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Thursday, April 30, 2015

Wood Heat, Part 2

Last week I went over the basics of wood heat and I got a few questions and requests for clarification, so I'll try to expand on the areas that I didn't cover in detail. It was kind of hard to sort the questions into groups, so there is no logical flow to the answers.

Curing Firewood
Fire wood is “cured” by letting it sit in a place with good air flow and protected from precipitation. The actual curing is letting the wood dry out and letting some of the “lighter” chemicals (mostly alcohols and ketones) evaporate off, which promotes a more efficient burn. The water content of wood varies from 40% to 75% when it is green, and green wood tossed into an established fire will burn very slowly, because most of the heat of the fire is being used to boil off the water to allow the wood will burn. Water takes a lot of heat to convert to steam, so burning green wood makes for a very inefficient fire that will provide very little heat output and a lot of smoke.

Stoking (feeding) a Fire
Fireplaces are like campfires: feeding them consists of shoving a piece of wood into the existing fire and hoping it catches. Wood stoves have a few more steps.
  1. Open the air intake to get the embers as hot as you can. This will also help move any ashes away from the air intakes.
  2. Open the damper to allow the smoke out of the firebox. On windy days, or if your chimney isn't tall enough, smoke will come back down the chimney and can quickly fill a room. Trying to stuff a 16 inch long piece of wood through a 20 inch opening while your eyes are tearing up due to smoke is no fun. 
  3. Open the firebox door and use your poker to level out the embers and ashes. Having a level bed gives you a more stable place to stack your fuel. Unstable stacks tend to burn unevenly and it's disquieting to hear loud bangs coming from your heater, even if it is nothing more than the wood falling against the side as it burns and shifts. 
  4. Start placing firewood into the stove. If the fire has burned down to just embers, you'll want to start with thinner splits or smaller pieces and add the larger pieces on top of them. This give the embers more surface area to ignite the new fuel, just like building a campfire from twigs and increasing the size of the firewood as you build it up. 
  5. Close the door and reset the damper and air intakes to positions that will provide the heat you want.
Banking a Fire
When you're getting ready for bed, or going to be leaving the heated space for a while, it makes sense to “bank” the fire. Banking a fire means consolidating the hot embers into a pile in the stove and cover them with ashes. This insulates the embers and cuts down on the air that can get to them, slowing the fire. Cutting back on the air going in and closing the damper creates an environment where the fire can barely burn, which will give you a better chance of having a bed of embers to start your next fire. Banking also gives wood a chance to cook off the volatile chemicals and turn into charcoal, which burns clean and evenly.

Overnight Fires
When I cut firewood, I always leave 10% of the pieces unsplit. These pieces, between 10 and 15 inches in diameter, are my “night logs”, and are the last piece of wood fed into the wood stove before I go to bed. Large pieces burn slower (less surface area), and provide more fuel per unit of volume, than splits. This is  another way of ensuring that I have a bed of hot embers in the morning to use for starting a fire that would heat the house. Lifting a heavy piece of wood to burn overnight beats having to wake up during the night to make sure the fire is still burning, or waking up in the morning to a freezing cold house.

Cleaning a Wood Stove or Fireplace
There are two ways to clean a wood-burner: hot cleaning and cold cleaning. A hot cleaning is when you let the fire burn down to embers and ashes and use a shovel or rake to separate the two, removing the ashes but leaving the hot embers or coals to use as a starting point for the next fire. You can also run a brush through the chimney with a banked or low fire.

Cold cleaning is letting the fire burn out completely and emptying the firebox. Cold cleaning is better at clearing room for more fuel (important if your stove is small), but requires starting a fire from scratch every time.

Chimney Sweeping
Whether you have a chimney or stove pipe, you will need to clean it. Burning green, uncured wood will create more soot and residue than burning dry, cured wood will, and the soot (carbon residue) and creosote (tars and gums that condense on the cooler chimney walls)  will be deposited on the inner surface of your flue. Keeping the chimney clean serves two purposes:
  1. A clean chimney allows better air flow or draft to carry the smoke away from the fire box, leading to a more efficient fire. As the chimney fills in with deposits, you'll have to open the damper more to get the same draft, so be careful with the first fire after a cleaning.
  2. A chimney full of soot and creosote is an invitation to a chimney fire. 
If the built-up crap in your chimney catches fire, you'll hear a roar like a jet engine as it sucks heated air through the firebox and you'll see flames and sparks shooting out of the top of the chimney. A chimney fire is a poorly contained blast furnace and can generate the same levels of heat. Chimney fires are dangerous because they can get hot enough to melt metal stove pipes and ignite nearby wood, destroy the mortar holding a brick chimney together (leading to leaks and collapse), and spew sparks all over your roof. I've seen quarter inch steel glow red, and several houses destroyed, from chimney fires. In case of a chimney fire, shut down all sources of air to the fire and close the damper to try and smother the fire. 

For fireplaces, there are extinguishing blocks that you can toss into the fire. They work by burning rapidly and consuming the oxygen before it gets to the fuel on the walls of the chimney. Don't relight the fire until after everything has cooled off and you have inspected the flue for damage.

Tools to Use
  • All of your tools for working with fires and hot materials should be made of metal, preferably steel. Aluminum and thin pot metal (mostly zinc) have a much lower melting temperature than steel and may not last long in daily use. 
  • Chimney brush. Get a brush of the proper shape and size for your chimney. 8 inch round brushes are common, but you can get square ones for masonry chimneys. Your brush should have handles that can be added onto each other to extend the length, allowing you to store and carry them with less hassle. Flexible twisted-steel and sectional fiberglass handles are cheap and last a long time with minimal care. 
  • Ash bucket. Find a metal bucket or coal scuttle for carrying ashes from the firebox to wherever you are disposing of them. Plastic buckets will only last until you get one hot coal mixed in with the ashes, which will happen. 
  • Metal scoop. A coal shovel or small metal shovel will allow you to dig out the ashes and dump them into your bucket. 
  • Rake or poker. A metal rake will help separate the ashes from the embers, while a poker will help rearrange burning wood to get a better burn or make room for more fuel.

Tools to NEVER Use
  • Never use anything plastic. Plastics just become more fuel when working around fires. 
  • All metal, solid handles. If you're working with a fire for a while, a solid handle will allow heat to travel up the handle and burn your hand. Look for tools with insulated or ventilated handles.
  • Painted tools. Paint will burn off and the fumes released may not be healthy to breathe. Burning paint may also spook a user into dropping the tool, creating a fire hazard. 

Disposal of Ashes 
Ashes have a variety of uses, so they're not just a waste material. If you refrain from burning trash, the ashes produced by a fireplace or wood stove can be used as a base for making lye (for home-made soap) and as a preservative for meat (more in a later article). Even if your ashes get contaminated with minor bits of metals, inks, plastics, or other trash, you can still use them as a fertilizer for gardens and lawns. Ashes are all of the minerals left over once you remove the water and carbon from plant matter, so it makes sense that they would be usable as a source of those minerals for other plant growth. Use them sparingly, since the addition of water will create lye, a very strong base that will raise the pH of your soil.

If you're not going to re-use your ashes, make sure they are disposed of in a manner that will not allow any run-off or leachate to damage plants or structures downhill from them. Dumping them into a deep pit or on a gravel lane are two safe ways to dispose of them, and they'll keep weeds from growing in gravel if you get the ashes thick enough.



Tips for a more efficient fire

Flame Height
The flames you see rising from a fire are actually the combustion of gasses released from the wood as it heats up. Flames are pretty, but anything over 6 or 8 inches high is a sign that you're burning the fuel too fast. Flames generate light but not much heat, you get more heat from the dark red glowing coals of a fire.

Fuel Size
Fire is a chemical reaction, and therefore takes place mainly on the surface of the fuel. Small twigs or sawdust have a lot of surface area and will burn much faster than a solid piece of wood of the same weight. You'll need some small pieces of wood for kindling when you start a fire, but if you use the hot clean method of removing ashes, you'll rarely have to start from scratch. Use wood cut to a length that will fit through the opening of your firebox and as big around as you can keep burning. A slow, deep fire will put out more heat and be more efficient than a roaring bonfire in a box.

Air Control
Balancing the air going in through the intakes, and the smoke going out through the damper, takes a bit of experience. You want to keep enough air moving through to remove the smoke, but you want the fire to burn as low and slow as you can to stretch your wood supply.

Circulation Fans
Fans move air; that's what they're for. Wood heat is generally good for heating only the room that the heater is in, and maybe the room above it. Having fans to move the air around will make a multi-room dwelling more comfortable. There are some neat heat powered fans on the market that use the heat of the stove to run a small fan -- no electricity needed. Ceiling fans are low-powered and an excellent way to move air near a wood-burner. Having air movement allows a smaller fire to heat a larger space more evenly.

Construction Tips

Chimney vs Stovepipe
Depending on if you are moving into a house with an existing wood-burner, or are adding one to an existing home, you may have a choice between a metal stove pipe and a masonry chimney. Pipe is much cheaper and easy to replace, but a masonry chimney will last for decades if not centuries. The house I grew up in had a metal stove pipe and it has been replaced twice in about 40 years, while the house I live in now has a lined brick chimney that has been in use for more than 100 years with little to no maintenance.

Wall/Roof Penetrations
Anywhere the stove pipe or chimney goes through a wall or roof, special construction methods are required to keep the hot flue from igniting the wood framework of the house. Stove pipe building codes vary slightly, but most require a double- or triple-wall section of pipe for any penetration. Triple-wall pipe is quite expensive (~$50 per foot) but is cheap fire insurance.

You will also have to pay attention to the flashing around the pipe or chimney that ties in with the shingles or other roofing material. Cheap flashing will allow rain and snow melt to run down the pipe and cause water damage to your roof and ceilings.

Hearth
The hearth is a non-flammable material placed under and in front of a wood-burner. Its main purpose is to catch any hot bits that may fall or fly out of a fire without catching fire, but it also does a good job of making cleaning around the heater easier.

Spark Screen
As I explain below, a spark screen is a must for fireplaces and stoves designed to run with the door open. Made of fairly coarse metal mesh, chain mail, or miniature chain-link fence, they provide a physical barrier to sparks and embers flying out of an open fire.

Clean-outs
Most chimneys are built with a door on the side below the firebox as a reservoir for ashes. Opening the door provides a convenient way to clean out ashes, but also provides a large opening for air to enter the firebox.

If your wood stove has a horizontal exhaust pipe, there may be a "T" in the pipe where it goes vertical. This serves as a handy drain for debris when you clean the pipe with a brush.

Flue Height for Proper Draft
There is a requirement for how high the top of the chimney/pipe must be above the peak of your roof for proper air flow (draft) and safety. Read the linked article for a good explanation of all of the factors involved.

Rain Caps
Simple rain caps are just a metal plate suspended above the top of the chimney/pipe, but they are usually not enough to satisfy insurance agents and fire marshals. You need to look for one with a spark arrestor screen around the openings, but be aware that the screen will plug with soot and tar fast. You'll be cleaning it several times a season, so make sure you can get to it even with ice on the roof.


Downsides to Wood Heat
Having control of your fuel supply and being able to keep your house as warm as you like (provided you have the wood for it) are nice benefits, but there are some disadvantages to wood heat.
  • Your insurance company will want to inspect the installation. Expect an increase in your rates (or cancellation of the policy) just for having a wood burner. If you have a wood (shake shingle) roof, you may have to treat the roof with fire retardant in order to get coverage.
  • Fire/burn hazard. This is especially true of fireplaces, but some wood stoves can also be a hazard to young children and clumsy adults. Single-walled wood stoves get hot on all sides; the top will be the hottest, but the sides can get hot enough to cause serious burns. Double-wall stoves and masonry stoves are a better choice if you expect this to be a possibility. 
  • Fireplaces without a spark screen are stupid. I don't use that word often, but it is warranted in this case. I equate lighting a fireplace without a spark screen to changing targets on a rifle range while people are still using it. Wood with any knots or burls in it will trap pockets of moisture inside the wood. When that moisture heats up and expands, it literally explodes that portion of the wood and sends hot embers flying. Even with a proper hearth, having hot coals spit out of the fire is never a good idea. Children and animals like to lie next to fires and can be seriously injured by the flying bits of fire. 
  • They're dirty. Wood fires create ashes, which tend to be considered “dirty” even though they have many uses. Any smoke that escapes from the flue will stain fabric and walls/ceilings. It's a slow staining (sometimes takes years to see), but it's there. I've seen an OCD sufferer wipe a child's hand print off a wall and end up cleaning every wall in the room because the clean spot was too obvious. 
  • Our wonderful EPA doesn't like wood stoves and is trying to place so many restrictions on them that they will be too expensive to buy. Local municipalities have also placed restrictions or outright bans on them in some areas due to the smoke and fine particulates they create. Even though wood heat is carbon-neutral (trees grow back using the CO2 released by burning wood), some environmental groups lobby against their use. 
  • Wood smoke has a distinct odor and can travel a long way. From an OPSEC standpoint, a wood-burner is a waving flag that someone has heat and fuel. 
Finally
Everything considered, I like having wood heat available. It works even if the electricity goes out. It is a pleasant heat with the advantage of giving a room a temperature gradient from hot near the stove to cool near the far wall -- this means people can always find a place in the room that is the “right” temperature.

The Fine Print


This work is licensed under a Creative Commons Attribution- Noncommercial- No Derivative Works 3.0 License.

Creative Commons License


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