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wofati plus a rocket mass heater  RSS feed

 
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What if the RMH exhaust goes through a bench and then out through an exhaust under the thermal mass that is part of the wofati

My thinking is that with a RMH, the exhaust is steam and CO2.  Plus, when you run a fire, aren't you trying to warm your environment?  And if you are trying to warm your environment, that suggests that your thermal mass isn't doing the trick.  So it might be nice if your thermal mass was a little warmer.  So if I route my RMH exhaust through the downhill umbrella I'll be warming my thermal mass and getting of my heavy exhaust.  It seems that once the fire is out, all of the exhaust is gone and the combustion chamber has cooled (hours after a fire) then the airflow would reverse.  Fresh, warm air would come into the house.

??
 
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I'm not too sure what the downhill umbrella is. Are you thinking of running the exhaust under your floor for a ways?

I think that the warm CO2 and H2O vapor are lighter than air - until they cool to near the same temp as the air.

You are not suggesting that you want to return the CO2 back into the living room are you?

It does seem that the air flow would reverse ,at some point, as the mass cools. I would be interested as to how people have dealt with this. If I put a damper in the exhaust ,to close after the burn was complete, I would forget to open it before the next burn.
 
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Seems like a good idea overall.

Where are you thinking the condensation will go? Will it be able to drain without soaking into any of the soil or insulation under the umbrella?
 
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The only way I can see the flow reversing is if the outside outlet of the exhaust flue were to slope downward which would seem to be a bad thing in trying to get rid of even marginally warmer than outside temp exhaust gases.

Not saying that running the exhaust under/in the thermal umbrella isn't a good thing/idea, just not sure about using the exhaust flue to bring warmed outside air back into the dwelling. 

Now if you were to add an earth tube (? I think this is the proper terminology but please correct me if I am wrong.) in the trench below or beside, but separated by 6 inches, or preferably more, of dirt/rock/thermal mass, from the exhaust flue, when burying it, to use for warming outside air and bringing it back in I think that would work.  Again in the case of the earth tube, the outside air inlet would need to be lower than the inside air outlet for proper flow to take place without the use of a fan or some other assist.  And you would also be able to have fresh air coming in while using the RMH.

Dave
 
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ronie wrote:
I'm not too sure what the downhill umbrella is.



Are you familiar with the wofati article?

The "umbrella" covers the dry dirt mass.

ronie wrote:
I think that the warm CO2 and H2O vapor are lighter than air - until they cool to near the same temp as the air.



The exhaust video shows steam about 120 staying about level - not going up or down.

ronie wrote:
You are not suggesting that you want to return the CO2 back into the living room are you?



I wish to minimize CO2 in the house.
 
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Joel Hollingsworth wrote:
Where are you thinking the condensation will go? Will it be able to drain without soaking into any of the soil or insulation under the umbrella?



I definitely want it carried beyond the umbrella.
 
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Dave Patty wrote:
Not saying that running the exhaust under/in the thermal umbrella isn't a good thing/idea, just not sure about using the exhaust flue to bring warmed outside air back into the dwelling. 



My feelings are the same.  And yet .... it seems like it should be just fine.

 
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Dave Patty wrote:
The only way I can see the flow reversing is if the outside outlet of the exhaust flue were to slope downward which would seem to be a bad thing in trying to get rid of even marginally warmer than outside temp exhaust gases.



Sorry, I was confusing my ideas a bit.  I have been toying with the idea of making the exhaust go down and out instead of up. In Paul's post, it seems that he is thinking of turning the exhaust down as well.

You are right Dave, that a regular updraft wouldn't reverse flow. But what i am thinking is that  the updraft mass heater would reverse function and cool the room and the mass (at some point after the fire went out) by the continued flow of air through the heater drawing warm air out of room and the mass. It seems that it would need a way to close off the air flow by either closing the intake  (load chamber) tightly, or by a damper of some kind in the exhaust.


Paul, I don't think that you would want to draw air into your room through the heater's exhaust. The burnt smell inside the  RMH would not be very good to draw into your living room. 

Are you thinking of exhausting the burn gases downhill instead of up a chimney?


 
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I've been wondering for a while you couldn't run a fresh air pipe into the feed. Use a screw type damper to open/close it as many wood stoves do. In addition put a door on top of the RMH feed
 
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ronie wrote:
Paul, I don't think that you would want to draw air into your room through the heater's exhaust. The burnt smell inside the  RMH would not be very good to draw into your living room. 



That was my first thought too.  And then I got to thinking about how clean it is inside of the exhaust pipe.  Would there be a burt smell?


ronie wrote:
Are you thinking of exhausting the burn gases downhill instead of up a chimney?



Yes!  In fact, with a RMH, my understanding that a proper chimney would not work.  So sending the exhaust straight out a wall is normal.  Sending it down seems smarter.  Sending it upward would lead to problems.


 
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Silver wrote:
I've been wondering for a while you couldn't run a fresh air pipe into the feed. Use a screw type damper to open/close it as many wood stoves do. In addition put a door on top of the RMH feed



I think this has been thoroughly beat up in a few different threads here.  In a nutshell, you want the fire to always run as hot as you can - which means that you never restrict the air flow.  Some people have put in a fresh air intake, although the value of that is debatable.

 
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paul wheaton wrote:
Yes!  In fact, with a RMH, my understanding that a proper chimney would not work.  So sending the exhaust straight out a wall is normal.  Sending it down seems smarter.  Sending it upward would lead to problems.



All the design diagrams of RMH's that i saw had upward exhausts. That is why  Aprovecho  had recommended a 250 degree exhaust - so that the gases could make it up and out - at least that is what i thought.  And Ernie is talking about 70 degree exhaust in some of his posts... so i'm left scratching my head some times.

I sure don't want to exhaust 250 degree heat unless i just have to, so i was thinking that a downward exhaust could be a little longer and lower temp exhaust. (Like the 70 degrees that Ernie gets.)

The downward exhaust presents some problems as it seems that it would reverse flow, at some point, and return gases back toward the intake.

One of the exhaust gases is CO2 and the other major exhaust gas is moisture vapor...While both gases could be beneficial to plants (as long as the temp wasn't too high) both  CO 2 and moisture could present problems if they came into a living area.


paul wheaton wrote:
That was my first thought too.  And then I got to thinking about how clean it is inside of the exhaust pipe.  Would there be a burt smell?



I do think that there would be a burnt smell.
 
paul wheaton
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I think that if there is a difference of opinion between aprovecho and ernie/erica/ianto, I would side with ermie/erica/ianto.

 
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I appreciate your vote and it means a lot... I don't doubt the advice that i heard...I just try to figure out why the differences and it takes me a while to figure it out...I believe that both camps are sincere and are trying to give the best info as they see it.
 
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Absolutely true!

I've met lots and lots of folks that really meant well, and utterly insisted that were 100% right - and I knew they were wrong. 

 
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whereas we know that we don't know,
and we tell you so,
and then we make up a bunch of reasonable-sounding guesses that dazzle you into believing us.



Aprovecho has some good research logged.  The main thing to remember is we are talking about two different animals, and we don't know each others work that well. when you know which 'stove' someone is referring to, a lot of the discrepancies resolve.

Forgive my not following too close, I've been working on a different project.  Wish you were still a little more local, Paul, so I could pick your brain about Web design.  Know any good PHP tutorials that I can learn in a week or two? 

For what it's worth:
Yes, a RMH can have a cooler exhaust than most any other stove - especially if it's got a horizontal exhaust option.  or at most a low vertical rise.  The internal combustion chamber of the RMH provides the primary draft, and can 'push' exhaust onwards.  Most other wood-fired systems including masonry heaters have to rely on the exhaust being hot enough to pull draft up through the tallest part of the chimney. 

I've been a little confused by the references to 90 and 70 and 250 in conversations. 
I heard 90 degrees was the minimum exhaust temp for a (vertically exhausted) masonry heater; but do they mean 90 F, or 90C (which is closer to 200F).

When Ernie and Ianto say 70 or 90, I think they're mostly meaning Fahrenheit, and the RMH's they're working with have an internal heat riser that's a significant proportion of the chimney height itself.  They're often in low, one-story buildings, and even then, they often exhaust horizontally out a convenient wall into low-traffic garden alcoves.

...
I didn't know the approvecho folks were doing heating stoves.  From what I've seen of their cookstoves, they would need to use normal woodstove or appliance exhaust principles.  They design the heat riser to maximize heat transfer to the cooking pot, not draft.  Most of their cooking stoves either exhaust right up around the pot like a natural gas stove, or out through an external chimney like a woodstove.

Hot Stack Draft = height * temperature (technically not "=", but proportional - "<><"

So if they are saying 250 for a lightweight cook-stove, and the masonry folks who've been at it longer (and maintain a pre-warmed chimney for most of the heating season) say 90, I bet the masonry folks mean 90C (200F), and I am content. 

RMH's allow the lowest exhaust temperatures if your situation allows you to use dirty tricks like horizontal and alcoves.  If you have to do a 2-story vertical exhaust, follow the conventional wisdom and plan on sending out more heat for safety.

And for what it's worth on the cooling flow:
Our stove does continue to draft for some time after its fire goes out.  We like this, because it means we don't have to worry about smoke-back.  But we don't like the idea of it cooling off all our well-warmed thermal mass.
We shut the lid on our fuel feed down almost all the way at the end of a burn, and then close it fully when the fire is completely out.

This is a pretty humane solution to the damper problem, 'cause it's hard to start a fire without opening the fuel-feed no matter how forgetful you are.

 
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If i knew what i didn't know and i tried to tell you...you would need to be free for a few years. (I am from Mark Twain's state).

Thanks for all the info..I had concluded that the differences were in the details. You stopped short of dealing with a downhill exhaust. I am interested in thoughts on how long the downhill exhaust might be.
I may want to run it down through several outdoor cold frames to provide a little warmth and moisture to those plants.

Here is the link that John gave us for Aprocecho http://www.aprovecho.org/lab/pubs/arcpubs  At the bottom of the page is Designing Improved Wood Burning Heating Stoves. That is where i got the info about their RMH and other heating stoves.

 
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Erica Wisner wrote:
Forgive my not following too close, I've been working on a different project.  Wish you were still a little more local, Paul, so I could pick your brain about Web design.  Know any good PHP tutorials that I can learn in a week or two? 



Look for the Head First book.  Call or email anytime.



 
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ronie wrote:
If i knew what i didn't know and i tried to tell you...you would need to be free for a few years. (I am from Mark Twain's state).

Thanks for all the info..I had concluded that the differences were in the details. You stopped short of dealing with a downhill exhaust. I am interested in thoughts on how long the downhill exhaust might be.
I may want to run it down through several outdoor cold frames to provide a little warmth and moisture to those plants.

Here is the link that John gave us for Aprocecho http://www.aprovecho.org/lab/pubs/arcpubs  At the bottom of the page is Designing Improved Wood Burning Heating Stoves. That is where i got the info about their RMH and other heating stoves.




The amount of parsing that goes into that quote, explains a lot about how Twain got away with his parodies of small-minded people.

A long, gradual downhill exhaust can be very do-able.  Once your gasses get close to your outdoor temperature, they tend to drift downwards anyway.  But at that point, the low heats aren't worth much for most situations.  Your cold frames might be the perfect place to try, though.

Leave a 'cleanout' between frames, so you can try it at different lengths.  With a very long exhaust, it may not draft properly in certain weather conditions.  (You are relying entirely on the combustion chamber draft, and if you are lucky in your temperature gradients you may have a slight downhill 'siphon' effect from the cold end.)

And as with any exhaust, make sure there's a convenient place for it to escape without endangering people or being blown back in by prevailing winds.
 
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Thanks Erica, great info as always... I appreciate all you and Ernie and Paul do to get info out to the planet.  It seems that so many people hoard info these days and charge people for it.
 
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paul wheaton wrote:
Some people have put in a fresh air intake, although the value of that is debatable.




This is a common misconception, a fresh air intake is always a good idea.  If you don't have one the air vented in the exhaust has to come from somewhere.  This is usually the already heated, at least partially, air in your house.  This then is replaced by cold air coming into the house through cracks around doors windows etc  that you have to heat again and it creates drafts, uncomfortable, as well as using more fuel.  running the fresh air intake down through the 2nd drum to preheat it then across to the fuel section just above the fire chamber provides controlled make-up air, prevents drafts, allows the fuel chamber to be sealed and insulated preventing accidents with small children and is all-round safer. 

MK
 
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Exhausting downhill?? hmm  That would require one of 2 things, either a powered exhaust or cooling the exhaust to below the ambient air temperature.  Hot air rises, without a fan or an exhaust freezer a temperature inversion would result backing up the CO2 laden air and putting your fire out.  One of the neat things about rocket stoves is they don't need power, for the relatively small net gain in heating this scheme involves you pay a great deal for the electricity to run the exhaust downhill.
 
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mekennedy1313 wrote:
This is a common misconception, a fresh air intake is always a good idea.  If you don't have one the air vented in the exhaust has to come from somewhere.  This is usually the already heated, at least partially, air in your house.  This then is replaced by cold air coming into the house through cracks around doors windows etc  that you have to heat again and it creates drafts, uncomfortable, as well as using more fuel.  running the fresh air intake down through the 2nd drum to preheat it then across to the fuel section just above the fire chamber provides controlled make-up air, prevents drafts, allows the fuel chamber to be sealed and insulated preventing accidents with small children and is all-round safer. 

MK



When you look at the thread title you see that Paul is talking about a WOFATI. Paul and others concluded that they want some fresh air drawn into the room. I don't think that they have a misconception, but rather want fresh air in their living space.
 
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mekennedy1313 wrote:
Exhausting downhill?? hmm  That would require one of 2 things, either a powered exhaust or cooling the exhaust to below the ambient air temperature.  Hot air rises, without a fan or an exhaust freezer a temperature inversion would result backing up the CO2 laden air and putting your fire out.  One of the neat things about rocket stoves is they don't need power, for the relatively small net gain in heating this scheme involves you pay a great deal for the electricity to run the exhaust downhill.



You are correct that this downhill exhaust could not be done without extreme care. The gas flow could reverse under certain conditions.

The dangers, that the downward exhaust presents, may not be worth the small heat gained in the cold frames.

The power to the exhaust is in the vertical heat riser (and atmospheric pressure).. or you could call it the rocket engine, or the rapid gas expansion chamber or how about the gas/heat pump?
 
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ronie wrote:
When you look at the thread title you see that Paul is talking about a WOFATI. Paul and others concluded that they want some fresh air drawn into the room. I don't think that they have a misconception, but rather want fresh air in their living space.



Not disagreeing with the idea of having fresh air in the room.  The question is whether to control the fresh air infiltration or have it uncontrolled.  If air comes in where you don't want it, it a) often brings moisture which results in mold (ick + sick) and b) you can't preheat it with the energy in the air that is leaving resulting in significantly higher fuel costs.  His example is living in a zip-lock bag, no air in or out.  This too is a bad situation, turnover of the air in a structure is a must for a healthy environment.  What I'm pointing out is that controlling that turnover (where and how much) has advantages both for health and the economics of energy (and by extension the environment).
 
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So are you saying the RMH needs to have a outside air source piped from outside directly into the burn chamber? And if yes, then are you also saying that the fresh air for the room comes from another process or is included some way using an air exchanger?
 
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ronie wrote:
So are you saying the RMH needs to have a outside air source piped from outside directly into the burn chamber? And if yes, then are you also saying that the fresh air for the room comes from another process or is included some way using an air exchanger?



Yes, separate sources.  If you don't have a controlled source then infiltration occurs around windows, doors etc.  This is what leads to drafts, cold zones, condensation, mold and so on.  By controlling where the air comes from you can grab some of the heat in the air going out and use it to warm the air coming in.  A much better situation!  If a home becomes "stuffy" it isn't getting enough fresh air.
 
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mekennedy1313 wrote:
This is a common misconception, a fresh air intake is always a good idea. 





At the first workshop I attended there were five instructors because the workshop was doubling as an instructor reunion. 

I made the same point you just made.  And five sets of eyeballs rolled combined with five heavy sighs.

and .... wait .... I'm sure I've told this story here before .... yes, here it is: 

http://www.permies.com/permaculture-forums/1450_0/alternative-energy/rocket-mass-heater-air-from-outside



 
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mekennedy1313 wrote:
Exhausting downhill?? hmm  That would require one of 2 things, either a powered exhaust or cooling the exhaust to below the ambient air temperature.  Hot air rises, without a fan or an exhaust freezer a temperature inversion would result backing up the CO2 laden air and putting your fire out.  One of the neat things about rocket stoves is they don't need power, for the relatively small net gain in heating this scheme involves you pay a great deal for the electricity to run the exhaust downhill.



Your path is longer than you think grasshopper.

First, you are right:  hot AIR rises in a pool of cooler air.  But what about warm C02 and steam?  If your air temp is 20 and your CO2 and steam is 60, what happens?  C02 and steam are both much heavier than air when they are the same temp. 

Don't forget, that while the RMH is running, you do not rely on draft/draw beyond the combustion chamber.  You have a really nice push going on.

 
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ronie wrote:
You are correct that this downhill exhaust could not be done without extreme care. The gas flow could reverse under certain conditions.



There's the rub.  When the fire is out, would the flow reverse.  Are there ways to mitigate that?  And, possibly, maybe we like the idea. 

I would, at the very least, like to conduct an experiment. 

Have the exhaust pass through hundreds of tons of wofati mass and a slight downhill slope.  Assuming that the end of the burn is very clean, and then the fire goes out.  A few hours later, the flow reverses.  Fresh air comes in, passes through the hundreds of tons of warmed mass and fresh, warm air comes into the home. 

Naturally, one question is:  what will it smell like?  Will there be some sort of icky residue that we don't want to smell?  My impression so far is that the RMH stuff burns very, very clean.  I suspect that there would be little, if any smell. 

But, I could be wrong.

 
ronie dee
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Your hot thermal mass can radiate heat into your living space without convective air flow. It might be possible to do what you are suggesting, but if you are not there to personally carefully get it right every time... or if you make a mistake. It could  put too much CO2 into your living space and cause serious problems... One mistake may be all you get.

The moisture, from combustion, may be problematic in the downward exhaust... at what point does the moisture vapor become a liquid? Where does this liquid wind up?   

If the exhaust is long through tons of mass, does the CO2 have time to completely get out before the flow reverses?

When the flow reverses, cold outside air is drawn over your nice warm thermal mass rapidly cooling it.

I vote that the exhaust is one way:

[move][font=Verdana][size=20pt]OUT[/size][/font][/move]

If you get it to work the way you are suggesting maybe you would be considered a mad genius:

[move] [/move]

If it goes wrong once you might be out of the BLUE and into the BLACK.
 
paul wheaton
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you would be considered a mad genius



That would totally rock.  I would get stationery made up that said so.

Suppose we are talking about an eight inch system.  I think the steam and co2 would me small and heavy and pour out.  And I think the combustion chamber would be warm enough that it would continue to push air through for a while after the fire is out. 

The slope would be gentle enough that if any air does come back, the amount of flow would probably be rather gradual.  The trick is getting past the combustion chamber.  It would have to have enough force to go straight down through the combustion chamber.  I suspect that that won't be the case.




 
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paul wheaton wrote:
That would totally rock.  I would get stationery made up that said so.

hahhahhaha

Go ahead and make the stationary..

If it is heating that you want, then the warmed mass will move heat up and towards your cooler spaces without any movement of air back through your exhaust... the air moving back through your exhaust will include cold outside air that will cool your mass faster.

If it is fresh outside air that you are wanting you could open the door or use an air exchange system like Dave and MK  have suggested.

I guess now I will have to go and find the tread and explain why i abandoned the idea of an underground envelope for an easier system.
 
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paul wheaton wrote:
   But what about warm C02 and steam?  If your air temp is 20 and your CO2 and steam is 60, what happens?  C02 and steam are both much heavier than air when they are the same temp. 



This would be very difficult to do,  you would need a very long cooling path and remember as the path lengthens so does the inertia of the air mass making it more difficult to push.  The energy gained at heating is finite and will be drained after a while.  Additionally if the exhaust is lower than the air intake, flow will reverse at some point in time even if the system is heating.  Additionally if you look at

http://www.youtube.com/paulwheaton12#p/u/21/wyeGvxfWkfY

Even as cool as the exhaust is it still goes up after exiting the pipe, to go down at ambient temperature, the steam would have condensed giving up the heat of vapourization and heat the CO2 requiring further cooling.  If Your cooling enough to get condensation (there is no steam at 60 celsius or farenheit), similar to a high efficiency condensing natural gas furnace, the exhaust still goes up.  The laws of thermodynamics are not to be triffled with.  Remember the exhaust is only relatively low percents of CO2 and steam and still relatively high percents of inert nitrogen which will move in the normal way.  Why try for relatively small low temperature gains when a backdraft represents a high level of risk??
 
Max Kennedy
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As for a house being a ziplock, thats better than it being a sieve and not having enough fingers to plug the holes. At least in a ziplock I can poke holes where I want them and can control them.  Just watch Mike Holmes for what happens when air infiltration is not controlled.  Thermal mass is great, far superior to heating just the air and with enough heat you can even heat a sieve but why would you want to waste that energy by having holes where it will leak out or where cold coming in can cause problems?  A lot has been learned over the years, why not take advantage of the benefits of both systems?  Air turnover is good, like many things however, uncontrolled turnover is not good!  Don't just discount something, understand why things are that way. Comfort and safety levels are much better than in the past, Indoor air pollution is worse.  Overall we live longer and healthier.  Comparing our breathing and CO2 production to a fires is silly.  A fire uses far more air than we do in breathing (calculate the volumes and mole fractions for complete combustion if you don't believe me).  Thus what is sufficient for breathing is woefully inadequate for a heating fire.  This is a red hearing comparison typical of some of the worst thinking on why global warming isn't generated nor influenced by human activities.  I've given specific reasons why uncontrolled infiltration is bad (a previous post) perhaps you can give more than just the "envelope" argument.
 
paul wheaton
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mekennedy1313 wrote:
Even as cool as the exhaust is it still goes up after exiting the pipe,



That's after hours of running a fire in order to dry it out.  That's why the thermometer was registering such a high temp.

That steam was far hotter than normal.
 
paul wheaton
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mekennedy1313 wrote:
As for a house being a ziplock, thats better than it being a sieve and not having enough fingers to plug the holes.



I think this is a debatable space.  For some people, they will want to go to the extra trouble to create the ziplock bag and then do some sort of air-to-air heat exchanger.  Others will build something ..... looser, therefore probably much cheaper. 

 
Max Kennedy
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paul wheaton wrote:
I think this is a debatable space.  For some people, they will want to go to the extra trouble to create the ziplock bag and then do some sort of air-to-air heat exchanger.  Others will build something ..... looser, therefore probably much cheaper.



Agreed, a debatable point.  Preferences aside though the expense isn't all that different though the attention to detail during construction is.  Given some info Erica and Ernie recently posted about the very low fuel consumption of a rocket stove the extra effort might be a moot point so long as the infiltration is such it doesn't cause mold (disease) or structural decay.
 
gardener
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air infiltration wont cause mold or disease making your home into a baggy will.

here in portland we see this all the time with restorations of victorian homes. every time some yahoo with no experience seals up the house in the "proper way" the house has mold and rot problems. eventually the owners start asking around and they get an ear full about how down right stupid it is to put a 200 year old house in a ziplock. the only way the ziplock idea works is with a hvac system/ central heating. the filters alone will run several thousand over the typical turn over time of 5 to 7 years.

nice easy experiment is to take a couple pieces of wood put one in a zip lock and the other next to it on the counter and see which one decays faster. (dont cheat let both bits of wood sit out for a week so the contamination and moisture are the same) if you want another variable put another bit of wood in a baggy and a soda straw seal up the baggy around the soda straw so the only way the wood gets air is through the straw.  or you can go down to the local wooden boat yard and talk to the folks about rot problems in wood boats. what you are going to hear is lots and lots about air flow and moisture exchange.

hmm wonder why? might be cause molds dont like airflow and they dont like dry. come to think of it carbon monoxide dont like air flow either nor do the several hundred gasses that come out of the materials used in the modern home. if they cant accumulate in a nice dead zone your exposure risk goes down.

then we have the issues of fire I used to love going into a modern sealed up home. the scenery was incredible; jets of flame where the central heating system ducts where located cause the only place to get air was through those ducts, rooms that explode when you open a door cause the owner closed off the vents to the room. heavy smoke concentrations up on the ceiling that would ignite and look like a orange and black blanket.  the view inside an old home that had lots of leaks was kinda boring. the stuff that was burning was burning and the stuff that was not was just kinda smoldering. no plumes, little chance of exploding rooms, and for the most part much lower smoke build up. course the old house could burn faster but it didnt tend to explode into a conflagration.

but i am old fashioned, modern folks might like the exploding house.
 
paul wheaton
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Hey there "m", remember the part where I said there were all of these eyeballs rolling at my question?  I would like to introduce you to one set of those eyeballs:  ernie. 



 
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