Lawrence Wood

Kyrt Ryder wrote:Lawrence, is it possible the baseline your working from is heating the air of the space in those calculations? RMH waste only a small portion of their energy heating gasses, most of it is radiated directly.

Then there's also the question of whether you're using the raw energy to raise heat values, or extrapolating needs based on wood stove knowledge.

Actually I'm using industry HVAC heating values so this would be forced air heat.  Really I think this is ultimately where the disconnect is. So I've read Paul's comments, looked at the video, did some other research and what i've come up with is the following:
1. RMH are used differently. More on this in a moment.
2. Comparing RMH to wood stoves, oil heat, electric (in other words radiant/convection/conduction) is pointless
3. Adopting an RMH system is a change of lifestyle not just switching out your heater

So what we have is a situation where if you adopt an RMH that you are also buying into changing how you think about heat and how you are willing to change your lifestyle to adapt to those changes. You have to redefine what you consider to be heating your home. With forced air heat you heat the air and move it around. You as a person live within that heated air which has a relative degree of uniformity.  With an RMH you have a mass of heated material that slowly radiates the heat into the space in a much more localized manner. I don't doubt that you can blow a fan across it to move the air around and heat things more uniformly.  If you are one who likes a uniform heat throughout your house I doubt an RMH would fit your desires. if however you can adapt to having heat in one or two localized places then this is a potentially a good option. Note in the video paul posted the heat is extremely localized and the room is cold. The subject feels fine. She had to wait a bit for her hands to warm up on top etc but overall she felt good  What you notice is the positioning of the RMH in locations to place them close to where you will be so you will be heated.

I compare RMHs to a high efficiency boiler like a Garn (www.garn.com) that is also highly efficient but considerably more flexible since you can put radiators all over and or have radiant floor heating. They have their downsides for sure since a Garn or similar unit is considerable more expensive, requires electricity and could leak like any other water based unit. Now I spent a few decades in Cascade county MT. It was cold as hell at times but on the bright side it was low humidity. Today, as a resident of Western WA I have to deal with high humidity. As a result I have had a few instances where lack of air circulation has allowed cold wet spots to promote the growth of mold. In my quest to research RMH heaters for deployment in a workshop, house and possibly a chicken coup i have concluded that i need to account for localized heat and while I would love to use as little heat as possible like the video demonstrated, that would be insanity in this humidity. I had originally thought that all I needed was dehumidifiers to lower that humidity but then I learned about dead spots and had to rip out some moldy drywall and carpet. Ironically the air drying effect of a standard wood stove is quite helpful in my situation.  At present the only combination that is effective is some heat, dehumidifiers and air circulation. Unfortunately with the layout of my current house the locations of the RMH compared to where I would need to have heat are incompatible. As such a hybrid scenario can be employed where the RMH would heat the main area and some more localized heating for other areas.

In my original question I wondered how 1/2 cord of wood would be enough to heat a structure. it makes sense when you consider you are not heating the air as a forced air unit does. You are not heating the house per se you are heating those areas where you are living and from what I can tell, to a lesser extent. In my case I doubt I could get by with so little fuel but i think I can reduce the consumption considerably.

David Livingston wrote:Lawrence
I think you may be missing my point .
I doubt not your math
Some people find they use an eighth of what they used previously BECAUSE they had a crap fire to begin with !
If they did not have a crap fire to begin with they they wont I dont expect to use an eighth less but I do expect to use a lot less .

David  

Actually your point is the one I think I get:-) and actually I quite agree. I think there are a lot of really inefficient wood stoves out there that are going through fuel like mad and have a lot of heat going right up the chimney.  I'm also wondering about how many are backing their stoves with some other kind of heat like electric etc. With that situation they would easily be using less wood fuel and not making for good comparisons.

David Livingston wrote:Hi Lawence
I dont quite understand your argument .
I burn about six units of wood a year in my home here in France . I have a conventional home conventional stove  . If I get a RMH and I build it my self I use less wood than I do at the moment I still heat my house . May be a tenth  may be a quarter of what I burned before . The important word is LESS the rest is just detail .

David  

I don't doubt at all that an RMH would use less fuel. Of course my question lies in how much less but I'll get to that in a moment. I'm a fan of RMH, don't get me wrong. Heres how I see it:
1. Typical wood stoves suck.
    A. Dirty
    B. Not efficient
    C. Overrated efficiency numbers
    D. Some need expensive catalysts
    E. Reburn chambers are good but add complications to design
    F. Heat capture is too low
    G. Heat transfer is too low
    H. Expensive, comparatively
2. Rocket Mass Heaters are impressive
    A. Efficient
    B. Cost effective
    C. Less fuel needed
    D. Flexible design

Now back to my question. Amount of fuel needed.  So far we have the following pertinent points:
1. Estimates are fuel usage is 1/8 to 1/10 that of a conventional wood stove or as one example in this thread from 4 cords down to 1/2 cord.
2. Per paul there are thousands of these heaters built and they are consistent with the 1/2 cord claim

So heres my question, restated from another direction.
Does 1/2 cord of firewood contain enough btus to heat a house for an entire winter?

From what I have been able to determine:
Not adjusted for any particular climate, or insulation factor and assuming the heat duration is 3 months at full heating, 4 months of half heating, 4 months at 1/4 heating
1. A 1400 sq ft house takes approx 24,000 btu / hour,
2. Estimated usage: 3 months at full heating, 4 months of half heating, 4 months at 1/4 heating
Using the following calculations and 100% efficiency:
Months Days Hours/Day Hours
3        90 24         2160
4        120 12         1440
4        120 6          720
 Total        4320

Total BTU 103,680,000
BTU/Pound 10800
Total Pounds 9,600
3. Most common species of wood weigh between 3000 - 5000 pounds when green. Estimate 1000 - 3000 pounds when dry.

So to get the btus we need 9,600 pounds and dividing that by 2000 pounds average dry weight we get shy of 5 cords. Another way to look at it is 1/2 cord is 10,800,000 btu about 1/10 of the btu needed for a years heating.  So while I have no doubts whatsoever about the efficiency of the RMH i just don't see how you can get enough heat out of that amount of fuel regardless of how it is burned.

Glenn Herbert wrote:You may remember that they talk about their wood use per day, and figure about a cord for four months of deep winter in the mountains near the Canadian border. This is for their pre-existing small house, construction standards unknown but probably standard stick-framing at best.

This reduction in fuel is an interesting claim. if we can simplify things and simply multiply the %efficiency * fuel btu per unit weight. Now if we have 100% efficiency then we should have the full btu value/unit mass which means we have our maximum theoretical value for a fuel. But no matter the fuel that btu value is limited. So no matter the technology or the fuel there is a limit to the amount of heat produced. Because of that I have doubts about people being able to heat their home with 1/8 the wood when it seems that it is unlikely that for a given home that they would have a fuel that produced enough heat, even at 100% efficiency.  Now I haven't run the numbers but even with very good insulation and high btu fuel I doubt that a situation where 7 cords were burned before can heat a home with 1/8*7 cords. Anyone else wonder about this?