State University Extension
Home Maintenance And Repair
Energy Tips - Choosing the Right Furnace for Your Home
take control of your home heating bill in two ways: by making your
house as energy efficient as possible, and by making the heating system
more efficient. It’s best to address your house first since low cost
options exist for saving energy and methods exist to maximize the
efficiency of your existing heating system. But if your furnace is very
old, it’s wise not to spend money on major furnace modifications.
Instead, consider buying a new energy efficient furnace that will save
money on heating and air conditioning, conserve fuel, and keep your
home consistently warm in winter and cool in summer.
Selecting the Best Furnace for You
Choosing the ideal heating/cooling system can be challenging. In the
final analysis, you may find that issues such as comfort, aesthetics,
air quality, or other environmental concerns matter as much, or more,
than dollars spent. But invest the time needed to make a considered,
informed choice. That is the best insurance to be sure that you meet
your home heating, financial, and environmental goals. The major
• fuel type
• furnace efficiency
• correct sizing of the unit.
The first decision to make when selecting a heating system is fuel
type. The choice is often one of personal preference, but it can be
dictated by your home’s location. For example, if you live in the
country, natural gas may not be available as a choice. Gas and oil
furnaces and electric heat pumps are comparable options for central
forced air heating. Once you decide on fuel type, unit efficiency and
the unit size are the next considerations.
What is efficiency? In brief, an efficiency rating tells you how much
of the total energy used is delivered to the home as heat. The higher
the efficiency, the better the furnace functions. For example, a
furnace that is 80% efficient delivers 80% of the fuel consumed to the
house as heat. The rest, 20% or 20 cents on the dollar, goes up the
chimney flue. Your heating bill, therefore, depends on the cost of the
fuel and the efficiency of the furnace.
Most gas units use a power burner or an induced draft system to
regulate the flow of combustion air. Oil units have flame retention
burners and improved heat exchangers that result in a smaller, hotter
flame and reduce the amount of hot exhaust gas escaping up the flue.
Electric furnaces are considered to be 100% efficient because the
electricity is converted entirely into heat.
Some furnaces are classed as high efficiency and super efficiency.
These units recover more heat from the flue gases by condensing water
vapor and may use two or more heat exchangers. They also may use
outside air for combustion giving significant savings when the furnace
is located within the heated space.
Sizing the Unit
The best way to be sure you purchase a properly sized unit is to
calculate the home’s heating requirements (Btu/hr). This calculation is
fairly easy, but lengthy, and involves determining the area and the
insulation level of each part of the home, i.e. floor, ceiling,
exterior walls, and windows. You also need to estimate air
infiltration. If this calculation is beyond your skills or your desire,
consult your furnace dealer or an energy consultant to have this done
professionally. Computer programs and web sites are available to help
with this calculation as well. The important thing is to take the time
to do this or to have it done by a skilled professional.
Just like autos, the actual efficiency of a furnace depends on how it
is operated. But an oversized unit will waste energy by excessive
cycling on and off, sending even more heat up the flue. An undersized
furnace may not provide enough heat for comfort during very cold
weather. Most often, units are oversized so beware of this pitfall in
order to save fuel, save operating costs, and save money.
Remember—properly matching the size of the furnace to the heat load of
the building helps you save money in two ways:
• The furnace unit operates more efficiently saving costs on fuel
• You don’t pay for a larger unit than you need.
Duct Systems and Thermostats
After you select and install just the right furnace, be sure to check
your duct work. You will lose money instead of save it if the hot air
produced is lost or cooled before it reaches your living space. Ducts
should be located inside heated space to minimize conductive heat loss.
Avoid putting ducts in unheated crawl spaces and attics. After
installation, make sure all joints are sealed with duct tape to prevent
air leaks and insulate any ducts that are located outside heated air
You might consider investing in a new setback thermostat to save heat
and money as well. You can by a clock thermostat and set it for hourly
levels, or buy a programmable thermostat that has more options and it
more finely tuned. Or just set the thermostat back by hand each night
or when you are away, as long as you remember to set it back up again
when you return!
Furnace Efficiency Ratings
These ratings can be a useful tool to compare different furnace models.
But be sure you are comparing the same types of efficiencies. The two
most common are Steady State Efficiency and Annual Fuel Utilization
Efficiency (AFUE). Steady state efficiency refers to how well the unit
performs once it is warmed up and running. The AFUE considers the fact
that a furnace cycles on and off, and accounts for the amount of fuel
used to heat up the furnace itself. The AFUE will always be lower than
steady state efficiency.
Another tool is the yellow Energy Guide label required on all new
furnaces. This label provides the AFUE rating, an estimate of annual
energy costs, and a comparison of the unit to others of roughly the
same size. Also, the Gas Appliance Manufacturers Association (GAMA)
publishes a Consumers’ Directory of Efficiency Ratings.
Obtaining Bids and Planning for
As with any major purchase, plan to talk with at least three heating
contractors and get written bids and copies of warranties. Be sure you
are getting bids on comparable units and systems so you won't be
surprised when the bill comes. Check references for the contractors'
work as well.
Furnace air filters should be changed at least twice a year and once a
year, vacuum out the fan blades and lubricate the motor. Be sure your
pilot light is properly adjusted or you will waste fuel and lose money
at the same time.
Paybacks and Tradeoffs
To see if you will really save money, you have to anaylze two factors:
the heat load and fuel price. The higher efficiency furances cost more
to begin with, but they may save enough in fuet costs to pay the
difference rather qucikly. Fuel costs vary from type to type, region to
region, season to season, and for other hard to calculate influences
that may be poltical and/or environmental. A general rule of thumb is
that the higher the heating load and fuel price, the more you can
justify investing in a higher-efficiency furnace.
How to Figure Payback (in years) on
To compare the payback on two gas furnaces, try this method. Unit one
costs $1,800 and is rated at 75% AFUE efficiency. Unit two costs $2,400
and is rated at 86% AFUE efficiency. To calculate payback, use this
Payback (in years) = Extra
Annual Savings ($/year)
If your annual heating bill is $800, and your furnace is 75% efficient,
your bill could be reduced to $698 at 86% efficiency ($800 x .75/.86).
This would save you about $102 each year, assuming no differences in
The actual payback in this example would be just under six years
($600/$102 per year = 5.9 years), where $600 represents the difference
in purchase price and $102 represents the savings by installing a more
efficient furnace. If fuel prices rise, paybacks (number of years) will
be shorter making the decision to install the more efficient furnace an
even better one.
The following chart compares AFUE efficiencies for different types of
Range of Efficiency
|Older gas or
efficiency gas or oil
Efficiency gas or oil
Energy Tips - Choosing the Right Furnace for Your Home, Michigan State
Washington State University Cooperative
Extension Energy Program, Jack Brautigam and Edwin Valbert, EB 1836e.
Society of Heating, Refrigerating, and Air Conditioning Engineers,
Star, Money Isn't All You're Saving, U.S. EPA, http://www.energystar.gov/
Energy Efficiency and Renewable Energy, U.S. DOE, http://www.eere.energy.gov/
Environmental Technologies Energy Division, Lawrence Berkley National
Contains the do-it-yourself energy audit tool.
Gas Appliance Manufacturers Association
contains the Consumers' Directory of Certified Efficiency Ratings
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June 30, 1914, in cooperation with the U.S. Department of Agriculture.
Thomas G. Coon, Director, MSU Extension, East Lansing,MI 48824. This
information is for educational purposes only. Reference to commercial
products or trade names does not imply endorsement by MSU Extension or
bias against those not mentioned.
information.was reviewed as
of June 2008. For more information about the contents please
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