burn while sleeping or while out and about. We want the surface temperature to be low both for efficiency and so the possibility of skin burns is reduced.

Hi, the phase change material referenced above is probably sodium acetate; inexpensive food grade is the ingredient in re-usable hand warmers etc. see wikipedia

Andor Horvath wrote:Hi, the phase change material referenced above is probably sodium acetate; inexpensive food grade is the ingredient in re-usable hand warmers etc. see wikipedia

That could be, melts at 136F or so. Exactly 140F is a bit too round to be true. Raises the boiling point to 252F, so have to be careful of the maximum temperature it is exposed to. I would not want to have it sitting on top of the riser, for example.... but as part of a bench it could work very well.