Best way to measure amount of wood used over the winter?

I want to brainstorm some ideas on how best to measure the amount of wood we use over the winter to heat our house with a rocket mass heater.

Volume isn't going to be a good guide, especially as we'll be burning as many scraps and offcuts as possible.

One tub of small sticks doesn't actually represent very much wood.



We also get left-overs from roofing jobs. Which would make a much heavier tub!



And then there are very thin strips of wood used as packaging for roofing tiles...



And we are likely to have to supplement that with some 'real' wood cut from actual trees.

I think weight is the only way to make it even vaguely scientific, but how to actually record it? Weighing each log we put in the firebox, or even weighing each bundle we put on, is going to get old really quick. And there's so much danger of forgetting to actually write anything down.

I thought maybe we could have three tubs of burnables - of small stuff for kindling, one of big wood offcuts, and one of 'real wood'. Then weigh each tub, record it, note the date we start burning it, and weigh it again when we refill it. That way we can stoke the fire with whatever we feel like by grabbing it out of one of the tubs and it really doesn't matter if we don't note it down each time.

Maybe some days we can be more detailed and record actual weights burned each day, or something simpler like how many times we refill the firebox. Maybe take some notes on outside temperatures.

Does anyone have any ideas on how to make the measurements as useful as possible? What information do people actually want? And what system of recording is likely to actually be used rather than just forgotten when someone decides to thrown just one more log on the fire?

Brainstorming and suggestions welcome...

Hi Burra,

I agree that weight is the way to go.

Seeing that bucket my first thought was one of those scales to weigh your luggage. You could hang that either close to the stove or where the firewood is stored and add a little book with a pen next to it to take notes.

I like your idea of weighing the whole tub, but how would you do that?

As to what measurements people are interested in I would think maybe kg/h to roughly calculate the heat output of the model of stove you built?!
For you the overall consumption would be interesting I guess, but couldn't really be applied to other houses or different stoves.

I would rely on actual weight, but I would also guesstimate volume and weight. Over time, your estimates will become more accurate.

I can’t connect this train if thought with wood, but in calculating work performance I ran guesstimates, production outcomes, and a pretty scientific method of calculating predictive outcomes.   At the start, guesstimates were the least accurate.  Over time, all were in agreement.

The advantage is that eventually you will able to monitor your wood use without weighing.   Of course, an occasional spot check is a good idea to make sure you are not drifting too far off in your estimates.

How about a simple bathroom scale by the door where you bring in the wood?

Record the weight of person + wood. When wood is dropped off, weigh the person. This will account for boots, coats, hardware in pockets, etc. Record both values.

The problem is that bathroom scales always (ahem) seem to read heavy.

I like your idea weigh the bucket/container when you refill it and record it with the date.

Before the season starts you can weigh the containers empty so you know how much to remove from the weight you get each refill.

OK, I think we have a solution.

A friend visited me today, with some wood for the rocket mass heater, some tubs of donkey poop for the garden, and a lovely old locally made harvest basket for me to use as a log basket.

Also Poppet, but I had to give her back...



The plan is to weigh a tub of wood, write down the date and the weight, then stash the wood in the basket. We can end up with a mix of different size wood in the basket and can just grab what we need without having to worry about weighing or recording while we actually use it. Then when the supply in the basket starts to run low we weigh out another tub and decant it into the basket. I've ordered some luggage scales, which hopefully we can set a tare on to allow for the weight of the tub.

We're both allergic to bathroom scales, and in any case we are likely to go outside, pee, grab some fruit to sample, and generally mess up plans for any weighing system that involves us standing on scales.

I think it will work without too much daily hassle, and if we feel like it we can record more accurately on particularly wet days or whenever the fancy takes us.

Also, I have the most awesome friends.  I mean, who else has the sort of visitor that rolls up on a two wheeled  tractor pulling a trailer loaded up with gifts like that!

Looking forward to the results!

I agree with weight.  Though you might want to weigh different types of wood separately, as different woods give off different btu's.  I really couldn't tell you how much we use.  We burn mesquite and we usually cut once a week during the winter with an elderly family member who feels more comfortable with someone around.  I also pick up lots of little pieces of deadfall for starting fires and fueling outdoor cooking.  We don't cut during the summer because of the heat and the rattlesnakes.  

Judy Bowman wrote:I agree with weight.  Though you might want to weigh different types of wood separately, as different woods give off different btu's.  

I'm hoping someone can point me in the direction of reliable information on this, but I'm sure I learned once that the difference in btus from different woods is just down to their density and moisture content, and that if the wood is dry you get the same energy for the same weight of wood.

Does anyone have a good resource for this?

Burra Maluca wrote:

Judy Bowman wrote:I agree with weight.  Though you might want to weigh different types of wood separately, as different woods give off different btu's.  

I'm hoping someone can point me in the direction of reliable information on this, but I'm sure I learned once that the difference in btus from different woods is just down to their density and moisture content, and that if the wood is dry you get the same energy for the same weight of wood.

Does anyone have a good resource for this?

Agreed. A pound of wood is a pound of wood. However, you might get better efficiency out of a denser, slower burning, wood as the masonry will have more time to “saturate” with heat.

I'm wondering if you might be coming at this the wrong way, or maybe you want to come at it a couple different ways.  Someone touched on it, what I'm thinking anyway, but not the way I'm thinking about it. BTUs were mentioned. I'm guessing you are trying to figure out how much wood you will use to keep warm. Rather, you might want to try to figure how many BTUs is needed (or that you use) to keep you warm. Your weight of wood might be a way to go, but also the volume of wood might be a way to go.  The typical unit of measure that I'm familiar with used for firewood is the 'cord', which is a volume of wood. (A stack of wood X feet tall by Y feet wide by Z feet long. Depending on the kind of wood, the amount of time a cord will last you will vary considerably. If you are going to use weight (and I'm not telling you not to) consider that dimensional lumber that is kiln dried is going to have very little water weight while a tree cut in the woods will have a higher water weight and possibly a considerably higher water weight depending on how long it has gotten to dry. Personally, I'd look at trying to measure volume or BTUs, but don't ask me how to measure BTUs (might give best answer though) someone here might know or have an idea.

just a note. Don't burn treated wood. A couple reasons. It might not be healthy, and/or it might gum up/contaminate your stove.

Mark Reynolds wrote:I I'm guessing you are trying to figure out how much wood you will use to keep warm. Rather, you might want to try to figure how many BTUs is needed (or that you use) to keep you warm.

What I'm actually trying to do is to have a ballpark figure to give people when they see the rocket mass heater and ask "So how much wood do you burn on that thing every year?"

Cords don't mean anything here. Cubic metres might, and firewood is generally sold either in big 1m3 builders bags, or from a truck carrying 2.5 m3. It's a very rough and ready measurement as the wood is never stacked, just thrown in roughly with an awful lot of empty space.  Also most of the stuff I burn is thin twiggy stuff that doesn't lend itself to being easily measured like that. Also some is old half rotten stuff (not treated, don't worry!) that takes up space but doesn't weigh much any more.

BTUs are also likely to leave most people here staring blankly, and for that I'd need to know weight anyway.

Though having said that, when the guy that dropped the donkey poop and the baskets of wood yesterday he immediately started trying to remember about how to calculate BTUs using pans of water and measuring how long they take to boil. Which says something about my friends. Mostly they live like peasants but are secretly engineers and scientists...

What I would *really* like is to be able to say something like "I burned 500kg of dry wood, which I gathered myself using my trusty pair of loppers, and we kept warm on it all year round" or somesuch.

And then everyone will start arguing with me about how their house is bigger, or made of stone not brick, and how they have to buy their wood, or how they like their house warmer than mine. But just one fairly accurate measurement of the wood used in this house, this winter, on that stove will give us a starting point to talk to people that live near here and might be considering building a rocket mass heater similar to ours.

For one, all and every wood species has the same combustion value by weight. Within some percentages, that is. Surprisingly enough, conifirous softwoods are slightly better than oak, for example, because of the resin in them.
And yes, big logs burn slower than small branches but that's not an advantage per se. All these (batch)rocket heaters are burning quite fast anyway, whatever the fuel is they are fed with. Which means that the temperature difference between the hot gases and the masonry mass is huge. Due to that effect, the heat absorption by the surrounding masonry is very high as well.
Also, the dryer the fuel, the less water need to be evaporated, which in turn is better for overall efficiency.

For Burra: every kilogram of reasonable (>15% moist) dry wood of whatever species, burned completely, will generate roughly 4 kWh of energy. Chimney losses and so on are already deducted, so this is what stays in the house. Provided the chimney temperature is reasonably low, of course.
Talking to EU citizens about fuel consumption is best done in the above terms, in my humble opinion.

One kilogram is equal to 2.2 lbs, 4 kWh is equal to 13,600 BTU's. So, every pound of whatever woody fuel could potentially generate 6,181.8 BTU's of energy.

I had wondered about the measurement in cords as a volume, which is why I mentioned dimensions, but I gave dimensions in feet (sort of) which sort of ended up in the same category as using cords being that a cubic meter would have been more appropriate for that side of the pond. I succeeded in conveying the idea of using volume at any rate, although it may not be the best answer. I wasn't aware that BTUs corresponded so closely to weight, and that weight would be needed for the calculation for BTUs (was thinking BTUs could be calculated from the temperature of the room, rate of temperature change, the size of the room, and the time at a given temperature) but my science of study is not physics and I don't know much on this topic) at least if the wood is relatively and uniformly dry.

Yes, there is a lot of voids, dead space in trying to measure a volume of irregularly shaped wood. That is taken into account, or is part of, (typically) the volume measurement of a stacked cord. A random pile may actually have more voids.

I think I might ask around and try to find out what the cubic metre bags of different types of wood actually weigh. Then, assuming the wood inside them is relatively dry, I can do a few conversions of my own figures into the equivalent number of bags that I would have had to buy.

It's so hard trying figure out how to give the information out in a way that the people who would most benefit from it will actually understand. And accept. I remember Paul's claim that he uses only 10% of the wood he used to use when he switched to a rocket mass heater is widely rejected because people simply don't believe it. But being Paul he refuses to change the claim because he knows it's true.

So I think -

-  weight of all wood burned

- rough equivalents of cubic metres of different types of wood.

might be the best metrics for me to concentrate on for now.

Peter - thank you so much for your input. I'm tempted to get it framed and hung on my wall somewhere near the rocket mass heater!

While you're weighing stuff, have you also thought about weighing the ash(or 5 gallon buckets for volume)?  Less cleanup might also be a selling point.

That's a good idea. Maybe I'll keep a lidded tub and I can just show a photo of it at the end of the burning season. And maybe a photo of me sprinkling it on the garden.

"Burra's Hearth Soap"

Million dollar idea right there, lol.

I did actually consider setting up making soap to sell when I first came to Portugal, though I never got past the stage of making it for myself and for gifts to give to friends.

Since then it seems that half the people who move here set up selling soap, so there really isn't much point any more.

There is one place that excels at it though, and I get all my laundry liquid and stuff from them - aldeia sabão

Here's one of my little dragons, Rosa, with some of my soap, and a pretty Portuguese bowl, and a soap bag I made.