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Seven 4'x8' solar collectors installed along the driveway are covered with tempered glass to capture the heat of the sun. When the collectors are hotter than the water in the solar storage tank in the basement, the system switches on, and an anti-freeze solution is heated as it is pumped through copper tubes in the collectors. A heat exchanger then transfers this heat to the solar storage tank, which in turn either provides hot water to the domestic hot water tank or transfers heat to the exercise pool and/or to the radiant floor heating system, as needed.

Additional heating is provided by natural gas.

The solar collectors produce 500-1,000 BTU/sq.ft./day, depending on the amount of sunlight that day. With seven collectors, the array produces roughly 100,000-200,000 BTU/day, the equivalent of 1-2 therms of natural gas/day.

The Illinois Department of Commerce and Community Affairs (now called the Department of Commerce and Economic Opportunity) contributed $5,000 towards the cost of the system through its Renewable Energy Resources Program. (The maximum rebate amount has now been increased to $10,000.)

Radiant Heating System
This house is heated with a hydronic (liquid) radiant floor heating system. The tubing for the system is set into gypcrete, installed on top of the subfloor, to provide thermal mass. There are six zones--two each for the ground floor and the second floor, one for the third floor, and one for the basement. It has not been necessary to activate the floor heating in the basement zone because sufficient heat radiates from the exercise pool to keep this level warm.

Two ENERGY STAR-certified Weil-McClain GV Gold (Series 4) boilers serve as the primary heat source for the system. Although relatively small by some standards, they are over-sized for this house, given its tight construction and the homeowners' energy-conscious lifestyle and practices. (Each boiler consumes up to 105,000 BTU/hour to generate up to 92,000 BTU/hour.)

According to the U.S. Department of Energy, some national surveys have determined that well over half of all HVAC contractors do not size heating and cooling systems correctly--and the most common mistake is oversizing. Unfortunately, that was the case for this house. Despite the homeowners' oft-stated assurance that the house would be exceptionally air tight and energy efficient and despite repeated questioning about the size of the proposed heating and cooling system, the HVAC contractor maintained that the system was sized appropriately.

When the contractor finally provided a copy of his calculations (long after the over-sized ducts and boilers were in place), it was clear that the unique features of this house had not received careful consideration. In addition to the size of the house, key factors for correctly sizing a heating and cooling system include:

• Insulation levels. During construction of this house, special attention was given to achieving high levels of insulation.

• Window area, location and type. Although windows comprise a significant amount of the exterior wall area here, they are high-performance, triple-glazed, low-E, argon-filled windows. There is also good passive solar gain from the south-facing windows in the cooler months.

• Local climate. For space heating purposes, climate is measured in Heating Degree Days (HDD), which indicates how much colder the temperature is outside compared to a baseline temperature of 65 degrees Fahrenheit. (This baseline reflects the observation that houses generally require heating if the outside temperature is lower than 65 degrees and cooling if the temperature rises above 65 degrees.) If the average temperature for the day is 35 degrees, it is said to have 30 HDD. The Heating Degree Days for each day of the year are added together to give a measure of how cold the year was. The average number of Heating Degree Days in the Chicago area is roughly 6,000 HDD/year.

• Occupant life style. The homeowners are diligent about closing the window shades on hot, sunny days in summer and after dark in winter. They also make good use of the airlock at the front entry during colder weather.

• Types and efficiencies of lights and major home appliances. Most of the lighting in this house is provided by highly efficient compact fluorescent lights.

Advanced Combustion Wood Fireplace
Conventional wood fireplaces are extremely inefficient. When a fire is burning, most of the air heated by the fireplace--approximately 80 percent--rises up the chimney rather than being distributed into the house. As the warmed air escapes, outside air is pulled into the house through gaps around windows and under doors. This in turn causes the main heating system to work harder to heat the cold outside air. The result in many cases is a net loss of heat.

Moreover, the poor combustion in these fireplaces allows large volumes of unburned compounds to go up the chimney or even spill into the house. These incomplete combustion products become creosote, which can cause chimney fires, and also turn into fine particles and toxic pollutants, which are a major source of air pollution and can compromise indoor air quality.

Advanced combustion fireplaces, on the other hand, make use of new technological developments to achieve dramatically improved operating performance, energy efficiency and safety as well as reduced emissions of air pollutants. They use two separate paths of combustion air to feed two simultaneous combustion zones. One source of air is fed directly to the burning wood while the second is aimed above the main fire where it ignites the incomplete combustion products that would otherwise be released up the chimney.

The use of two combustion zones reduces emissions of incomplete combustion products ten-fold compared to a conventional fireplace. In addition, the low levels of incomplete combustion products and creosote virtually eliminate the potential for chimney fires.

Other design features contribute to the high performance of these fireplaces. Outside air is brought into the firebox for combustion via an insulated air duct. Indoor air is heated as it is drawn in through a grille under the firebox and moves through a heat exchanger before being sent back into the room through grilles at the top of the fireplace. Truly airtight, gasketed doors prevent room air from being drawn up the chimney, and a special ceramic glass window allows most of the infrared heat from the flame to be transmitted into the room.

The unit installed in the living room of this house is an RSF Onyx Fireplace. With an EPA rating of 3.8 grams of particulate matter/hour, it is certified to the same standards as the best freestanding wood stoves. It supplements the radiant floor heating during the colder months, raising the temperature of the house several degrees above the thermostat settings of 65-68 degrees for the different zones in the house. The firewood that the owners burn comes from a sustainably managed tree farm in Missouri.

Wood burning and the environment
A by-product of combustion is carbon dioxide, a greenhouse gas that contributes to global warming. When fossil fuels such as oil, natural gas and coal are burned, carbon that was buried deep in the earth for millions of years is released, resulting in an increase in the concentration of carbon dioxide in the atmosphere.

Although carbon makes up about half the weight of firewood and is released as carbon dioxide when the wood is burned, this is part of the natural carbon cycle of trees. As they grow, trees absorb carbon dioxide from the air and store it in the wood as carbon. When a tree dies and decays on the forest floor, it slowly releases carbon dioxide back in the atmosphere; if the tree is burned as firewood instead, the same amount of carbon dioxide is released. In both cases, the carbon dioxide is absorbed again by young trees, and the carbon cycle is repeated, again and again--without increasing atmospheric carbon.

Key to the sustainable use of this renewable fuel is to ensure that our forests are well-managed and that the firewood we burn comes from trees that are harvested using sustainable forestry practices. And using an efficient, clean-burning appliance, such as the advanced combustion fireplace discussed above, also helps make heating with wood a good choice for the environment.

Links and Resources

• Healthy Heating is an industry service site educating consumers, contractors, architects and designers about radiant-based heating, ventilation and air conditioning systems. Site features include articles, newsletters, discussion groups, and other resources.

• An article in the September/October 1994 issue of Home Energy Magazine Online--Fireplaces: Studies in Contrasts--critiques the design of conventional fireplaces and discusses the energy-efficient, environmentally-friendly, and safe alternative of newer advanced combustion wood fireplaces.