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Post by faraday on Sept 30, 2012 18:06:55 GMT -8
Building a 1500 SF high thermal mass home (concrete with external insulation). Would like to include a rocket mass heater but can't figure the chimney thing out (searched but can't find the info).
What we would have is concrete slab, vaulted ceiling about 17 feet up, 5 feet of attic, and 3 feet of external stack. So the system would be about 3 feet of stove height, 14 feet of black stove pipe, and 8 feet of Class A insulated chimney pipe, or 22 feet of stack on top of heater. This would be vertical coming out near the peak of the roof.
Understanding that a RMH has a relatively low temp output, will a 22' chimney be too tall?
Any recommendations on internal diameter of chimney pipe?
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Post by Dan (Upstate NY, USA) on Oct 1, 2012 3:39:57 GMT -8
I have an 8" RMH with a external 6" triple wall stainless steel 30' vertical exhaust. Keeps the heat bench at a negative pressure so the little air gaps in the clean out always suck air in, even when the stove is not running. Ensures no CO2 getting into the living area when its being fired.
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Post by koldenburg on Oct 1, 2012 4:20:25 GMT -8
I've been involved in the construction of many high performance homes. RMHs are not sealed combustion so if you house is tight don't plan on running bath fans or a dryer while your stove is burning.
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Post by faraday on Oct 1, 2012 5:36:14 GMT -8
wolf1004 - I assume you meant 6" ID pipe, running external to the home. Glad it doesn't smoke. Thanks for the info.
koldenburg - I'm planning on burying a couple of air intake pipes under the ground to temper incoming air, so things can breathe. The home depends on stored summer ground heat (hot attic air circulated under slab) to supply a portion of winter heat. How "untight" can RMH's be and still function? In otherwords, do they depend of tight packing of cob to seal them or could something like sand be used in the bench area?
I'd appreciate a few other responses from folks with tall chimneys to get a better feel for that issue.
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Post by koldenburg on Oct 1, 2012 9:02:57 GMT -8
faraday - I don't want to be dismissive of thermal storage but hot air from the attic may not be stored all the way 'till winter for you. For every 1000sf of concrete slab 4" thick you're looking at storing about 185,000 btus for every 10 degrees above ambient temperature. That's only equivalant to two gallons of propane storage. I would highly consider installing lots of insulation under and around the perimeter of the slab and rather than using hot air, install some pex tubing for a future solar panel that could add heat as the sun shines.
Don't forget that the Earth is an infinite heat sink and you won't overcome heating a slab without loosing most of the heat to the ground unless it's well insulated. There's a reason that the European passive houses are using 8-10 inches of insulation under the slab.
How much fresh air needed for combustion is going to depend on how tight the house is and how large exhaust fans are in the house. Moreover, do you plan on operating the fans at the same time as the stove? My house can depressurize to about negative 25 pascals with the dryer and bath fan on - that means the chimney would have to have lots of draw in order to overcome that pressure. Even though I have what was sold as a sealed combustion stove it will smoke the house with a low fire and the dryer on since there's just enough openings from the controls which make it not actually sealed combustion. In my case, my wife forgets that there's a fire and turns the dryer on making the house smell smokey. This is much more of a safety concern for a stove without a sealed fresh air supply and a dying fire as it could bring a substantial amount of carbon monoxide into the living space.
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Post by matthewwalker on Oct 1, 2012 10:33:49 GMT -8
Faraday, mine is plumbed into an existing masonry chimney. About two feet of stove height, 5 feet of black stove pipe, then into the chimney which rises another 10 feet or so. It's an 8" system, and once the mass is warmed for the season it never smokes, and has constant draft even when it hasn't been burned for a day or two. In my opinion these things aren't sealed well at all, and work just fine that way. I agree with Wolf, once warmed up it's running at negative pressure and pulls in at any gap rather than leaking out.
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Post by faraday on Oct 1, 2012 13:13:28 GMT -8
Hi koldenburg - I completely understand where you're coming from, IF you depend on the slab for storage, and assume the earth is an infinite heat sink. But, there's another way to look at things, and I'm going to do a little experiment with it. Here's the concept in full: 1. Treat the ground under your house as your thermal friend, rather than enemy. 2. Insulate the ground from your wall out, going out 10 to 15 feet all around your house. This would be down say 24 inches below the surface, and sloped away from the home. 3. Put a waterproof membrane over the insulation ALL around the house. Diligently prevent any precipitation from getting through this layer, because precipitation both cools the ground, and increases its thermal conductivity. Use rain gutters, downspouts, and pipe to channel roof water past the zone of the protective thermal umbrella. 4. Buy the theory that heat travels through dry earth at a VERY slow rate (like 15 feet in 6 months). See below... 5. Either inject heat 10 to 15 feet below the surface of the home and let it slowly rise to and through an uninsulated slab, OR inject it just a few feet below foam insulation below the slab and force it to take months to get the perimeter where it then heats the house from the footing area. As absurd as this may sound, there's a fellow who's done this and it works. See below... 6. Take really hot attic air during the summer and circulate it through thin wall plastic pipes buried in the earth. Turn the fans off, only when the attic temp drops below ground temp. If you do some quick calculations based on influx of solar radiation over the area as large as a roof, you'll see that you have a considerable amount of energy, and even with 5% or less getting stored to the earth, you could probably coast through a winter. 6. Wait 2-3 seasons for the ground temperature to stabilize (this is the bad part), while the house gets warmer each winter. Just in case it doesn't work I've incorporated hydronic heating, and hopefully will also have a rocket mass heater. The system as I've described originated (I think) with a fellow by the name of Don Stevens from Spokane, Washington who calls it "Annualized Geo-Solar Heating" or AGS. A white paper for it can be found here: www.greenershelter.org/TokyoPaper.pdf
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Post by Donkey on Oct 1, 2012 15:37:41 GMT -8
is is wandering a bit off the topic of stoves, but I've just gotta add in 2cents..
koldenburg, air tight houses have been shown to contribute to a list of negative health effects. I hope you've allowed for adequate air exchange with the outside.
Fortunately my climate is so forgiving, all I need are some windows facing south, a bit of insulation in the ceiling and a little thermal mass.
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Post by koldenburg on Oct 1, 2012 17:21:36 GMT -8
Donkey, the more modern, best practice, versions of tight homes rely on knowing how tight they are with actual measurement and planning for fresh air for people (and modern offgassing materials). Many of the older sick house syndrome homes failed to deal with excess humidity and condensation issues making for a toxic environment. Without measuring it's purely speculation if the house is tight, and if if you want it not too be "too tight," do you put in random holes to approximate how leaky it should be? You're definitely correct that some climates are much more forgiving than others.
Faraday, great you're up for some experimenting! I have read reports of similar methods to your own, and been involved with construction of similar concepts, with very differing degrees of success. Again, your climate is probably much more forgiving than mine.
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Post by faraday on Oct 1, 2012 19:51:42 GMT -8
Does anyone else have experience with a tall (23 foot) insulated stack? If it looks like a go, I need to buy the roof flange, put it up, and finish shingling.
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Post by grizbach on Oct 2, 2012 0:55:42 GMT -8
faraday, I've got a 2.5 story house with a small RMH(5.75"- 500lbs). It's piped into my exsisting 10" double wall chimney. The exhaust is hot enough that I usually close down the damper to about 1/3. I love that if it's windy out or if there is some flame back from the burn chamber, I can open the damper and have it pull as strong as I want.
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