heat exchanger metal for RMH

Hi all.
Like most of you, I share your frustration with trying to live better despite regulations and insurances that prefer we don't. And so, I can't legally build a RMH in my home without spending $2000 plus on a certified masonry heater builder. Apparently any old wood stove is ok if it's EPA, because they care about the environment if it's in the house, but if it's outside their environmental value dissipate quickly and I can build anything i like 10 feet from the house?? Anyhow, *rolls eyes* i'm trying to build a rocket stove batch type heater with pipes going through it as a heat exchange that will vent hot air into the house from 10 feet.

Lots of design issues to discuss and things to try for sure, but my main one is about the toxic fumes most metals give off when heated to high temps. Galvanized pipe is out of the question. Copper? I've heard mixed information. Mostly because copper is used for hot water, but can it withstand the heat inside a rocket stove? Aluminum will melt. So, i'm currently considering just clay pipe inside a big clay bell and only attaching the aluminum pipe onto the outside of the clay where it is colder.

Ok, I realize this explaination is just confusing. i'm going to just draw a picture. What i'm looking for is something for the green pipe. or maybe just make it a void in the clay.

Main question: best metal if any to use inside a rocket stove that won't give off fumes. remember this is for venting into the home not a chimney where it will go "up in smoke"

Secondary question is about wool and clay. i've heard good R-value comes from straw or paper clay rocket stoves. I found one source that used wool in the clay, but lost it. Will wool just burn out like straw and paper or does it provide extra insulation. I like wool, but i'm guessing it will just burn out and some type of rock, like rockwool, vermiculite or perilite is the only thing that can withstand the heat.

Hi, I have the same situation , so I have given this some thought.
You are moving heat in the form of  hot air, but one of the benefits of a rocket mass heater is the mass.
I would recommend building a rocket exhausting through a  bench or bell mass,inside an insulated shed,and pulling heated air from the space(plenum) above the mass, to the lowest occupied space in your home.
The heat inside the shed will be fine for the most basic materials, think anything you would use for dryer duct.

If you need/want to run the duct inside the mass itself, again recall that the solid mass only reaches certain temperatures.
Galvanized off gasses at 392 °F (200 °C), so bury it in your cob or cement and I think it will be fine.
Expanding aluminum dryer duct wont hold much weight, so something that sets up hard will be preferable.
If you really need to expose the  hot air ducting directly to the exhaust gasses, I would try actual stove pipe, or perhaps brand new muffler and exhaust parts.

I like that idea. Perhaps instead of putting one pipe in the bell, I should lay 10 pipes on top of the bell and then cover them with insulation. Put the whole thing inside a green house. Yup. I like it. And it's a lot easier then trying to create pipe like voids in a long section of clay.

If you want to move warm air, I would recommend building an enclosure all around your mass, insulating that very well outside, and running the duct to your house; also a similar duct low from the house back to the base of the mass enclosure, You will likely need to situate the whole heater shed considerably lower than the house if you want heat to flow without added fans.

I think heating water or antifreeze would be more practical and work better, requiring radiators or convectors inside the house, but much more flexible in layout and less space-gobbling. I would use a large unpressurized tank sunk into the mass/bell, with a coil of copper pipe submerged in the tank and circulating to the house. This will avoid the risk of pipes bursting from steam explosions.

I definately agree that water here would be the best way to transport heat. However, I'm killing 2 politicians with one stone here in that I need positive pressure inside the house for another reason so I need to be pushing warm air into the house at a low rate via fans anyhow.

Maybe you could use water as the main heat transfer mechanism, and have an enclosure with fan to push what heat escapes the mass into the house? Much less air volume needed, and if positive pressure is required, no need for return ducts...

Well.. the heat needs to move 10 ft before it's inside. I'm thinking of making the bell a good long 6ft leaving only 4 ft of well insulated pipe to brave the outdoors. I'm not sure how close can build the enclosure aka greenhouse to the home. I doubt i'll be able to build it right against the outside of the home. That's is technically an extension and would put my RMH inside the home legally. But I believe i can get close so the pipe will not have to be exposed too much.
The issue with using water to bring the heat up to the exterior wall is that I would then need to add air at that point not giving much time for the heat transfer to take place and it would be in a place more exposed to climate. This was my reason for the long bell. I'm no sure but perhaps you are correct that a radiator basically in the window with a fan might be most effective. I can certainly try both. that said if I did use water, it would be copper pipe. So, i'd be back to the same question. Doesn't copper give off nasty fumes when heated to rocket core temps?

I'm still liking the 10 x 1/2" air pipes layed on top of the bell and covered with something simple like rockwool.  

You can't have copper exposed to rocket core temps - it won't give off fumes as such, but will soften and melt; any joints inside the core would be destroyed at much lower temperatures. If the copper is carrying water, the water would flash to steam and explode with any hiccup in flow rate.

I would not just run water to the exterior wall, I would run it to all the places in the house that want warmth, with radiators at appropriate places. Or is it just one big room you want to heat? Running piping in the floor will give gentle radiative warmth, as long as your building construction is compatible.

Air ducting sufficient to heat your house will be pretty bulky, and insulating the stretch between shed and house would make it even bulkier, and may be considered by an overzealous inspector as connecting shed to house and knocking you out of compliance. After all, if a lot of air can move into the house, so can flames (the fact that you can't get flames into the duct if it is properly built is irrelevant.)

Has anyone used hot water from a RMH or RS to a air heat ex-changer?  This could solve the remove RMH from the house and you could put insulated pipe under ground and run it into the house that way

Would this water to air heat ex-changer work well inside a HVAC return air channel?  

You could use the HVAC air handler to push the warm air around the house.