Furnaces

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Introduction

Furnaces, which burn fuel to produce heat, are the most common type of heating system in the United States. Today's furnaces burn either natural gas, propane or heating fuel oil. A furnace with a minimum efficiency rating of 78% (the federal minimum standard) annually uses 790 therms and costs $320 to operate. Many older furnaces in the United States are less efficient than that. By upgrading your furnace to a new high-efficiency model, you can save as much as $900 over the life of the furnace.

Technology Options

Oil, Natural Gas and Propane
Oil, natural gas and propane furnaces are similar. The difference is the fuel they burn to produce heat. Natural gas is the most common fuel used for furnaces nationally. In California, oil furnaces require annual servicing mainly because oil does not burn as cleanly as natural gas or propane. Oil produces a lot of sulfur and soot when it is burned, which needs to be cleaned out of the heat exchanger and passageways. On the other hand, their efficiencies do not differ remarkably.

Furnace Components and Efficiency
There are four main components to a furnace: a burner, a heat exchanger, a blower and a vent pipe or flue.

    The burner is where the gas or fuel oil is delivered and burned to produce heat. Most gas furnaces today use an electronic ignition switch as opposed to a constantly burning pilot light. The switch works in a similar fashion to a spark plug. When heat is called for, the switch ignites gas in the burner with an electrical charge. This saves gas by not burning it between ignition cycles.

    The heat exchanger transfers heat from the burners to air that is circulated throughout the system. They come in a variety of shapes, with a serpentine configuration being the most efficient, and a variety of gauges, with the heavier gauge metals absorbing heat better.

    The blower is a fan that distributes the air heated by the exchanger into the duct system, through each vent in the building.

    The vent pipe or flue releases gaseous by-products of combustion to the outside through a PVC pipe or chimney.

Types of Gas Furnaces
There are three types of gas furnaces: a conventional warm-air furnace, an induced draft furnace and a condensing furnace.

    Conventional warm-air furnaces work by burning natural gas or propane in the burner. The heat produced by the burner is stored in one or more heat exchangers. Cool air is heated as it is forced past the heat exchanger. Heated air is then forced out of the furnace by the blower into the ducts of the central heating system. Cooler air is then drawn into the return ducts where it is returned to the furnace to be heated again and returned to the house or building. This cycle continues until the desired temperature set on the thermostat is reached. Air is drawn into the flue for venting, where it mixes with the hot gaseous by-products and is drawn outside.

    Induced draft furnaces differ from conventional furnaces in their venting method. Induced draft furnaces use a fan to draw the combustion products into the flue. This artificially created draft increases the efficiency of the furnace.

    Condensing furnaces contain a second heat exchanger that condenses water vapor in the hot flue gasses, extracting additional heat. The gases are then vented directly outside through a pipe in the wall. Condensing furnaces are the most efficient on the market but they are also the most expensive. Almost all ENERGY STAR qualified furnaces are condensing models.

Efficiency Rating
Furnace efficiency is rated by annual fuel utilization efficiency (AFUE). The AFUE measures the amount of fuel converted to space heat in proportion to the amount of fuel entering the furnace. The federal minimum efficiency standard for furnaces specifies an AFUE rating of at least 78%. ENERGY STAR qualified furnaces must have a minimum AFUE rating of 90%. The most modern and efficient heating systems can achieve an AFUE of as high as 97%.

For a list of the most efficient furnaces in all output categories, visit the American Council for an Energy-Efficient Economy (ACEEE)'s guide to top-rated furnaces.

Efficiency Improvements
Some improvements that have been made to the components of furnaces in recent years are two-stage burners and higher-efficiency blower motors.

    Two-Stage Burners — When the heating demand is low, two-stage burners allow you to run your furnace at a lower burn rate most of the time, using less gas than a single stage burner. These use a special gas valve that fires the burners at either the low-heat or high-heat level. When heating needs increase, the burner fires at the high-heat level, increasing the burn rate. These burners save energy by using a smaller amount of gas when demand is low, thereby providing more even heating throughout the day.

    Fan Blower Motor — Depending on the size of the furnace, fan blower motors can use between 84 kWh/year for the smallest gas furnaces (25,000 Btu) to almost 550 kWh/year for the largest models (118,000 Btu). Some newer furnaces have an electronically commutated (ECM) fan blower motor that is more efficient than a standard blower motor. Variable-speed blower motors also have been in use in newer models. By using only the power needed for the immediate heating demand, these motors use 75% less electricity than standard motors. They also adjust to changes in air pressure among rooms, heating the space more efficiently.

Efficiency Benefits

A new ENERGY STAR qualified furnace with an AFUE of 90% will save $600 over its lifetime vs. a furnace with an AFUE of 78%, the federal minimum efficiency standard. The highest-rated furnace, with an AFUE of 97%, will save $900 over its lifetime vs. the national standard. See the table below for specific natural gas savings.

Cost-Effectiveness Example
Performance Base Model ENERGY STAR Base Best Available
AFUE 78% 90% 97%
Annual Energy Use 790 therms 685 therms 635 therms
Annual Energy Cost $320 $270 $250
Lifetime Energy Cost $4,600 $4,000 $3,700
Lifetime Energy Cost Savings - $600 $900
Source: DOE, Federal Energy Management Program
*Based on national average gas price of $0.40/therm. To evaluate specific savings, multiply the annual energy use in this table by your gas rate: Example: 790 therms x your rate for natural gas

Your actual energy use will vary depending on your climate. Furnaces in milder climates, where heating loads are smaller, will use less energy.

Calculate the amount of money you can save with the purchase of a new energy-efficient furnace. Compare the costs of different heating fuels.

Manufacturers

Carrier, Bryant, Thermo Pride and Lennox are some of the manufacturers of ENERGY STAR qualified high-efficiency furnaces. For a complete list, visit the ENERGY STAR website.

Purchasing Tips

  • Before buying a new furnace, make low-cost energy efficiency improvements to the home or building. For example, add insulation and make sure windows and ducts are tightly sealed and do not leak air. You can hire a qualified home performance or heating contractor to help you evaluate your existing system. Energy efficiency improvements will save money on the furnace, because the home or building will need a smaller furnace.
  • Evaluate the comparable costs of oil, natural gas and propane. Since they all have comparable efficiencies in furnaces, the deciding factor in your choice will be how available each fuel is in your area and how much each costs.
  • When buying gas and oil systems, specify sealed combustion. Sealed-combustion furnaces bring outside air directly into the burner and exhaust flue gases (combustion products) directly to the outside, without the need for a draft hood or damper. They generally burn more efficiently and pose no risk of introducing dangerous combustion gases into your house. In furnaces that are not sealed-combustion units, back-drafting of combustion gases can be a big problem.
  • Hire a contractor to run a heat-loss, heat-gain calculation to accurately size the new furnace. Among other important considerations, make sure that the calculation takes into account:
    • Size and configuration of the home
    • Levels of insulation in walls, ceilings and between floors
    • Number and type of windows in the home
    • Orientation of the home to the sun
  • Evaluate the cost-effectiveness of buying a new furnace. If you live in a cold climate, it usually makes sense to invest in the highest-efficiency system available. In milder climates with lower annual heating costs, the extra investment required to go from 78 to 90-95% efficiency may be hard to justify.
  • Ask for an ENERGY STAR qualified furnace.

Emerging Technology

The MicroHeater, developed by the Pacific Northwest National Laboratory, could redefine the way in which homes are heated. The size of a palm, the MicroHeater is capable of combusting 4,000 to 120,000 Btu of gas per hour. They can be installed in baseboard heaters, and an array of them can efficiently heat an entire house. The MicroHeater can reduce energy loss from a typical central heating system by 45%.