Metal woodstoves are a significant improvement over open fireplaces from the standpoint of producing more usable heat. They limit incoming air, thus avoiding heating air not needed for combustion. Another improvement: they use a lengthened heat exchange pathway to improve heat transfer from the heated combustion gases before they exit the chimney.

Unfortunately, metal woodstoves must operate at low, inefficient, and polluting combustion temperatures. Why? Because wood combustion requires high temperatures to be clean and efficient. Wood burns starts to burn cleanly at around 1200 degrees Fahrenheit, with continuous improvement up to about 2000 degrees. Cast iron begins to glow red and fail at about 1200 degrees. See the problem?

Further, woodstoves don’t have enough mass to store the heat they produce. If installed in a house without lots of thermal mass, they heat only the air in the home, which can store very little of the heat. To compensate, woodstove users – like most fireplace users – opt for a slower, cooler fire that yields a modest amount of heat for longer. In doing this, they get less heat from their wood and send the rest, as smoke, up the chimney.

What about catalytic stoves? Catalytic metal wood stoves use clever tricks to raise combustion temperatures and lower particle emissions: brick-lined fireboxes protect the metal to permit somewhat higher burn temperatures, and catalytic converters reignite smoke at lower temperatures, for less pollution. However, since users of these stoves don’t necessarily have a way of buffering and storing heat, they frequently opt for the slow, dirty burn. This tends to foul the catalytic converter and produce pollution.

Lacking thermal mass for heat storage, and industrial-strength, high-temperature combustion chambers for efficient combustion, metal woodstoves are not optimal for whole-house heating in most homes.

The next article in this series will describe a relatively obscure, centuries-old appliance capable of producing clean, efficient wood heat: the masonry stove.

Previous Articles in this Series:

• Heating Your Home: Why Fireplaces Don’t Heat
• Heating Your Home: Thermal Mass
• Heating Your Home: Forced Air
• Heating Your Home: Heat 101
• Heating Your Home: Radiant Heat, Wood Heat

Photo credit: Linnell Esler via stock.xchng

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Author’s note: the following article on home heating is the fifth in an eight-part series.

Open fireplaces have a reputation for polluting air. Actually, a fireplace, when burned hot and fast, creates very little pollution. The trouble is, a hot fire in a fireplace sometimes yields less heat than a smoldering fire. Where does the heat go?

The optimal amount of combustion air contains just enough oxygen to burn all combustible gases liberated by the heat. Any additional air grabs heat and sends it up the chimney. Under some circumstances, fireplaces can so far exceed this air-to-fuel ratio that they suck more heat out of a house than they radiate back into it. The fire actually makes the house colder!

The usable heat produced from the fireplace is primarily radiation, the same heat you feel on your face when you look at the flames. While fireplaces often contain lots of thermal mass (masonry), the unrestricted flow of cool air across this mass prevents it from capturing much heat. Nevertheless, if the damper is closed as soon as the fire burns out, a significant amount of heat will radiate back into the room instead of going up the chimney. Unfortunately, when the fire burns out, many fireplace users give up and go to bed without taking this critical step.

Here’s where pollution enters the picture: instead of burning a quick, hot fire and closing their damper, most people elect to burn their wood slowly to meter out heat.

Slow combustion means that the wood is burning at a lower temperature. At a lower temperature, a smaller portion of the combustible gases actually burn. More gases leave the chimney as smoke and soot (pollution).

With fireplaces, you really can’t win: if you burn a hot fire, you lose most of the heat up the chimney. If you burn a slow fire, you get very little heat, and lots of pollution.

The next article in the series talks about woodstoves.

Previous Articles in this Series:

• Heating Your Home: Thermal Mass
• Heating Your Home: Forced Air
• Heating Your Home: Heat 101
• Heating Your Home: Radiant Heat, Wood Heat

Photo credit: Wikipedia Commons

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The previous article discussed the disadvantages of using forced air to heat your house. Another approach is, literally, to heat your house –- not the air cycling through it. Why would you want to do this?

Well, for one, when you heat the building itself, you can open all the doors and windows, let all the warm air escape, close everything back up, and, instantly, be warm again – without having to add more heat.

The Empty Fridge

Warm masses heat you like the sun does: by sending you radiant heat. As I explained in Heating Your Home: Heat 101, heating and cooling differ only in perspective. If you can get your head around that, then the Empty Refrigerator Effect explains why heating air is an inefficient way to heat a home.

Like it says in the fine print, refrigerators achieve their rated efficiencies only when they’re full. Every time you open the door, warm air enters. If the fridge is empty, the inside temperature may go up five or ten degrees. On the other hand, if the fridge is full of food when you open the door, warm air still enters, but the food absorbs the heat and gets warmer by less than a single degree. It takes many such door openings before the fridge has to turn on. With proper design, your house can be a full refrigerator working for you.

To make your house a full fridge, it has to have substantial thermal mass. Thermal mass is usually heavy, but that’s not the point: things with high thermal mass absorb lots of heat before they get warmer, and lose lots of heat before they get cooler. More traditional building materials – stone, brick, and earth – have high thermal mass. Concrete (and water) also have high thermal mass. Unfortunately, drywall, studs, paint, fiberglass insulation, and the other flimsy stuff we generally use to make houses in the U.S., don’t.

To do you any good, the mass has to be on the inside of your home – ideally, with insulation behind it. Unfortunately, few walls are built with adequate thermal mass, and of these, fewer still have insulation behind that mass. This is not so in northern Europe, Scandinavia, and various cold, but otherwise enlightened, countries.

Heating Floors: Radiant Floor Heat

If not in the walls, then where do you put the mass? The floor. Slab floors are a pragmatic and very effective way of adding thermal mass to houses utilizing contemporary stick framing. Typically, tubing, or less commonly, plenums (duct-like voids) run through the slab. The slab can be heated by pumping warm water through the tubing or warm air through the plenums. This setup with water circulating through PEX tubing is called a hydronic floor heating system, or a radiant floor. For weight reasons, the heated slab for radiant heat has to be included in the original design for a suspended floor/crawlspace system. Thin slabs, sometimes poured, self-leveling gypsum concrete (gypcrete) – with correspondingly less mass – are more typical in retrofits because many suspended floor systems can support them with little or no framing changes.

Overall, radiant floor heat is amazingly efficient and effective. People and pets find they are comfortable at a lower ambient air temperature when standing (or laying) on a warm surface. This means you can maintain the same level of comfort with a lower thermostat setting, saving energy and money. Radiant floor heating systems generally use natural gas or another non-renewable fuel to heat the water.

Upcoming articles in this series talk about wood heat: why fireplaces and woodstoves are inefficient and pollute, and what to use instead to get clean, efficient heat from wood.

Previous Articles in this Series:

• Heating Your Home: Forced Air
• Heating Your Home: Heat 101
• Heating Your Home: Radiant Heat, Wood Heat

Related Articles:

• Retrofit Radiant Heating
• Green Building Elements: Warmboard

Photo credit: lyng883 via Flickr

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Forced air systems are the most common heating systems in California and are used in most new construction elsewhere. They have two big advantages: they are cheap to install, and they provide heat at a moment’s notice. Having “instant-on” heat is vital for intermittent use spaces like ski cabins. Otherwise, forced air is the least energy efficient and least comfortable way of heating a typical home. Why?

Ventilation and Heat Loss

For the health and well-being of its occupants, a home must exhaust stale air and refresh it with new air drawn from outdoors. Forced air systems heat and blow this air, via ducts, throughout your house. Since new air is continually entering and leaving, you are heating the outdoors.

There are ventilation systems specifically designed to make HVAC less wasteful. Heat Recovery Ventilators (HRVs) transfer heat from warm outgoing air to cold incoming air. (A comparable device called an Energy Recovery Ventilator can cool incoming hot air in hot-humid climates.) Running for some fraction of each hour, HRV fans typically exchange enough air with the outdoors to keep inside air fresh. However, HRV is rarely considered cost effective in our climate.

Even in the most air-tight houses, air is lost via unsealed combustion devices, bath or range exhaust fans, and the opening and closing of doors.

Ducts, Mold, Mildew and Rot

The ducting that moves the heated or cooled air is a network of hidden spaces that can collect and redistribute debris and moisture. When the heat cycles off, warm air cools and condenses inside the ducts, forming liquid water. The moist, protected environment is an ideal space for mold and mildew to breed. Broadcasting any dust, mold, mildew or pathogens growing in the duct work, forced air is often at the heart of Sick Building Syndrome.

Ducts also leak. PG&E estimates that the ducts in a typical home leak as much as 30% of the air they move. This warm, moist air can escape into an unconditioned space (for example, an unheated attic or crawlspace), cool, condense, and deposit water where you don’t want it and can’t see it, like your roof or floor framing. Subjected to enough regular wetting, this wood rots. When heated or cooled air escapes into unconditioned space, it heats or cools the outdoors, not the house.

Dry Sinuses, Skin and Eyes

Many consider forced air inherently uncomfortable. Since the blown air needed to heat the rest of the air in a room must be substantially warmer than the ambient air temperature, many people find that it dries their eyes, skin and sinuses.

As a homeowner, you have alternatives to forced air. The next article explains thermal mass and radiant heat.

Previous Heating Your Home Articles:

• Heating Your Home: Radiant Heat, Wood Heat
• Heating Your Home: Heat 101

Photo credit: pixelviz via Flickr

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According to the U.S. Department of Energy, heating and cooling amounts to 46% of all energy consumed by our homes. Water heating uses another 14%. In coastal California, where extreme heat is rare and winters are mild, a properly sited, well designed passive solar home can generate its own heat and hot water, and do without air conditioning.

Historically, few homes are so well sited or built. Since our area has more heating days than cooling days, most homeowners need a heating system. What few know is that many indoor air quality problems can be by-products of forced air heating, ventilation and air-conditioning (HVAC) systems installed in their homes.

Too often, homeowners are unaware that they have better options. Radiant heat delivery is more efficient than forced air and has the potential to solve a number of indoor air quality (IAQ) problems. Fewer still know that wood-based radiant heat, besides avoiding these IAQ issues, has even more benefits. Significantly:

• wood heat is usually less expensive than fossil fuel
• heat produced from wood is carbon-neutral
• with the right equipment, wood will burn cleanly enough to meet stringent air quality standards

Some municipalities also are unaware that wood can be burned cleanly, and prohibit wood burning appliances in new homes. If we don’t take the time to educate others about the benefits of wood heat, one of the next best alternatives to solar heating is in danger of being banned everywhere.

The next article in this series explains what heat is and how it’s used to warm a home.

Photo credit: Temp-Cast Enviroheat LTD

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What is Heat Exactly?
If we’re going to talk about better ways to heat a home, we’d better have some idea of what heat is. What you experience as heat is just the energization of the molecules in your body. Heat is the energy that gives those molecules kinetic (vibratory) energy.

Obviously, your body produces its own heat through the metabolic process (burning calories); the important thing is that your environment neither inundates you with excess energy (when it’s too warm), or draws too much energy away from you (when it’s too cold). This begs the question, how does your environment give or take energy from you?

Physicists and engineers call the process heat transfer. There are three different mechanisms: radiation, conduction, and convection. Radiation is what you feel when you stand in front of a fireplace or in sunlight. Electromagnetic waves, primarily in the infrared portion of the light spectrum, strike you and transfer their energy. Conduction is how a coffee cup warms your hand: the kinetic (vibratory) energy of the coffee mug creates a resonance with your molecules, transferring energy. Convection heats by moving a warm fluid (typically air) across a cooler surface (you). In each case, the warmer object loses energy to the cooler one.

Another way of looking at heat transfer explains cooling in terms of heating: if you decide something is too warm, you cool it by finding something colder and facilitating heat transfer between the two. Strangely enough, for substances like gases (say, air), you can apply mechanical energy to drive this flow of energy. When you compress a gas, you raise its temperature; now the heat will tend to go some place cooler (indoors, if you’re trying to heat, or outdoors, if you’re trying to cool). When you remove the pressure, the gas is now cooler than its surroundings. Heat pumps, including refrigerators and air conditioners, work this way, more or less.

Unfortunately, the occupants of a house can’t depend on contact with warm, solid surfaces as a primary source of heat (despite what you might hear from people who deal in heated counter tops and toilet seats). At best, conductive heating is a side benefit (or a luxury feature). Thus, home heating is primarily about radiation and convection, known as radiant heating and forced air heating, respectively.

The next article in this series explains the drawbacks of using forced air heating, cooling and ventilation systems (HVAC).

Previous articles in this series:
Heating Your Home: Radiant Heat, Wood Heat.

Related articles:
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Frank Schiavo’s compact, tract-built, three-bedroom ranch-style home in a modest San Jose neighborhood demonstrates that remodeling to create a cutting-edge green home is neither difficult nor expensive. Heated with sunlight and cooled by night air, his home is comfortable, quiet and tasteful, filled with light and local art. With only modest investments in a sun room, extra insulation, new windows, a very small array of rooftop photovoltaic and solar hot water panels, his electricity bill for the coldest, cloudiest months of the year averages a few dollars a month. His gas bill is even more modest.

What’s most impressive about Schiavo’s house isn’t that it’s so comfortable and practical for him to own, it’s that it demonstrates that lofty resource conservation goals can be achieved on a modest remodeling budget.

Passive Solar Energy is Inexpensive

Schiavo’s remodel performs so well, and for so little, because it focuses on conservation, not features. San Jose has plenty of sun, so Schiavo’s house exploits passive solar design. First, Schiavo thoroughly insulated. Next, he added heat-collecting thermal mass (in the form of a small sunroom addition) to store heat energy in the winter and stabilize temperatures. In the summer, he stores the cool of the night air. Interior walls sport an unusual finish detail that, at first brush, appears to have been motivated by modernist aesthetics. Stacks of black, rectangular solids suggestive of consumer electronics protrude from interior walls extending from the floor to chair-rail height. As Schiavo explains, these are actually five gallon metal cans that have been painted black and fitted into steel support racks in key wall sections. The cans are filled with water, which has terrific thermal mass for its weight and volume. Many of these cans are situated in an interior wall that separates the interior from a south-facing sunroom. The water-filled cans store heat in the winter (and the cool of night air in the summer) and release it into the interior of his house.

Passive Solar Heating/Cooling: Operating the House

In the winter and early spring, Schiavo lowers special insulated doors in his sunroom, exposing the water-filled cans. Sun enters the windows of the sunroom and heats the brick-in-sand floor. The warm air in the sunroom then heats the water-filled cans. At night, Schiavo closes the insulated doors, and the water-filled cans radiate heat back into his house. This is an implementation of a passive solar Trombe Wall.

An added benefit of the sunroom space is that it makes an ideal place to hang laundry to dry. Schiavo admits he does use his gas dryer: about a minute or two per load, with no heat, to fluff-up his clothes and remove lint.

Schiavo Himself

A sustainability activist, passive solar design consultant, and retired environmental studies instructor from San Jose State University, Schiavo doesn’t shrink from publicity. A recent article in the San Jose Mercury News (4/5/2008, Is that a lion in the yard? S.J. fence-mural draws second looks) covers the extensive mural in Schiavo’s front and side yards, painted by a friend.

Schiavo first found the public eye in a well-publicized struggle with his local garbage company. Through a combination of disciplined purchasing habits, composting in his yard, and extensive recycling, he has virtually ceased to produce any trash. For years, he continued to pay the local garbage company for a service he wasn’t using. The mayor of San Jose found out and ordered the garbage company to stop billing him. His example led to the City’s composting program, run, incidentally, by a former student.

If you live near San Jose, you can see Schiavo’s house and mural at 1186 Bayard Drive. Look for footprints painted on the sidewalk, position your feet in them, and watch mural, building and landscaping meld into one large piece of art.

Related Articles:

• Life Cycle Costs
• Vancouver Adaptive Reuse
• Super-Insulating Vacuum Glass
• Patrician Place: an Experiment in Energy

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1. Ingredients

• 3 cups unbleached white bread flour (e.g., Whole Foods bulk item #5100)
• 1/2 tsp salt
• 1/4 tsp (no mistake!) instant yeast
• Approximately 11 oz of water

• Optional: cornmeal, bran, coarse or fine flour to sprinkle on top

Equipment

• 8-10″ wide round or oval pot, with sides at least 5-6″ tall, with tightly-fitting lid. Cast-iron dutch ovens work well
• Pizza stone, ceramic tiles, or bricks in the oven for thermal mass (I got two 16″ tiles from the Cupertino HomeDepot for about $7)
• Shallow roasting pan

In ceramic or glass mixing bowl, mix dry ingredients with a rubber spatula (or a wooden spoon). Add water and continue mixing to create a very wet dough that sticks to everything. If it seems too dry (more like a normal dough), add more water, a bit at a time. Just get it thoroughly mixed. Don’t bother kneading; the yeast and long rising time will do the rest.

Fit a dinner plate over the bowl and let rest at room temperature for 12-18 hours, or until dough has roughly doubled in size and shows signs of bubbles on top.
Use the rubber spatula (or spoon) to scrape and press the dough repeated from the sides of the bowl, working out the C02. Cover with a cloth. Let it rise again for about 2 hours, until it has again doubled in size.

About 1 hour before the dough finishes its final rise, arrange the roasting pan on the bottom shelf of the oven, then arrange the stone or tiles or bricks on a shelf immediately above it. Finally, put your pot with the tightly-fitting lid on top of the stone/tiles/bricks. Pre-heat the oven to 500 degrees F. Note: if you don’t pre-heat the pot with the oven, bad things can happen: your stones/tiles/bricks may shatter when you set the cold pot on them; your pot could shatter; and your bread will fuse to the pot and become impossible to remove intact.

When the dough has risen and the oven has hit the target temperature, pull the pot (with mitts!) from the oven and set it on a stove-top burner. Pull the lid off and set it on another burner. Using the rubber spatula, carefully pour and scrape the dough, trying to heap it as much as possible in the center of the pot. Sprinkle optional flour, cornmeal or bran on top of loaf. Now pour 1/8 cup of water (approximately) into the pot, trying not to let the water touch the bread before it turns to steam (too much water reaching the dough at this point can cause the loaf to stick). Slam the lid on to trap the steam, then put the whole thing back in the oven. Slide the roasting pan partway out from underneath, and pour 1 cup of water into the roasting pan, shove the pan back in, and quickly close the oven door. Set a timer. Baking time should be between 30 and 40 minutes. Add more water to the roasting pan at 8-10 minutes into the bake.

Steam in the first 15 minutes forms the thick crust and causes the unconfined loaf to rise properly.

At 30 minutes, remove the lid and check the loaf for done. If the loaf is relatively light and hard, pull out the pan and remove the loaf. Place loaf on a wire rack (or cool stove burner) to cool for at least 20 minutes. The finished loaf should be relatively light and well-browned. Caution: cut it too soon, and you’ll ruin the loaf! The heat trapped inside is needed to finish the baking process.

Bon Appetit!

Variations:

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