Building Science 101: Wood heat and a sustainable Maine home
Burning wood for heat is a time-honored tradition in Maine. There’s nothing like the radiant warmth of a fire on a cool night, which humans have been enjoying for several hundred millennia. But is burning wood appropriate for an energy efficient, low-carbon home?
The answer is a resounding, “It depends.”
One consideration is the type of appliance used for burning wood. An open fireplace is charming and traditional, but as a heat source it’s been outdated since cast iron wood stoves came into use 500 years ago. They waste a lot of heat, with the chimney drawing large amounts of air from the room. They operate at low efficiency, sometimes using more heat than they produce (while they radiate heat to warm people and objects directly, they use conditioned air from the room to burn). They are also dangerous, frequently the cause of fires due to burning embers escaping, and sometimes allowing a lot of smoke and particulates into the room.
In short, there is never a good reason to have an open fireplace indoors, especially if you care about energy use. You can improve the efficiency a bit with a chimney-top damper and by adding doors to the firebox, but it will still be an energy waster. If you have an existing fireplace that is rarely used, you can put a purpose-made foil balloon in the flue to block most air flow. If it’s never used but you don’t want the mess of major demolition, you can install wallboard over the firebox, but first be sure to seal and insulate the firebox opening.
A better approach for an existing or new fireplace is a fireplace insert, which is essentially a wood stove in the form of a fireplace and greatly improves the efficiency and safety of an open fireplace. Most have blowers to distribute warm air, and some can operate without the blower in case of a power outage. Inserts are available in many styles, efficiency levels and price points.
Another popular appliance is a freestanding wood stove. From colonial- and Victorian-era cookstoves to the highly efficient units now available from Europe, wood stoves cover a lot of territory. Look for models that burn very efficiently — designed for nearly complete combustion of the wood — with doors that seal tightly.
Alternative forms of wood heat
PELLET STOVES AND BOILERS: using hardwood, softwood or other biomass, kiln-dried and compressed into rice-sized pellets, this type of heat has been gaining popularity in fits and starts over the last twenty years. They require electricity, and the pellets are either bought in bags or delivered into a large storage container.
MASONRY HEATERS: also called Russian, Finnish or Scandinavian heaters, they include a small fire chamber and a flue system specially designed to extract the most heat from a quick, hot fire, and radiate it steadily for many hours. They are simple and low-tech, but relatively expensive, and it’s hard to find ones that would not overheat a small, highly efficient home. (Maine Wood Heat has one made for smaller homes.)
WOOD BOILERS: these can be an efficient way to heat both indoor spaces and hot water, and they can be combined with a fossil fuel boiler for backup. The best are called wood gasification boilers, with some units designed for indoor use and others for outdoor use. When used with dry wood they are virtually smoke-free. Non-gasifying wood boilers are also available but are more polluting, especially outdoor versions that can release a lot of smoke relatively close to the ground.
Guidelines for wood stoves and fireplace inserts
GET A UNIT CERTIFIED BY THE ENVIRONMENTAL PROTECTION AGENCY (EPA). It should have either a catalytic element or secondary combustion to significantly increase efficiency and reduce pollution. Secondary combustion is generally considered preferable for ease of operation and lower maintenance requirements. Some units combine both combustion types.
CHOOSE A UNIT THAT WILL NOT OVERHEAT YOUR SPACE. Manufacturers often specify appliances by the number of square feet they will heat. However, an efficient home may need a fraction of the heat output that a less-efficient home would need to stay comfortable. In fact, it can be hard to find units small enough for very efficient homes, which may only need 10,000 to 20,000 Btu/hr on a cold winter night, and less in moderate temperatures (most wood stoves are rated for 30,000 to 100,000 Btu/ hr. maximum output). Look not only for a unit that will meet your maximum need, but that can run efficiently at lower outputs.
USE PROPERLY DRIED HARDWOOD. Wood that has not been dried can use half of its available heat cooking off the water before providing useful heat, and it burns at a lower temperature, creating more soot. Dry wood for one to two years before burning. When you buy “seasoned” firewood, it’s often only been drying for a few months and often outdoors or in large piles where air flow prevents fast drying. Dry wood burns hotter and cleaner than damp wood. Contrary to popular belief, you can burn softwood, but it contains less fuel than most hardwoods.
PROVIDE MAKEUP AIR, ideally directly to the appliance, but if that is not an option on the model you choose, locate an air inlet nearby where hot embers can’t get drawn in. The exterior intake for the makeup air should be at or below the elevation of the stove. To maintain building airtightness, consider including a valve to turn off the air supply when the stove is not in use. Once up to temperature, stoves typically draw 10-30 cubic feet of air per minute, so without makeup air you will be sending a lot of conditioned air up the chimney. (Even “airtight” stoves are not totally airtight.) 10-30 cfm is not a large flow but it’s enough to depressurize a tight house and starve the fire for air.
PLAN ON OPENING A WINDOW OR DOOR slightly when starting a fire in a tightly sealed house. Even with dedicated makeup air, when starting a fire the chimney is relatively cold and may not draw well until up to temperature. Some people have a hard time accepting this concession, but it’s simply necessary when burning wood in efficient homes.
KEEP THE CHIMNEY INSIDE THE BUILDING ENVELOPE — a warm chimney draws much better than a cold chimney. Terminate the chimney near the house’s ridge so not too much is exposed on the exterior. And please, please keep chimneys away from roof valleys — they are hard enough to fully waterproof as it is.
Chimneys are usually either masonry (brick, block or stone) or insulated stainless steel. Masonry chimneys should have clay tile or stainless steel liners. There is more heat loss through masonry than through insulated metal chimneys, but the difference is not as great as you might expect, since steel is better than masonry at conducting heat. In any case, it’s very important to air-seal the chimney at your home’s airtight layer or air barrier.
Environmental impact of burning wood
Maine is by far the most heavily wooded state on a percent- age basis — we’re “wood rich.” We are not rich in electricity generation, and we produce no fossil fuels. Yet we send about $800 million out of state annually for fossil fuels to heat Maine homes, plus a portion of the $740 million that we spend on electricity (source: US Energy Information Administration, eia.gov), when we could be spending that money locally. So, how does burning wood com- pare to the environmental costs of other types of heat?
Burning wood is typically considered carbon-neutral, because essentially all of the carbon that makes up wood (roughly 50% of its composition) is taken from the air, sequestered where it can’t con- tribute to climate change. If the wood was cut and left to rot on the ground it would re- lease the carbon back into the air. Further, one or more new trees can be replanted to take its place, or the wood may be part of a thinning operation that leaves a healthier forest that will continue to take up carbon. But it is not without some considerations. While burning wood is considered a “carbon-better” option than fossil fuels, burning wood instantly releases decades’ worth (or more) of stored carbon, and it takes decades to regrow the biomass burned in one winter. And soot-emitting wood stoves contribute to global warming.
According to climate scientists, we only have a few years to quench emissions enough to stop runaway climate change. So, invest in a high-efficiency appliance, installed properly; use dry wood; air-seal and insulate your home so no more wood is burned than necessary.
