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Fireplaces and wood-burning stoves

Most fireplaces are used for creating a relaxed, cozy atmosphere rather than heating. Usually their use as an auxiliary source of heat cannot be economically justified, since they are very inefficient. A fireplace requires a large volume of air for combustion. Normally this air is drawn from the heated air in the house. Since the combustion air flows up the chimney, all the fuel that had been used in heating the air is wasted. To minimize this problem, a fireplace should have dampered air vents that provide combustion air from the outdoors. Although newer fireplace installations have provisions for outside air, most existing fireplaces do not have this feature and therefore waste more heat than they generate. In fact, in terms of the total heating value of the wood being burned, a fireplace does not generate much heat at all. Only about 10 to 15 percent of the available Btus actually find their way out into the room, while much more warm air disappears up the chimney.

Even when a fireplace is not being used, it is a potential source of heat loss. If the flue is not blocked, heat in the house will be drawn up the chimney. This condition can normally be minimized by closing the damper. Check the fireplace to see whether there is a damper and if so, whether it properly seals the flue opening. Fireplaces in many older houses do not have dampers. If a movable damper is too costly to install, you can always block the flue with a piece of sheet metal supported by guides on the sidewalls. It can be inserted and removed on an as-needed basis.

Some fireplaces have built-in convection ducts around the firebox that function as a heat exchanger. Cool air entering the bottom of the ducts is heated as it circulates around the firebox. The warm air rises and flows out into the room through openings in the upper section of the ducts. This type of fireplace has a greater heating efficiency than the conventional open-faced masonry fireplace. Nevertheless, a much more efficient way to heat the house is by using a wood-burning stove. A good-quality stove will allow only the air needed for combustion into the unit. Consequently, heated room air is not lost up the chimney. If you control the airflow through the stove, a log will produce more heat by burning for a longer period of time. In addition, since the stove is physically located in the room that it is intended to heat, the entire surface of the unit will heat up and warm the room by convection and radiation.

The cost-effectiveness of a wood-burning stove for heating will depend on the cost of the wood being burned. In those areas where wood is inexpensive, a stove is an economically viable method for providing auxiliary heat and reducing your overall heating bill. Even in those areas where wood is more costly, as utility rates increase, the use of a wood-burning stove becomes more attractive. Different woods have different burning characteristics. For example, wood from conifer trees (softwood) such as pine, spruce, and fir burns more quickly and gives less heat than wood from deciduous trees (hardwood) such as maple, oak, and beech. To aid you in making a proper selection for your fireplace or stove, TABLE 19-2, prepared by the Maine Bureau of Forestry, shows the various characteristics of wood.

Heating and air-conditioning systems

During your energy audit, you have been mainly concerned with those items needed to reduce heat loss. Another item of concern is maximizing the Btu output of your heating system. As you walk around the house, check the radiators or heat registers to see that they are unobstructed. If your house is heated with a warm-air furnace, check the filter to see if it needs replacement. If the oil or gas burner for the heating system was not cleaned and tuned up prior to your energy audit, it should be done as soon as possible. A heating system out of adjustment results in greater fuel consumption, which can be quite costly. Heating systems require a periodic tune-up for efficient operation.

Various devices on the market to be fitted on or around the chimney flue are supposed to cut fuel consumption and conserve heat. Many of these furnace attachments, however, do not live up to the manufacturers’ claims of saving significant energy.

One device of concern from a safety point of view is the automatic flue damper. This device is designed to close the furnace chimney after the burner turns off. The intention is to conserve heat by preventing room air from being drawn up the chimney. However, if the automatic flue damper fails to open when the burner fires, poisonous carbon monoxide fumes will vent into the house. If your furnace has such a device, you should find out whether it has been approved by a nationally recognized testing agency and by your local utility company. If it has not been approved, you should consider its removal or replacement as a precautionary measure.

By lowering the thermostat setting 5 to 10 degrees each night before going to bed and raising it in the morning, you will reduce fuel consumption and save dollars. The amount of the savings will depend on the duration of the setback, the climate, and the fuel costs in your area. See TABLE 19-3.

Table 19-2. Characteristics of woods for use in a fireplace or stove.

Ease ofEmber  Heating
Species starting generation Sparks Fragrance value
Apple Poor Excellent Few Excellent Good
Ash Fair Good Few Slight Good
Beech Poor Good Few Slight Excellent
Birch (white) Good Good Moderate Slight Good
Cherry Poor Excellent Few Excellent Good
Cedar Excellent Poor Many Good Fair
Elm Fair Good Very few Fair Good
Hemlock Good Low Many Good Fair
Hickory Fair Excellent Moderate Slight Excellent
Locust (black) Poor Excellent Very few Slight Excellent
Maple (sugar) Poor Excellent Few Good Excellent
Oak (red) Poor Excellent Few Fair Excellent
Pine (white) Excellent Poor Moderate Good Fair

The degree day is a unit that expresses the severity of the climate in an area. The reference temperature for evaluating degree days is 65° F. Degree days are the number of degrees that the average (of the high and low temperatures for a twenty-four-hour period) is less than 65° F. For example, if for a twenty-four-hour period (during the heating season), the high and low temperatures are 50° F and 30° F, respectively, the average temperature would be 40° F. The degree-day number for that day is then 65, 40, or 25. The total number of degree days for an area, therefore, is simply the sum of the degree-day numbers during the heating season. You can check with the local utility company to find the total number of degree days for your area.

From a convenience point of view, if your heating system is controlled by a manual thermostat, you should consider replacing it with an automatic-lock thermostat. With this type of thermostat, you can regulate the amount of the setback and its duration. Depending on your requirements, thermostats with double setbacks are also available.

Table 19-3. Energy savings for lowering thermostat.

Percent of energy savings for an 8-hour temperature setback
Degree days5° F10° F
5,000 8.1 12.1
6,000 7.2 10.8
7,000 6.1 9.6
8,000 5.2 8.5
8,500 4.6 7.8

If there is a central air-conditioning system in your house, it must also be cleaned and tuned up to maximize the efficiency of operation. Prior to the cooling season, have the system checked to see if it needs a refrigerant charge. The system will cool even if it is low in refrigerant; however, it will operate inefficiently. Check the location of the compressor to see if it is in the shade and whether the airflow into and out of the unit is unobstructed. If the compressor is in the midday and afternoon sun, it will not perform efficiently. You should build a sunscreen to shade the unit if necessary. Be careful, however, not to obstruct the airflow.

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