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more questions about proper sizing of a batch box for my dome home  RSS feed

 
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So I have looked at the batch box style and think that is the way for me to go.

I have seen all the ways to figure up the size of the stove, with the exception of how to size it to the house.

I have seen one formula: Q=G*V*DT

Where Q is the amount of Kw required

G is an insulation factor.

V is the volume of the house (m3)

And DT is the difference in temperature from outside to inside.

I have a dome home that is 36' in diameter.

That gives me a Volume of 346 m3.

For my house I chose to use the insulating factor 1.6

and my DT is 25 degrees celsius.

According to that formula my Kw requirement would be 13840 or 13.8 Kw

According to the chart, there is no way for a batch box to heat my house.

The bigger the internal diameter, the heavier the load of wood for each fire:
Internal diameter (mm) -- Wood load (kg) -- Mean power considering two fires a day (kW)

    125 mm — 3.5 kg — 1.1 kW
    140 mm — 4.9 kg — 1.5 kW
    150 mm — 6.0 kg — 1.9 kW
    175 mm — 9.5 kg — 2.9 kW
    200 mm — 14.2 kg — 4.4 kW
    230 mm — 21.6 kg — 6.7 kW
    250 mm — 27.8 kg — 8.6 kW

Considering that a 250 mm stove is a 10 inch stove.

There must be a problem with either the formula or my math skills.

My numbers are:   Q= 1.6 * 346 * 25    so Q= 13840

Can anyone advise me on this? What have I missed?

Thanks in advance.

I really want to use one of these.
 
gardener
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IIRC, those numbers are either instantaneous, or released kw/h numbers.

Are you sure your Q is not in watts? Better ask at Donkey's.

 
gardener
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I've checked the numbers, there's nothing wrong with your maths. You can get away with one single and one double fire a day in a 10" system though, that would deliver enough power to heat the house properly. Or build an even larger one, a 30 cm or 12" system. The bell size would be huge too, it seems to be a viable option to add a bell bench.

But I'm inclined to think your problem is the insulation of the house, as there's none. Better start to insulate the hell out of it. It's definitely work but it needs to be done just once, as opposed to processing enormous amounts of fuel every year, again and again.

For example: our house is about 650 m³, passive house quality insulation value, temperature difference about 28º C during two weeks per year (we like it quite warm). We use a 15 cm (6") system and alternating single and double firings to heat the house. Usage of softwood per winter: 3 m³ or just short of one cord, provided my calculation is correct.
 
Tom Riker
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Thanks for the replies.

Perhaps the insulation number is my issue. I chose 1.6 insulation factor because it was a guess.

Since a dome is all roof, I put grace membrane over the entire dome, so it is literally air tight with the exceptions of doors and windows. (  grace to seal around them too)

Since it is built from 2x6 lumber, my insulation is R-21.

All the literature I have read says a dome requires 30-40% less BTU to heat and cool. Add to that my complete membrane; I believe I should be very close to that.

Would R-21 and an air tight home give me a better Insulation factor than 1.6?

I currently stay in an mobile home that is VERY drafty and poorly insulated and my only heat source is an "Old Timer" wood burner with an 8" flue. This thing will run me out of the house if I fire it up and open the draft. I can only let it idle. I currently am burning around 4 cord per heating season.

I would like to use a batch box with a bell. I have enough room for a bench if needed. my floor is unfinished concrete, fiber reinforced at 4" thick. Height is not an issue because the ceiling at that location is 14'.

Thanks again for any help.

I have bought the book from Paul about building RMH. Is there a book that covers how to build batch box with bell? Maybe a set of blueprints for a known good design?
 
steward
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Location: Northwest Montana from Zone 3a to 4b (multiple properties)
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I found a post over at Donkey that has your calculation in it.

http://donkey32.proboards.com/thread/2174/size-batchrocket

My take, is that your G is likely at or below 1, it may be as high as 1.2. This takes your Q closer to an 8 or 9kw value.
 
Tom Riker
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Thank you for the reply.

Since I didn't know what an insulation value was or how to figure it, I took a guess.

The only figure in that list that I recognized was the straw bale, at .5.

I know that a straw bale averages R-30 here in the States. Considering that I have R-21 and the Dome itself requires less energy to heat and cool, I should be VERY close to that.

Using the .8 figure gave me a 9 inch batchrocket.

I am thinking of building an 8 inch batch with bell and a mass bench. Any thoughts? I also have a brand new Mitsubishi ductless heat/air pump that I can use as back up heat.

I also wonder how much mass to use.... is there a ratio of rocket size to mass? I am in the process of reading batchrocket.eu. It is probably in there.  I am guessing about 3000 pounds
 
gardener
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Location: Upstate NY, zone 5
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The ratio of system size to mass will vary depending on your climate. If it gets cold and stays cold for days, weeks or months at a time, you want a larger mass to even out temperature swings in the space. If you are subject to frequent temperature swings, cold one day, warm the next, you want a smaller mass that will respond quicker to running the fire or letting it stay out. I couldn't tell you the actual masses to use for a particular situation.
 
Tom Riker
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Thanks Glenn. I live in Indiana so it may be 65 one day and 30 the next.  I have access to plenty of wood so I am not concerned about the amount of wood I use, except for the simple fact that I don't like wasting it either.

My wanting to use this type of heater is that it burns so much cleaner and is more efficient with the fuel it does use.

I can fire three times a day easily.

Currently I burn continuously at an "idle". It is VERY dirty and I get ALOT of creasote build up.  Bad for the environment. Bad for the flue. And I use a lot of wood.

I could heat with propane, but I really like the wood heat.

Thanks for explaining about the mass sizing, I think I should keep the mass to a medium size because of the frequent changes in climate. ( Oh no....climate change)


 
Tom Riker
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Okay. I think I figured out what an insulation factor is. It is called a K-factor here.

You can figure it by dividing the thickness in inches, by the R-value.

A straw bale is roughly 16" thick. It has roughly R-30.

16\30=.53

My dome has R-21 in 5.25"

5.25\21=.25

I hope this will help someone!!!
 
Tom Riker
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With the new information, numbers look WAY better!

347*.25*30=2595 or 2.6kw/hr

I am in the 200 mm range!

Looks like I'm building me a RMH!

Now to get the videos!
 
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