Thursday, October 21, 2010

Fuel, Air, Heat

Fuel, Air, and Heat, are the "fire triangle".

They are the three elements you need to initiate and sustain the chemical reaction combustion; the rapid oxidation of fuel producing heat and light a.k.a FIRE.

Too much or too little of any of the elements, will tend to overwhelm or starve out the others and kill a fire.

To get a fire going, you have to get enough heat into your fuel, with enough air, that combustion can become self sustaining. Too much air, and the fire blows out. Too little it smothers out. Too much fuel, and you need more air and more heat to ignite it etc... etc...

It's a balancing act, an art, and a science.

As I've mentioned before, the primary heat in our house is a fireplace insert; which is in fact a modern wood burning stove.

It's one of the super-efficient ones the government gives tax credits on, certified green by whoever because it burns completely and emits almost no pollution blah de blah...

Since I know some of my readers are fireplace geeks (yes, there are fireplace geeks, and some of them are SERIOUSLY HARD CORE) the specifics of it are as follows:
Napoleon NZ26 with primary and secondary heat exchanger, secondary combustion chamber, and primary firebox burner bars, 3 chamber flue with six position remote adjustable thermostatically controlled damper, adjustable outside air intake, and under grate air inlet; and a thermostatically controlled forced hot air blower for the heat exchangers.
The setup, with the optional extras, is rated at around 80% efficient with a maximum output of 70,000 btu/hr (continuous maximum fuel burn), or a full firebox rating of 27,000 btu.

What all that means, is that the stove effectively acts like a blast furnace, taking cold outside air, and forcing it up through the bottom of the fire at high velocity to make the wood burn hotter and cleaner (it's actually got a manual downdraft damper you can hold open, and it's like hitting the power blower on a forge. If I burned charcoal in this fireplace I'm pretty sure I could forge metal with it... at least until the flus melted).

The heat of the primary combustion gassifies more wood, then the unburned wood gasses flow up into the secondary combustion chamber where they are ignited by the concentrated heat of the firebox, and the exhaust gasses. The hot burning gas then rushes out through the burner bars back into the primary firebox, burning again, adding more heat to the primary fire, igniting more unburned gasses, and generating even more wood gas..

Finally, when there isn't enough easily combusted fuel energy left to burn in the gasses, they flow back up through the secondary combustion chamber again and out through the heat exchangers; given up much of their heat to air forced across the heat exchangers by blowers.

For those of you NOT stove geeks, all that crap means you end up with a theoretical efficiency of something over 80%... and in the real world you actually recover something like 60-70% of the heat energy of your fuel (if we had remote ducting forced hot air, instead of the free air blower, it would be more like 70-80%, and the fireplace exhaust would barely be hot to the touch with nearly all the heat going into the ducts).

So, with a relatively small (and very well insulated) firebox, that can only hold maybe six quarter splits; it puts out a hell of a lot of heat (if I stoke it up to full heat, with a full firebox, with a good lay, and the dampers fully open, I cant stand four feet in front of the thing for more than a few seconds).

That efficient burning, combined with the hot air blower, heat exchangers, and our vaulted ceiling with ceiling fans; warms up the whole house from 50 degrees to 75 degrees in less than an hour.

One completely full firebox load of fuel, starting from a hot coal bed, will burn for about 4 hours with the dampers wide open (high throttle), up to about 7 hours with the dampers closed down; and it'll keep the whole house nice and warm that whole time, even when it's well below freezing outside.

There's just one real problem with the thing, it's tiny.

As I said, the firebox is really small. Technically the firebox is about 20" wide, 18" deep and 20" high at the door; but that doesn't into account the fact that the firebox is trapezoidal (the better to reflect heat out into the room), or the 3" of fire grate, and the thick refractory brick insulation on the thing.

The actual stackable area in the box is basically a 17" cube.

Back to why that's a problem in a minute...

In the U.S we buy wood in cords. The cord is a rough volumetric measure, not a measure of mass or heat energy; as every type of wood, as well as woods of different degrees of seasoning (drying out), will weigh entirely different amounts, and put out entirely different amounts of energy per unit volume.

This can vary from as little as 11,000,000 btu per cord (and about 2000lbs weight) for white cedar seasoned less than a year; up to over 30,000,000 btu for the densest hardwoods, seasoned over two years (and weighing about 4000lbs per cord).

So, as you might assume, different woods command vastly different prices. Wood labeled as "unseasoned mixed firewood", might get as little as $80 per cord this year (and would probably not be usable in the year you bought it); while two year seasoned Tamarack (generally regarded as the best softwood firewood commonly available in our region) might get as much as $180 per cord this year (nobody burns much hardwood up here unless they cut it themselves; there's so much relatively cheap softwood. You occasionally see someone offering birch, but it's rare).

Our "firewood" around here is usually a mix of various fir, with some black birch, hemlock, juniper, cedar, pine and tamarack thrown in. So it's all over the place for heat value, for sparking, for pitching up or creosoting (the intial tendency to give off wet smoke with residues that smell bad, and clog up fireplaces. Creosoting is basically eliminated by complete combustion in a modern wood stove, ONCE you get the fire hot enough) for ease of lighting etc...

At any rate, what you're buying when you buy "a cord of wood" isn't exactly uniform. Not only isn't it uniform in heat, or weight, it's not even really uniform in volume.

By definition, a cord of wood, is a pile of wood 4' high, 4' wide, and 8 foot long; or 128cu feet of wood; however, the actual volume of wood to airspace can vary greatly depending on how the wood was cut and split.

Again, unless you specify otherwise (and pay more for being specific), you're going to get a mix of whole bucked logs, half splits, and quarter splits; of anything between 6", and about 20" in diameter (anything 8" to 16" is almost always half split. Anything bigger than 16" is almost always quarter split)

Unless you get quarter splits, you aren't actually getting anywhere near to 128cuft of wood. If you're getting all 12" rounds, you might get as little as 80cuft of actual wood, in a 4x4x8 pile.

And of course, rounds (unsplit logs) don't season well, nor do they light very easily. Quarterd logs stack best, they season best, and they ignite easier than halves or wholes; but quartering every log takes a lot more time and effort. Most woodcutters only bother quartering the logs over 12"-16" diameter unless you specifically ask. In general, you want your load to be split up in all quarters, or mixed halves and quarters... though it can be nice to have an unsplit round to use as a chopping block, or when you want a long slow overnight burn.

Finally, there's the length issue.

A "cord" unless otherwise specified, can have logs bucked to any length between 12" and 24".

Conventionally, cords are cut and stacked into three "faces"; of 4' high by 8' long by 16" wide logs each. 16" is a pretty good log size, and three faces (to make a 4' stack) are convenient to stack.

Usually, cutters don't bother with 12" unless you specifically request it, because it's a lot more work, and 4 faces is harder to stack, and deal with (for one thing, logs narrower than 14" may fall through standard racks). Some however will offer "stove cuts", with logs cut to 14" to fit smaller stoves and inserts. The downside is, you still only get three faces, which means you're paying the equivalent of 16cuft (1/8th cord) extra, to have them cut short.

Much more common, is that logs are cut oversize, to 18" or 20"; which is small enough for most fireplaces, but will be too long for a lot of stoves, unless inserted at a funny angle.

The upside to that is, you usually still get three faces; so in exchange for the inconvenience of overlength logs, you're getting 1/8 cord for free.

Now, you'll recall above, our firebox has only 17.5" usable width? Yes, it's 19.5" wide at the widest point, but it quickly narrows.

This means any log cut longer than about 16" won't fit, unless it's split into quarters. If it's quartered, we can fit it into the front half of the firebox, or we can slide it in at an angle etc... etc...

Earlier this year, we bought a cord of mixed one year seasoned firewood; mostly fir, some birch, some cedar, some pine; mixed whole, half, and quarter, and cut to between 16" and 20"; for about $100. We've used about 1/3 of it so far, but the 1/3 we've used has been the shorter cut pieces, and the quarters. What we've got left either doesn't fit, or barely fits into our stove without recutting, or resplitting.

Both are serious pains.

We're going to need another 2 cords over the winter most likely; and if the winter ends up longer or colder than typical, maybe another 3. We plan on buying 2, cut to 16" and split in quarters, and they'll run us something like $150 a cord.

Honestly, heating our house the whole winter for under $500, I'm pretty happy with. We've got a couple good local firewood places to deal with, and we should have no problem getting in two cords of well seasoned and properly split wood.

But there's still the irritation of the oversized wood to deal with. And starting fires in general.

Now, I've been lighting fires since I was a little kid. All the houses I grew up in had fireplaces; and my grandparents place in New Hampshire was wood stove heat only. I have no problem getting a good fire going, even with marginal kindling and tinder; presuming properly split and seasoned firewood.

Mel on the other hand... Her very first fire was this past March; and in general her success in firemaking has been.... shall we say, mixed?

Even for me though, dealing with the oversized logs is a pain. If you don't have the room to create a good firelay, you can't get the fire triangle going; and even with good wood, it's difficult to get a fire really cooking. With mediocre wood, you end up with a fire that uses up more and more tinder and kindling, and never really lights off, just smoldering out over time.

Now, there is a solution, splitting the wood into "splints"; which are basically small sticks split out of bigger logs. Splints light easily with little tinder, and burn hot, allowing you to start burning a larger log fairly quickly.

You can buy splints, but they aren't cheap, at around $5 per bundle, each bundle good for maybe 1/2 dozen fires. They aren't ridiculously expensive, but you can also just make your own.

Conventionally you do that with a sharp hatchet, and a hand sledge. It works great, but it's a pain in the ass; and you have to do 20 minutes of splintmaking for one or two fires.

The real solution is to resplit everything down to quarters or smaller. Eights catch a lot better, and are easier to stack into a good firelay than quarters for example.

Of course, resplitting three cords of wood is a LOT of work. More work than I want to do frankly.

So that's why I bought one of these (and the stand for it):

Some people say these little splitters are awful, useless, what have you, but they're wrong. The problem is they're using them for the wrong job.

These small splitters only have 4-5 tons of force, and can only handle a 20"x12" log... and realistically not even that if they're wet, or it's hardwood. So if you're trying to use one as your primary logsplitter, yes you're going to be disappointed.

If you're doing primary splitting, of BIG logs, wet wood, hardwood etc... you want a real gas powered 12 to 20 ton splitter.

But what we're doing here, taking small rounds down to quarters and eigths, splitting already seasoned wood, and splitting splints... it does them in a heartbeat.

Importantly, they're small, and very convenient. Instead of resplitting the entire three cords at a time, you can just resplit what you want for each fire, right outside your door.

We got ours in this afternoon, set it up right outside the front door next to one of the log racks (a convenient outdoor outlet being right there), and I split four 18" or so long 10" or so thick birch logs into eighths and splints, in maybe five minutes, with absolutely zero effort.

Using those eights and splints, Mel was able to set a proper log cabin firelay, and got a fully involved, hot burning fire, within 10 minutes, using just a couple paraffin chip balls for tinder.

I am now fully confident that Mel will be able to lay and light proper fires all winter, using the wood we have without recutting and resplitting it all in advance. That alone is worth the $350.

And come next spring, I'm going to rebuck (it's in six foot lengths right now) and split the wood we cleared off the lot this year. None of it is more than 10", and it's all good black birch. That little splitter should handle it just fine at 16" length. That's a half cord right there, for just a couple hours work.