7.7: Insulation and Home Heating Fuels (III)

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Fuel Choices

Various fuels such as natural gas, electricity, fuel oil, and so on, are used to heat a house. Table 7.7.1 shows the percentages of households that use each type of heating fuel.

Table 7.7.1. Choice of fuels by percent of households

Fuel type	Percentage used
Natural gas	49%
Electricity	34%
Fuel oil / LPG	11%
Other	6%

Source: Department of Energy

As you see, more than 50 percent (56% to be exact) of the households in the United States use natural gas as their main heating fuel, and about 26 percent of the households use electricity to heat their homes. Another 11% use fuel oil, and the last 10% use something other than natural gas, electricity and fuel oil.

Capacity and Consumption

Capacity

The amount of heat a furnace can deliver is called its capacity. Heating units are manufactured and sold by their capacity. The heating capacities of Natural Gas, Propane and Fuel Oil are measured according to BTU/h, and the capacity of Electricity is measured in kilowatts.

Consumption

The amount of energy a furnace actually uses is called consumption. In other words, we pay monthly bills for the consumption of a particular heating fuel. Heating Fuels are sold to consumers in different units of measure. For example, Natural gas is sold by cubic feet (ft³).

The video below explains the difference between capacity and consumption in a gas-heated home.

Fuel Comparison

Table 7.7.2 compares details about different types of fuel.

Table 7.7.2. Capacity, consumption, and other information about various types of fuel

Fuel	Capacity	Consumption	Additional Information
Natural gas	Measured in BTU/hr. Most heating appliances for home use have heating capacities of 40,000 to 150,000 BTU/hr. In the past, gas furnaces were often rated only on heat input. Today, the heat output is given.	Measured in cubic feet (ft³). This is the amount that the gas meter registers and the amount that the gas utility records when a reading is taken. One ft³of natural gas contains about 1,000 BTU of energy.	Utility companies often bill customers for CCF (100 ft³) or therms (1 therm = 100,000 BTU) of gas used. Some companies also use a unit of MCF, where 1 MCF = 1,000,000 BTUs.
Propane or liquefied petroleum gas (LPG)	Measured in BTU/hr	Usually measured in gallons. Propane has an energy content of about 91,300 BTUs per gallon.	Can be used in many of the same types of equipment as natural gas. Stored as a liquid in a tank in the house, so it can be used anywhere, even in areas where natural gas hookups are not available.
Fuel oil	The heating (bonnet) capacity of oil heating appliances is the steady-state heat output of the furnace, measured in BTU/hr. Typical oil-fired central heating appliances sold for home use today have heating capacities of 56,000 to 150,000 BTUs.	Usually billed by gallons. One gallon of #2 fuel oil contains about 140,000 BTUs of potential heat energy.	Several grades of fuel oil are produced by the petroleum industry, but only #2 fuel oil is commonly used for home heating.
Electricity	Usually expressed in kW. A kWh is the amount of electrical energy supplied by 1 kW of power over 1 hour. Electric systems come in a wide range of capacities, usually from 10 kW to 50 kW.	Sold in kWh	The watt (W) is the basic unit of measurement of electric power.

Heating Values of Various Fuels

Each unit of fuel when burned gives different amounts of energy. The energy that is released when a unit amount of fuel is burned is called the heating value. The heating value of a fuel is determined under a standard set of conditions. A comparison of approximate heating values of various fuels is shown in Table 7.7.3.

Table 7.7.3. Heating values of commonly used heating fuels

Fuel	Unit	Heating value (BTUs)
Natural gas	CCF (100 ft³) or therm	100,000
Natural gas	MCF (1,000 ft³)	1,000,000
Fuel oil	Gallon	140,000
Electricity	kWh	3,412
Propane	Gallon	91,300
Bituminous coal	Ton	23,000,000
Anthracite coal	Ton	26,000,000
Hardwood	Cord	24,000,000

From Table 7.7.3, it should be noted that if a gallon of fuel oil is burned, one would get 140,000 BTUs. Similarly, a CCF of natural gas would fetch 100,000 BTUs.

Heating Efficiency

An assumption is made here that all the energy from the fuel is released, and all the heat is available to heat the place. However, generally, when a fuel is burned in a furnace, not all the energy (heat) is available for the final end user.

The energy efficiency of a furnace is not 100 percent. Not all the energy from the fuel is released, and not all the heat is available to heat the place. For example, if a furnace’s efficiency is, say, 50 percent, then twice as much fuel would be needed to heat a home.

Looking again at Table 7.7.3, we saw that the heating value of fuel oil is given as 140,000 BTUs. However, if the furnace’s efficiency is 50 percent, then the actual heating value of fuel oil is 140,000 BTUs x 0.5 (efficiency) = 70,000 BTUs. In other words, when a gallon of oil is burned, 70,000 BTUs of heat is actually available to the user. This is shown in Table 7.7.4.

Table 7.7.4. Heating values of commonly used heating fuels with 50% efficiency

Fuel	Unit	Heating value (BTUs), 100% efficiency	Heating value (BTUs), 50% efficiency
Natural gas	CCF (100 ft³) or therm	100,000	50,000
Natural gas	MCF (1,000 ft³)	1,000,000	500,000
Fuel oil	Gallon	140,000	70,000
Electricity	kWh	3,412	1706
Propane	Gallon	91,300	45,650
Bituminous coal	Ton	23,000,000	11,500,000
Anthracite coal	Ton	26,000,000	13,000,000
Hardwood	Cord	24,000,000	12,000,000

Table 7.7.5 shows efficiencies of a furnace.

Table 7.7.5. Efficiencies of a furnace

Efficiency	Heating value of fuel oil (BTUs)
50%	70,000
55%	77,000
60%	84,000
65%	90,000
70%	100,000
75%	105,000
80%	111,000
85%	122,000
90%	125,000
95%	131,000
100%	140,000

It can be clearly seen that as the efficiency of the furnace increases, the amount of heat available increases proportionally.

The higher the efficiency, the less oil needs to be put into the furnace to get the same amount of heat output.

Most of the heating furnaces burn fuel and release hot combustion gases. The hot combustion gases heat the incoming cold air and go out through the chimney. In older furnaces, all the heat in the fuel is not released or not transferred to the cold air (or water, in the case of heat registers and water heaters), and therefore is lost through the chimney. The air or water that is heated distributes the heat throughout the house. Newer models of furnaces have gotten better at getting more of the heat into the cold air and, therefore, into your house.

The video below shows how a wood burning furnace operates at 75% efficiency.

Furnaces usually are not as efficient when they are first firing up as they are running at steady-state. It is sort of like a car getting better mileage in steady highway driving than in stop-and-go city traffic.

What matters over the course of the year is the total useful heat the furnace delivers to your house versus the heat value of the fuel it consumes. This is kind of like measuring the gas mileage your car gets by asking how many miles you drove this year and dividing it by how many gallons of gas you bought.

For furnaces, they call this measure the AFUE (Annual Fuel Utilization Efficiency). The federal minimum-efficiency standards for furnaces and boilers took effect in 1992, requiring that new furnace units have an AFUE of at least 78 percent and new boiler units have an AFUE of at least 80 percent. In comparison, many old furnaces and boilers have AFUE ratings of only 55 to 65 percent.

To find out how efficient your furnace is, look for an energy guide label like in Figure 7.2.3.

Figure 7.7.1. EnergyGuide label for furnace efficiency

Table 7.7.6 lists the efficiencies of most efficient furnaces that were available in 2002–2003.

Table 7.7.6. Efficiency range of furnaces in 2002-2003

Fuel	Furnace type	Efficiencies (%)
Natural gas	Hot air	93.0 to 96.6
Natural gas	Hot water	83.0 to 95.0
Natural gas	Steam	81.0 to 82.7
Fuel oil	Furnace	83.8 to 86.3
Fuel oil	Hot water	86.0 to 87.6
Fuel oil	Steam	82.5 to 86.0

When buying a new furnace, make sure its heating capacity (output) is appropriate for your home. If the insulation and/or windows in your home have been upgraded since the old heating equipment was installed, you can probably use a much less powerful furnace or boiler. Oversized furnaces operate less efficiently because they cycle on and off more frequently; in addition, larger furnaces are more expensive to buy.