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heat exchangers for a rocket stove

Rudy Mallonee


Joined: Dec 06, 2012
Posts: 9
Hi--- New to the site..
Relating to full home heating with the rocket stove...

Having read the first few pages, I didn't come upon anything regarding using a typical forced air furnace air to air heat exchanger in conjuction with the rocket mass heater.. Has anyone addressed this?

Couldn't this type of air to air heat exchanger be connected to the exhaust end of the chimmey? The rocket stove wood heat should be no different than gas or oil fire that is directed through the exchanger provided that a sufficient distance from the chimmey is allowed to prevent extreme heating of the exchanger....Yes, the fan system is needed to propel the hot air throughout the house duct system nornally used with a forced air system....
These furnaces (used) can be bought through Craigslist very cheap, I've seen some in my area (Alaska) that are given away..

Also, for a radiant water system, why not use the same principle using the boiler exchanger, expansion tank and pumps from a conventional gas or oil fired home heating water boiler? .. Just direct the flue gases from the rocket stove to the boiler chamber at safe distance from the exchanger.....
Again there are older used boilers out there for sale cheap....Weil-McLain, etc.... and use the same radiant floor system with the PEX tubing... The PEX tubing eliminates the hard water deposits in piping....
Yes, there would have to be a heat sink, hot tub, etc..... or a steam pipe vented to an outside exit with a pressure relief valve and tempering valve at the expansion tank exit before entering the PEX manifold, eliminating pressure in the PEX piping and keeping temperature down....
John Master


Joined: Dec 04, 2012
Posts: 114
Location: Wisconsin
    
    1
I think you are on to something, my plan is to make a rocket mass heater but have it as a yard mounted wood boiler type instead of in the home. This gets the fire out of the house which keeps my wife happy as she doesn't like to be subject to my tinkering and would worry about the kids. You probably could incorporate your ducting through the rmh as a way to carry the heat in your homes normal forced air heating, I have a switch on my furnace that just runs the fan without the burner, this circulates the air in the house and could circulate the heat in your house. Interesting to see it if you build one.
allen lumley
pollinator

Joined: Mar 16, 2012
Posts: 2805
Location: Northern New York Zone4-5 the OUTER 'RONDACs percip 36''
    
  40
Rudi and John : i am only going to take a small part of the questions raised here,There was an implied question about removing the Rocket stove exhaust to a safe distance ? this deserves a thread all by its self, there are wood burning furnaces that use an 'Air to air' or 'Exhaust gas to air' heat exchangers, the problem is that in a 'Fill the wood box and forget them' furnace to get extended burning/heating times we end up with smoldering fires i.e. something that is a wood stove but not a Rocket stove.
The rocket stove lest we forget, uses a Heat Exchanger, which allow for heat storage for later use, up to 24 hrs and longer depending on build.
The rocket stove is basically a 'space heater ' and while improvements will be found and whole (small) houses with stacked living spaces have been heated by a single rocket stove, the contemplation of a second Rocket Stove 'Space Heater' should bring us to the ' why do we need to heat another area? ' question.
Again I don't understand the 'sufficient distance' reference, we have heat exchangers that can handle the temps easily, or did you want to remove the Rocket stove to a ''safer location''
Were we even considering going Air to Air from a remote location, i.e. with an outdoor heater, then go to your forced air furnace figure out the interior area of your hot air ducting, double it for the cold air return, this is the size of the hole you would have to cut into the side of your house and then supply insulated ductwork through your house to your hot air furnace- subtract the area of a small closet from the useful space in your house per floor that the ductwork would have to run on/through to reach your hot air furnace , also you will need to replace the blower motor to one that can run in a higher temperature environment , if it fails you have 2 furnaces and no way to circulate the heat ! this also doesn't consider booster fans to move ducted air long distance !
Remote boilers exist, and are creasote creators best kept away from the house for the same reason that they can never be Rocket stoves, see above. as best i can follow we are adding in more mechanical systems that ether will not work in a power failure esp with a outside boiler above ground level and the primary boiler in the basement, or will work very poorly It's a sad day when you contemplate the remains of a boiler with a cracked heat exchanger.
If I were given a wood boiler,used, free, loaded onto my trailer, I'd take it - right to a scrap yard, just saying - but like used cars, you're getting someone else's problem !

This is much more negative than I intended, but I serviced and maintained these systems for 50 years ! Allen L.


Success has a Thousand Fathers , Failure is an Orphan

LOOK AT THE " SIMILAR THREADS " BELOW !
John Master


Joined: Dec 04, 2012
Posts: 114
Location: Wisconsin
    
    1
I would help you lift it onto the trailer A bunch of my good friends have yard mounted wood boilers and the sheer size of their wood piles were a good sign to me that it wasn't the way to go. I think the only ones who still feed theirs are the farmers who have unlimited acres of tree line that needs to be maintained anyway. They still would be better of with one of these. From what I can tell the way to keep creosote down will be to gradually pull heat out into the water instead of a big water box with 175 degree liquid cooled walls covering everything. Going to take some r&d but I am thinking of either pellets auto fed, wood chips auto fed, long wood biomass logs or long scrap lumber sticks stapled together for extended burn.
Rudy Mallonee


Joined: Dec 06, 2012
Posts: 9
Allen --
As far as what I mentioned, I think you misunderstand what I'm thinking about.. Don't complicate it ... Forget the idea of using the complete furnace. Just use the heat exchangers... The rocket stove replaces the heat source..

As I understand it, and I'm new to thinking of rocket stoves and have a lot to learn about them, most of the heat exchangers talked about are thermal mass sinks... An ugly barrel sitting out in the middle of the house, along with the two or three tons of cob... The use of which presents a problem of design in a new house.... As mentioned by one of the repliers in this forum...

If you have a large house ( or are designing a new one) and it already/or will have a forced air system,(or radiant heat sysyem) then all you're doing is replacing the heat source... ducting (or tubing), fans, and return air duct is already in place and sized correctly or would be if you know what you're doing...
...
As to a safe distance, I was thinking of the uncontrolled heat being too hot for the exchanger.. If that isn't a problem, then the exchanger can be mounted directly to the rocket stove exhaust exit.. Blower duct fan/motors can be placed far enough from the stove to eliminate heat problems to the fan...

If there's room in the crawl space,(or design it into a new house) ducting could run into a thermal mass holding the heat and eliminating keeping a fire going all night.. fans on a thermostat can be placed in this area..

And as far as using a used furnace, I just saw a newer one for $200, in fine working condition that the owner was replaceing with a different system... So if you're got one to throw away,, just send it to me..
John Master


Joined: Dec 04, 2012
Posts: 114
Location: Wisconsin
    
    1
I think he was talking about the yard boilers and wanting to scrap them, not home furnaces. I think your solution may be part thermal battery to draw the heat out of the flue and alternately a set of ducting for your furnace to pass clean air through, and draw heat out of your thermal battery to the rest of your home?
Rudy Mallonee


Joined: Dec 06, 2012
Posts: 9
John
The heat from the rocket exhaust heats the air to air exchanger only.. no exhaust flue gases pass in the ductwork to the house.. then, any heat passing though the thermal mass will be clean..

And yes, the heat accumulated in the storage mass (a pile of rocks in an enclosed space in the crawl space would do it) can then be drawn on, coupled with a thermostat and duct vavle to help regulate temp.... There should be immediate heat available as soon as the stove is heating, as the ductwork, although passing through the rock pile is still going straight to the house rooms so long as the duct valve is open. Don't know if a valve like this is commercially avalable, but shouldn't be difficult to build one..

As I said, the ducting is a standard system with the exception of the rock pile.. same principle as with a passive system, thermwall to rock pile, to each room, etc..
I could be wrong... but principle seems workable and details such as placement of fans shouldn't present any problem...
This idea isn't !00% off the grid.. but should cut heating bills way down....

Anyone else have an idea on this kind of arrangement??
allen lumley
pollinator

Joined: Mar 16, 2012
Posts: 2805
Location: Northern New York Zone4-5 the OUTER 'RONDACs percip 36''
    
  40
Rudy Mallonee : Yes it can be done and the technology already is in common use in various other systems, however any system that takes heat off of a thermal mass by blowing air past it, has to have a ductwork designed large enough to deliver air at a slower speed of moving air as the temperature in the Thermal mass drops!
A clear example of this happens in the running of an air to air Heat Pump, This type of system does not try to match the temperature of the delivered air stream of a Fossil Fuel Fired Furnace as measured at the 'heat register' due to the best efficiency curve Electrical energy in / heat energy out, which can obtain a 1 : 3 ratio ! Instead a lower temperature, higher volume of air is flowed more slowly through the duct work delivering the same number of therms/B.T.U.s per minute as is delivered by the fossil fuel fired furnace. - again, just in a larger volume of delivered air.
I think the Heat Pump with its matching ductwork would be a better match for The Rocket Stove, than a Fossil Fuel Fired Furnace, but even that combination would require a back up energy source, OR some one dedicated to feed the Dragon in your Rocket Stove small meals over a 3-4 hr period at least once a day to generate/store 20-25 hrs of heat and do it 24/7/365, well not 365, how about using the heat pump, again that 1 : 3 ratio, to bring it down to 24/7/90 ? You are not eliminating a Furnace, you are Adding One or Two
At one time Heat pumps were designed with a thermal control that varied the flow of air, delivering warm but cooler air at a slightly slower speed to prevent complaints of 'my system is blowing cold air' this feature was being phased out the last I knew, just going to electrical heat and fossil fuel fired back-up systems !
Again blowing cool air over a Thermal Mass to pick up heat that is itself variable, will require a variable control due to the lower temps radiated off of the thermal mass, OR back-up heat. Taking heat off of the 55 gall drum will only require the use of a secondary fan limit switch, and back up heat. You will still need a blower motor rated for operation in higher temps or other secondary fan/blower motors!
While this system could be built and used 'off grid ' due to triple redundancy it would not be cheap, not much chance of seeing a payback !
All this is a very,very long way of describing the problems in trying to make a " Space Heater '' into a whole house Furnace ! Three long and lusty cheers for the crew that can make it work ! I hope this builds on what you were saying and fills in some gaps ! Pyro-Allen L.
allen lumley
pollinator

Joined: Mar 16, 2012
Posts: 2805
Location: Northern New York Zone4-5 the OUTER 'RONDACs percip 36''
    
  40
Rudy Mallonee : are you talking about using a remote sited Rockets exhaust to heat an Air to Air Heat exchanger, to then flow that heat to the thermal mass that you have moved into the house ? If I misunderstood that from your 1st sentence then I'm sorry, please expand on your plan here just a little ! Thank you, you have an Interesting way of looking at the way rocket stoves work that will cause all of us to question what we think about them! We all need to stretch our minds, if we are to make any improvement to the system ! Pyro-maticly yours Allen Lumley
John Master


Joined: Dec 04, 2012
Posts: 114
Location: Wisconsin
    
    1
I think rudy is talking about a labrynth of sheet metal after the burn chamber and have forced air carry the heat away instead of storing it in a thermal battery. Assumably indoors. My idea is to have the unit outdoors insulated and heating a liquid (was thinking recycled automotive antifreeze) then using the liquid to transfer the heat indoors to three different units, one exchanger in the garage, one in the basement and one coil in the home hot water heater. Mine would be smaller and running constantly like a conventional pellet burning appliance with an auto feeder to regulate fuel input.
Rudy Mallonee


Joined: Dec 06, 2012
Posts: 9
Allen,
I'm not expressing myself very well !! And just trying to see if my idea has merit. Have been thinking of how to keep it simple for heating a larger home with a rocket stove.. This is as simple as I have come up with so far...

But yes,essentially, you've got it right........ Get a good air to air exchanger from a used forced air furnace..The " labrynth of sheet metal after the burn chamber" as John Master calls it - is simply an air to air exchanger.... . Don't have to build one as there are a lot of used ones on the market, here in Alaska, anyway (Craigslist, etc.) as I had said, from free to a minimal $ amount....

Build your rocket stove... Duct the flow of exhaust gas to heat the air to air exchanger, then the clean heated air goes to the crawl space heat sink (in a presently constructed house that has a crawl space) or in a new house with the crawl space designed into it so that you have an enclosed heat sink there -- which could be a simple insulated concrete block box of sufficient size, filled with rock for the thermal mass -- duct in - duct out to rooms ------ return air duct (ceiling height) is just returned to the exchanger as in a normal forced air system... If there is a "auto feeder to regulate fuel input", as John Master says, then that might solve any problem with over/under heating ??

As John Master wants to keep the stove outside, it would still work in the same way with a boiler unit water heat exchanger.. With a constant, consistent heat source ("auto feeder to regulate fuel input", there shouldn't have to be a need for a heat sink as the amount of water/glycol in the system would be sufficient capacity.. Just plumb the system as you would for a normal radiant hot water in-floor or baseboard system..

Please improve and refine this idea -- if it's a feasible idea.. I haven't got it all thought out..
John Master


Joined: Dec 04, 2012
Posts: 114
Location: Wisconsin
    
    1
I think you could make the essence of an erica/ernie stove but instead of using the barrel for mostly radiant heat you could just make an insulated sheet metal box around it and and draw your air up and around the barrel to be carried away tthrough your hvac system. Could put a temp switch on it so that the furnace fan always runs when the rocket barrel is up to a certain temp, this way your furnace will be unharmed and still run off it's own thermostat as it does now. I would think you would want it to connect to the cold air side so you are not heating your cold rocket barrel with your gas furnace when it is not in use.
allen lumley
pollinator

Joined: Mar 16, 2012
Posts: 2805
Location: Northern New York Zone4-5 the OUTER 'RONDACs percip 36''
    
  40
Rudy Mahlonee : Talk about not speaking clearly - I answered your very 1st question with a 1960s mind-set and Greatly misled you !+


You used the phrase air to air Heat Exchanger and my '60s mindset automatically substituted ' low air temperature ' Air to Air Heat Exchanger !

You Do need a special Heat Exchanger, SPECIFICALLY a Ceramic Heat Exchanger !

No other H. E. will work at the Higher temperatures you need to transfer the Rocket Stoves heat energy to your Thermal Mass.

The transfer of heat from the rocket stove needs to be constant, when/if you got your thermal mass up to the temps that would let you store enough heat
for a whole house in a small-ish crawl space the heating/cooling cycle will cause your ductwork to 'Oil Can' with a BANG at start-up and shut-down

Any higher temp duct work would have to be isolated from the rest of the house - though a standard set of ductwork could then deliver heat It would pick
up flowing air through the crawl space, this would be where you would use the 'free Craigs list ' furnace as a 2nd heat exchanger to pick that heat up !!

A standard (low temp) Air To Air H.E. is technology from the 40s and it will not work at the temps That I wrongly led you to believe it would !
The mistake was mine . I apologize for my misunderstanding of the way you wanted to flow air ! Again I am sorry !



If you want to have a different whole house wood burning system than a Rocket Stove (which was made to be a space heater) you will need a truly massive
Masonary or Russian Furnace ! Again I apologize for my error that caused such confusion ! I am Sorry ! PYRO -Magically yours , - Allen Lumley
Geo Schoonmaker


Joined: Nov 04, 2012
Posts: 34
Forced air would be doable, but heated water is more efficient and versatile. If I rightly recall, a 1" pipe of water can carry as much heat as a 18" air duct. I'm on my second year of heating my doublewide with such a system. I'd have preferred to go with a RMH setup, but there was no good way to accomplish this with a doublewide that I still owe money on. By building a system that heats water, you can enjoy the benefits that rocket stove technology offers without needing the thermal mass to do so.

The fire is gravity fed and tends itself anywhere from 4 to 8 hours depending on the size/type of wood being burned. It heats water which is then pumped to a 18x18 heat exchanger. If you have an existing central furnace / AC, it can be installed right into your system. If you have existing radiators or baseboards, it will work with them too. Or you can do like I did and build a blower box and the air follows a loop through the house. Once the blower has been going for awhile, I can even it out to about a 5 degree temperature difference between the hottest and coldest room.

There is 110 feet or so of copper tubing in the chimney above the heat riser that takes the heat out of the exhaust and converts it into hot water. The top of the chimney feels like a dryer vent, so most of the heat is taken out. I plan to put in another 60' of tubing for even better efficiency. Here's some information on my system that I've built. Most of the bugs have been worked out and I now heat my home and water with it. Even with all the mistakes I've made in the building process, I have barely over $1000 in the entire system. If I built it all over again, I could probably do it for under $700. Keep an eye on that page, I'll be adding more to it in the near future.

I live out in the country and the few neighbors I have own outdoor wood furnaces. They burn wood like it is going out of style, easily 4 to 5x what I'm burning. You can see their smoke plumes for miles on a still moon lit night. They think I'm up to some sort of voodoo when they see no smoke from mine. They also think my woodpile is too small and the wood I burn is kindling compared to what they use. Hard to teach old dogs new tricks.

Anyhow, this might be a method you want to consider. My system isn't perfect, but it is getting better everyday. Let me know if you have any questions, I'd be glad to help you any way that I can.
allen lumley
pollinator

Joined: Mar 16, 2012
Posts: 2805
Location: Northern New York Zone4-5 the OUTER 'RONDACs percip 36''
    
  40
_ We really need a separate forum thread on Coppicing Here at Permies +A+
Geo Schoonmaker


Joined: Nov 04, 2012
Posts: 34
allen lumley wrote:_ We really need a separate forum thread on Coppicing Here at Permies +A+


Ask and it shall be given you lol, that is a great idea.
S Bengi


Joined: Nov 29, 2012
Posts: 1031
Location: Massachusetts, Zone:6/7, AHS:4, Rainfall:48in even distribution
    
    5
Using water(200F) or oil(600F) to transfer heat around the house is the best way to do it.
Just run the pipe through the ductwork and right by the vent have a coil in a thermal mass for radiant heat exchange.
You
 
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