EGEE 102
Energy Conservation for Environmental Protection

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 the table on the previous screen, 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.

Heating values of commonly used heating fuels if efficiency = 50 %
Fuel Unit Heating Value (BTU's) If Efficiency = 50 %
Natural Gas CCF (100 Cu. ft) or Therm 100,000 50,000
Natural Gas MCF (1,000 Cu.ft) 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
Efficiency of a Furnace
Click here to see the graph's tabular data

Efficiency of a Furnace

The table below contains the data from a line plot showing the efficiency of a furnace. The percent efficiency is dependent on the heating value of fuel oil (measured in BTU) and shows a strong positive correlation.

Efficiency of a Furnace
Efficiency of a Furnace Heating Value of Fuel Oil (BTUs)
50% 70000
55% 77000
60% 84000
65% 90000
70% 100000
75% 105000
80% 111000
85% 122000
90% 125000
95% 131000
100% 140000

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.

Click the play button and observe how a wood burning furnace operates at 75% efficiency.

How a Wood Furnace Operates

Click here to open a text description of How a Wood Furnace Operates

How a Wood Burning Furnace Operates

Cold combustion air from outside is pulled into the furnace combustion chamber by an electric blower. The wood fire in the chamber heats the air to 1 million BTUs. The hot combustion gasses heat the cold, outside air to 750,000 BTUs, which will circulate through the house. The combustion gasses (250,000 BTUs) travel out through the chimney.

Furnaces are usually 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 this:

Energy Guide Label with an idicator that the particular models operates at 94% efficiency.
Energy Guide Label.

The table below gives the efficiencies of most efficient furnaces that were available in 2002–2003.

Efficiency range of some of the most efficient furnaces in 2002–2003
Fuel Furnace Type Efficiencies (%)
Natural Gas Hot air 93.0 - 96.6
Natural Gas Hot Water 83.0 - 95.0
Natural Gas Steam 81.0 - 82.7
Fuel Oil Furnace 83.8 - 86.3
Fuel Oil Hot Water 86.0 - 87.6
Fuel Oil Steam 82.5 - 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.