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Water to air heat exchanger fed from my mass heater?

 
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I am wanting to add a water line into my RMH that is tied to a water to air heat exchanger in my furnace to circulate the heat throughout my house.  Mind you my mass heater will be in my basement next to my furnace area. The basement is finished and has 2" styrofoam insulation on the ceiling   Comments?  Suggestions? Help?  
 
gardener
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Hi Kevin;  Welcome to Permies!
Sounds like you have caught the RMH bug! Oh No !  Nothing you can do now, but build one!

Well you have big plans, let me give you my thoughts on them.
First , do you have a copy of the Rocket mass heater builder guide?  Readily available from Amazon. Highly recommended for first time builders.

Next I'll start easy.  Yes, pellets in small amounts can be burned in a rmh. Add to many at a time and you will cut off the air supply. Dry wood is  preferred.

Now you will hear this from many people.  Rmh's in basement's are problematical,they are not recommended as a first time build, if at all.  
First issue is, are you living / hanging in the basement ? If not it means repeated trips up and down stairs.
Next, air flow from the basement thru the home is usually poor. Things like exhaust fans / open windows upstairs can disrupt the flow of air into the rmh.
Without heat registers not much warm air will reach the upstairs.
All that said, folks have built in their basement successfully.  You could as well. I'm just trying to prepare you for some possible problems you might encounter.

Now on to the most important thing.
Getting hot water from a rmh has the potential to explode!  Water will flash to steam in a second ... expand and violently blow out of your pipes with scalding hot steam...  This is known as "Boom Squish"
You don't want to be around when that happens.
There are ways to safely heat water with a rocket stove.  Could it be plumbed into your furnace? Maybe.
Would it be hot enough to be used as your wanting ?  Average standing temp of a mass is around 100 F, I don't think that's hot enough for your needs.

I hope I haven't bummed you out about this. RMH's are incredible / awesome wood burners ... everyone should have one!
But there is a limit on what you can reasonably expect them to do.

 
 
Kevin Gouty
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Thanks for the info.....brings a lot in to question.  I guess basically my thoughts were using the RMH in place of a standard outside boiler.  I have a friend that has an outside boiler he built with the exchanger in his furnace, which is where I got the idea.  Just wanted to use the rmh in lieu of the boiler.  
 
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I'm thinking a rocket water heater may be more of what you're after.  A rocket mass heater puts all that thermal energy into a mass that slowly bleeds it into the direct vicinity.  You potentially want to put all of that thermal energy into water and then move the water to your forced air system.

So imagine a rocket heater that is heating up a 200 gallon tank of water (open to atmosphere so it can't go boom squish as easily).  Run the rocket to heat the water.  Run a line from that tank to your furnace for on-demand heat.  I'm making all of this up but I think it could be done and maybe has been.  A person could even do a co-heating system where you run the rocket whenever you can and then electrical or other fuel sources keep the water hot when you're gone or forget.  

I'm not sure if there's a way to run a rocket (non-mass) heater in a basement and exhaust it properly...

I'm probably giving you more questions than answers.  Sorry....
 
Kevin Gouty
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I think possibly the biggest question is can the rmh heat the water to a proper usable temp, and could the rmh be exhausted properly to achieve stratification......?
 
Kevin Gouty
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Maybe some more info would help.......the stairway to the basement is wide open (no door).....RMH would be 10' from stairs,   and what would be the difference in exhausting thru the sill plate to the outside vs through the roof?  If mainly the exhaust is steam?  Or would length of the chimney pipe  be an issue?  
    Maybe I have the wrong line of thinking for this idea.
 
Kevin Gouty
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Rocket heater is built.  I bought it from an individual who built it.  He used wood pellets.  The mass has not been built as of yet.  What if used the barrel over the burn chamber wrapped with copper line, and the proper safety devices to prevent boom squish and just added it to my existing gas furnace?
 
thomas rubino
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Location: latitude 47 N.W. montana zone 6A
1025
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Hey Kevin;
If you want to run your chimney thru the sill and then up,  that's OK but....   everything outside the house would need to be class A insulated. The length is not a factor, but the pipe must stay warm or your stove will stall.
Much better to stay indoors with an uninsulated chimney and just use class A to go thru the roof and outdoors.

What size Rocket stove did you get ? 8" J tube ?  Or is it a batch style?
Copper or stainless steel tubing would need to be 1" dia. or larger.
An open tank suspended / set over the barrel might be a less expensive  method.    Room humidity would jump though.

 
 
Kevin Gouty
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J tube......made with a water heater tank.  
 
Kevin Gouty
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4" j tube with 4" outlet.....going to change size of outlet to 6" minimum.   Maybe if I post a picture of it that may be of some help.  Or you could give me some advice on modifications that may be necessary.   Thanks for all the input and help.  
 
Kevin Gouty
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Pics
20200118_222735.jpg
[Thumbnail for 20200118_222735.jpg]
 
Kevin Gouty
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20200118_222725.jpg
[Thumbnail for 20200118_222725.jpg]
 
pollinator
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Location: Massachusetts, Zone:6/7, AHS:4, Rainfall:48in even Soil:SandyLoam pH6 Flat
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Idea 1-Direct
It's briliant, with the rocket mass heater "Bench" at 100F, if you run some pipes thru it, the water in the pipe will also get up to 100F. Just circulate that water directly under the floor for infloor radiant heating. There are some boom-squish concerns so two high pressure fail safe sounds like a good fix.

Idea 2- Seperstion/Buffer Tank
With this the Radiant Floor PEX pipe carries non-potable water at 100F constantly, it only adds a tiny bit of heat from the potable hot water tank, thru a indirect heat exchange. The hot water tank is heated by wood somehow. Zero risk of boom squish in the radiant heating PEX side though.

Idea 3
This is really an extension of idea 2, where the heating process for the domestic hot water side of things is explained.
A. We can add a coil of pipe around the outside of the exhaust pipe of the rocket mass heater almost doing away with a "cob bench"
B. Use the top of the barrel as a stovetop, by adding a stovetop-coil to the top of the barrel, similar to making pizza on top of the barrel, would be nice to add failsafe to elevate it if the tank temp get too hot.
C. Adding coil around the outside of the barrel.
D. Use a pellet/wood stove, it could also be rocket-ty too.
With these there is the risk of taking out too much heat too fast and this creates a dirty burn, maybe even back-draft.
Then there is the possible boom squish so we need to have two or so high pressure fail safe on the tank/coil. Maybe an electronic shutdown system once the tank water temperature get to 190F or some arbitrary temp to. Maybe a low-tech wax/bimetal that drops sand once the water temp get to 200F. If it is pellet auger feed, maybe the fuel source could be interrupted/turned off.

Idea 4



 
thomas rubino
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Location: latitude 47 N.W. montana zone 6A
1025
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Hi Kevin;
Have you run this rocket yourself yet ?  I'm wondering how hot that gets on the outside when burning?
Were you thinking of using pellets or wood ?  Hand fed wood or auger for pellets?
How long were you thinking for burn times ? With a 4" feed you almost have use pellets and an auger.
Hand feeding wood you would be running to feed it every 15 minutes.
I should warn you that the feed tube on that style (all metal) rocket can get hot enough to ignite pellets prematurely and start back drafting into the room.

The last thing I feel obligated to mention.
If your rocket is burning at true rocket temps, then your metal will eventually spaul enough to fall apart.   How long ??? Quite a while I would imagine, but it will happen.
 
Kevin Gouty
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I have not burned the RMH myself, but there are signs of metal warping on the base plate.  So, I am going to assume it gets pretty hot.  From all that has been said I am thinking the RMH that I have is not suitable for my idea of the water to air heat exchanger.   So, I think my best option is to use my RMH in my shop, and research other ideas for water to air option of supplemental heat.  I am open to any and all ideas.      My house is 2400 sq/ft.  Full finished basement 3 bed 2.5 bath.  Basement walls are 8" block.  2" styrofoam insulation on ceiling in basement.  Also has full kitchen in basement.  
 
thomas rubino
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Posts: 3749
Location: latitude 47 N.W. montana zone 6A
1025
cat pig rocket stoves
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Hey Kevin;
How do you heat now?  Your furnace ?  Hot water ? Hopefully not electric!

If your wanting supplemental heat, what about building a smaller brick batch box in your home ?
Downstairs or up it would help with heating costs , and by being a batchbox you load it up and walk away.
 
Kevin Gouty
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Propane gas forced air........I had the batch box idea in the back of my mind.  I just haven't pursued or researched it.    Any design ideas would be greatly appreciated.   Thank you!
 
thomas rubino
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Location: latitude 47 N.W. montana zone 6A
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cat pig rocket stoves
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Hey Kevin;
If I couldn't have wood heat , propane would be next on my list.
Tons of designs / styles of batch box's.
There are very small 4" brick units. A common size is  6" brick, you can build an 8" if you have the space.
Depends on where you can place in your home. In regards to chimney access and floor space. And how big an area you were wanting it to heat.

Here is a website all about batch box's and their development. http://batchrocket.eu/en/

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