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Outside Air Supply next order of magnitude improvement for rocket stove mass heaters  RSS feed

 
Scott Perkins
Posts: 32
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I have spent a few hours looking over all the RS mass heater info and I am seeing many at or past the point of diminishing returns
in efficiency. These devices as well as stoves and fireplaces that suck warm inside air into the burn chamber likewise suck
an equal amount of outside cold air into the dwelling. You guys and gals MUST seek pure outside air supply to jump
way up in efficiency. Well there is the inherent problem of having to babysit rocket stoves with the fuel isnt there ?
Not necessarily, look at the guy who has used the "fuel cartridge" long verticle input tube that gravity feeds wood sticks.

What can be done is to place this sort of input vertical feed tube in a closet or airtight cabinet on an outside wall
that is vented to the outside. This way combustion air can suck in cold air from outside without pumping out the
warm air in the house causing a negative pressure and sucking in cold outside air.

There will be a 20 to 30 percent increase in fuel efficiency and net heat output if this is practiced.

I learned this as a custom home builder with many stand along wood stoves. Only a small percentage of
them can be set up as airtight with outside air feed.

This is my first post and I wish you all Good Luck

Thanks

 
Chris Sturgeon
Posts: 91
Location: Yukon Territory, Canada. Zone 1a
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Interesting idea. Someone please chime in and let us know about how pulling in cold air directly into the combustion chamber affects the burn. Is the cold air going to reduce the efficiency? How about REALLY cold air, like Yukon cold (-30c)?
Further refinements on your idea, Scott, might be a small inside to outside access door to your feed tube, so though you may be letting a small amount of heat out of your insulated living space every hour or so, it wouldn't be a constant draw. Your wood could be pre-warmed and staged on the inside of your space as well.
This interests me as I live in a super airtight condo and I wonder about the available draw through my Heat Recovery Ventilation system.
 
Rob Torcellini
Posts: 51
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My situation is a bit different than heating a home, but this is my observations with heating my greenhouse. My greenhouse is fairly well sealed...as greenhouses are concerned, but it does have enough air leaking into it to provide air for the heater. I'm finding that during the first hour of burn, there's a lot of water vapor in the exhaust, which is no surprise since the greenhouse is housing an aquaponic system and can easily be over 90% humidity in there. After enough of the air "burns" through, there is no water vapor at all in the exhaust. (also I'm burning pellets which are usually drier than wood.) Once the air dries out, the heater really takes off and burns wicked hot.

So, the huge plus side for this is that I'm drying out the air, which greatly reduces the risk of disease on the plants (specifically powdery mildew). I"m also drawing in fresh air into the building. If I brought in outside air, I loose these pluses - and run the risk of burning more humid air if it's raining out or general high humidity. I know during the winter in most areas the humidity is very low, so this is probably a minor problem.

I'm also concerned about about the system backdrafting with a direct vent to the outside. I know on my house furnace, the air intake is right next to the exhaust. I inquired to the installer about this and he said it is to make sure that if the wind hits the house, it keeps the pressure in the same in the furnace. If the intake was on one side of the building and exhaust is elsewhere, there is an unequal pressure created. If you do this, I would recommend putting a damper in-line with the intake air somewhere....maybe even (gasp), a blower fan.

The history of my heater: Rob's Rocket heater Playlist
 
Scott Perkins
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The use of an automatic flapper valve damper in either the intake or the exhaust chimney is never a bad idea as under normal circumstances
its existence is invisible. My experience with outside air comes from installing about forty five stand alone wood stoves in custom homes I
have built. Backdraft is only a problem if you have hi pressure at the top of the chimney in those situations but even with stoves that
do not have outside air feed, I have never heard of a backdraft so bad that flame shot into the living facilities.
Now if you had a super strong backdraft that could blow the fire backwards, this is definitely where you would want outside
air feed because the flame ( if it were ever to happen ) would blow out the intake air duct instead of into your living room.

Now as was pointed out, if the exhaust port was located in an area that also contained the intake inlet, then large changes
in pressure would affect them both simultaneously and there would be no differential in pressure that could cause a backdraft.

As the green house manager, it is up to you if you want to bring in dryer colder air to replace some hi humidity warmer air. You are then
trading heat input for ventilation and I think I would rather use fans to power that instead of burning fuel to power the
air exchange.
 
Chris Burge
Posts: 90
Location: Spokane, Washington
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Scott-- You are correct in the assumption that most RMHs, by drawing in cold air from the outside, have a detractor to their efficiency quotient during the burn, but this is offset by the fact that either L-tube or J-tube rocket stove designs have a much more efficient fuel burn than even the best airtight pellet stoves. Plus, an RMH only burns long enough to 'fill' the thermal mass with energy which is then slowly released for many hours after the fire has gone out. While it may be true that if you rig up a conventional woodstove with an outside air intake that you might see a 20%-30% increase in overall efficiency, doing so with an RMH is almost a moot venture.

Mainly because, as Chris pointed out, cold outside air is just that: cold. To actually take advantage of the slight efficiency increase provided by burning exterior air, one would have to preheat the air to maintain proper temperatures in the burn chamber, which requires energy. Sure, you could rig up something that would get a little heat from the ambient air or heat exchanger, but all the effort for a slight energy gain that still must be paid for with a slight energy loss is a bit of a sisyphian undertaking.
 
Scott Perkins
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Cris-
I follow your logic but I think you are mis stating your data. In the wood stove world, I have seen texts conducted where the intake air
was 75 degrees different ( outside air vs inside air ) and the combustion temps were not affected or significanlly different. One way
this was explained was that colder air is denser and supplies a slightly greater amount of oxygen. They know this to be true
in race cars or any cars with turbo chargers ( and airplanes) where they try real hard to use the coldest possible intake air possible.

Of course it is to be assumed that the difference between outside air burn or inside air burn is only relevant during the
time that you have a fire burning and I admit that the rocket mass heaters should not need to burn as long as
or continuously as do wood stoves etc.

I just wanted to offer up a fresh look at an area that seemed to me to be overlooked when lots of other ideas are being
floated for marginal improvements. Nonetheless, decision making is about cost vs benefit and we want to be as
accurate as we can in defining both the costs in effort and materials and the benefit so better decisions can be made.

While we may differ as to degree of benefit, the effort to isolate the intake to use of outside air is very simple and
easy.



 
Chris Sturgeon
Posts: 91
Location: Yukon Territory, Canada. Zone 1a
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Good discussion. In my reserch into RMH I have definatly seen people look into outside air intakes. Not saying that your idea is old, Scott; you may have a different way of doing this than I've read about before.
Most systems I've read about consisted of underfloor ducting of extertior air to a standard placement interior feed tube. In every case these builders found it resulted in horrible back-draught issues, perhaps because intake and exhause were on different sides of the building creating a pressure differential? That last part is speculation.
I do know that most people going down this road have abondoned it. Care to do some further bush-whacking for us Scott?
 
Scott Perkins
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Chris Sturgeon wrote: Good discussion. In my reserch into RMH I have definatly seen people look into outside air intakes.

Scott's reply - Good, I think it is worth pursuing considering my extensive experience with fireplace inserts and wood stoves.

Chris quote:
Most systems I've read about consisted of underfloor ducting of extertior air to a standard placement interior feed tube. In every case these builders found it resulted in horrible back-draught issues, perhaps because intake and exhaust were on different sides of the building creating a pressure differential? That last part is speculation.

Scott's reply - Chris, I dont know why any back draught issues would be caused any differently by the intake air source or location. Whether it be outside
source or the interior feed tube, if a hi pressure hits the outsside or exhaust "vent", flow is going to go backwards regardless of where the intake is.

So if you were going to have a severe back draught event, I would rather funnel it to the outside instead of having a flame shoot out into the house.


Chris Quote:
I do know that most people going down this road have abandoned it. Care to do some further bush-whacking for us Scott?


Scott's reply - Chris, Respectfully I dont know how you can know that since it is extremely commonplace among major wood stove mfgrs
the world over. If you are talking about exclusively RSM heaters, I am a rookie and have no idea what the activity has
been or why. What I also see missing with the RSM heaters is any kind of airtight door that can be closed when not burning
to eliminate air flow through the system in any direction. Either direction of airflow is bad for performance if the fire is not burning.

Again, it boils down to the cost vs benefit decision. It becomes more attractive as the benefits increase and the effort decreases.
and vice versa. Both benefits and efforts have to be maximized in order to deem it worth it.

If I were doing it, I might consider a refrigerator on the outside wall and when you open the sealed door, you see
the fuel intake for the RSM heater. Discarded refrigerators being very common, much easier than building a air tight
closet or cabinet. Of course the refrigerator would have a duct from the interior of the fridge thru the outside wall
of the dwelling ( or any other outside location ) to the outside air source.









 
jay woo
Posts: 2
Location: N.E Victoria, Australia
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I'm every early into the design stage of a small dwelling and one of the thoughts I had, which was more to eliminate smoke inside the dwelling, was to have the feeder tube on the outside of the dwelling with the drum on the inside of the dwelling. I've not read any data on the length of the burn tunnel so it may not work, depending on the material used for the wall of the dwelling.
 
Ernie Wisner
gardener
Posts: 791
Location: Tonasket washington
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do not put in outside air supply! good god think about the design before you do something stupid.
the whole system of an RMH is designed as a lock just like the U-trap on your toilet the temp equalizes and you dont get any flow out or in.
as well why the hell do you want to breather worse air than your stove does? let it bring in clean air and exhaust the dirty air cleaned outside.
not only does this sort of foolishness cost you health in your house, done wrong and you have an extra chimney in your house to close to the feed and it heats up and acts as the path of least resistance.
Erica and I have said this a number of times, we have shown the problems caused in work shops for the last year. What part of its a stupid idea dont folks understand?

it causes smoke back if done in the feed tube, it causes Carbon monoxide because you cant get proper mixing of air and fuel, you get more free radicals in the exhaust. more air does not mean better at this point the system feeds itself just fine. bringing in the proper amount of air at the proper place. If you want more air in your house crack a window or drill a hole in your house on the other end away from the stove.

if (since i cant seem to get this out of folks heads) you want to play with outside air feeds, do so out side away from the ones you love, pay up your life insurance as well just in case.

Alright i am done with my rant and since it doesn't seem to get through; please dont subject your family to your unproven ideas till you have thoroughly proven them outside.
 
Chad Douglas
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This is my first post on this forum, so please correct me if I'm wrong -- I have not absorbed all the knowledge here yet.

To me, plumbing in outside air would be a great advantage, for two main reasons: (I'm basing this on physics, not practical experience, FWIW)

1. A RMH that uses inside air for the combustion is taking already warmed air from inside the structure and pushing it outside. Granted, you're able to reclaim some of that heat, but only if the air temp at the intake is higher than the exhaust temp where it leaves the building. Doing this will cause a slight negative pressure situation in the structure, and the structure will pull air from the outside to equalize. This will create a draft. So, you are wasting energy by taking 80-100 degree air and replacing it with -30 degree air. Pulling the air from the outside would eliminate the draft.

2. Air is denser when it's cold. This means that in any given volume of air, the colder it is, the more oxygen there is available for combustion. This is why people put cold air intakes on their cars, and why your car often feels "peppier" in the winter. So, if there's more available oxygen to burn, your fuel will burn hotter, which will drive up the efficiency of the RMH.

Now, figuring out how to ensure that the RMH gets ALL its air from outside could be a challenge, since there has to be some mechanism to put fuel into the burn chamber. I was thinking of making a pellet-based hopper like the one guy here did and having the hopper outside, as well as the air intake outside. then, to create a restrictor system, have some way of sealing the top of the combustion chamber and pulling in a little air from the outside as well, this having a completely sealed system.

Please explain to me why this is wrong.
 
Ernie Wisner
gardener
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Location: Tonasket washington
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1. you want air exchange of about 1/3 volume an hour at minimum. your entire house and EVERY product in it other than those that are totally inert off gas as the temp inside rises. most of this off gassing is toxic in concentration so exhausting it is a really good idea.
this is why folks spend a huge amount in air conditioning.

2. how much more efficient do you need the system to be; it is already metering its own air intake to optimum for the material being burned. in very few cases you will get more O2 in that could possibly help the burn and that would be when the stove is pulsing and then its just best if you let it slow down.

3. please build the test case out in the backyard and away from your loved ones, take the time to actually experiment with a stove built to spec before you try and fit your nice sealed up system to your house.
if you read the discussion not only in this thread but others in the part of the forum you will get this answer several times. Also if you do fit some sort of pellet feed that requires no electricity let me know.

have any of you with the bright ideas actually built an RMH? or is all of this supposition?
 
Chad Douglas
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Gravity-Fed Pellet Feeder:

http://www.permies.com/t/18515/stoves/Burning-Pellets-Rocket-Mass-Heater

Any gain in efficiency is good. That should translate into hotter temps in the riser, more complete combustion of gases, and more heat to sink into your thermal mass. Maybe it's a case of diminishing returns, and that the increase in efficiency is simply not worth the expense and trouble to build it with an outside air source.

I'm just thinking of my wife, who doesn't handle cold or drafts well. Maybe by putting a "controlled draft" near the air intake area would be a good compromise, so the rest of the structure would remain draft-free, and only the area right near the stove would be colder......
 
Rob Torcellini
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I agree with Ernie....
 
Ernie Wisner
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i had forgotten robs setup thank you for reminding me.
I need to check and see how his testing year is going so far.
Air feed close to the stove works well air feed into the stove IME not so well. closed system not a good idea when dealing with wood gas. whatever you do keep folks informed.
 
allen lumley
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Location: Northern New York Zone4-5 the OUTER 'RONDACs percip 36''
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Scott Perkins : Lets crunch some numbers, we are building very efficient Rocket S.M.H. now, lets say 85% perfect efficiency, as a believer I think its higher, but this is a number that other people can look at and want to believe !

So that would give us a less than perfect efficiency of 15% it is from this number that any increase in efficiency MUST come ! So a 30% improvement would make my Rocket, 4.5% more efficient ! Certainly a goal to strive for but not a big difference in the wood pile.

I stand with the people who have tried to make a cold rocket go, and not smoke at all,* and have noted extremely long times to achieve this goal when I am sucking in cold air through my feed tube!

You are not the only one to worry about the heated air that becomes my Rockets make up air and is used for combustion, however when it gets to my thermal bench it is taken up to be recycled as butt warming B.T.U.s which is more valued to me than warm air !

Here is a simple test to see if your Rocket is thermo-syphoning heat out of your system when you are not actively burning wood - get a ventilator cap the kind that spins to pull exhaust air out of an attic. The ventilator cap should be a close size match so that a piece of cardboard will allow it to sit over the feed tube opening, if the ventilator spins its thermo-syphoning , if not its not ! Contact me if you find one with a J-Bend that does !

If it does, set a couple of bricks or a paver over the Feed Tube opening after you are done burning your fire and make a mental note to check that out - most other wood stoves, unless you are making a smoldering stinking fire to begin with will greet you with wide open draft controls when you come back to them ! (Just comparing apples to apples )


A few words about negative pressure, do you have to open a window when you turn on the exhaust fan over your kitchen stove ( if you don't know, ask the cook) if not your house aint as tight as you think it is ! Pyro -AL

* Not smoke inside the house or from my exterior chimney ! ( burning bone dry small wood at start-up, that my standard and my bench mark ) Smoke tells me my efficiency sucks !
 
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