There are basically three ways you can compare the performance of different models and brands:
(1) How much energy they use to heat or cool your home (Efficiency); (2) How much you notice their presence (Sound Levels); and (3) How they make you feel (Comfort).

Efficiency

Furnaces. The efficiency of a furnace is measured in a rating known as AFUE (Annual Fuel Utilization Efficiency). A lot like your car’s miles per gallon rating, AFUE tells you how efficiently the furnace converts fuel (gas or oil) into heat. An AFUE of 80% means that 80% of the fuel is used to heat your home, while the other 20% basically goes up the chimney.
In 1992, the government established a minimum AFUE rating for furnaces installed in new homes at 78%. (In contrast, many furnaces manufactured before 1992 had AFUE ratings as low as 60% — so nearly half the fuel was being wasted.) Furnaces with AFUE ratings of 78% to 80% are considered “mid-efficiency”; those with ratings of 90% or higher are known as “high efficiency.” The maximum furnace efficiency available is around 96.6%.

In general, the higher the efficiency of the furnace, the more it will cost but the less fuel it will use to heat your home. If you have an older furnace (with an AFUE of about 60%), you could save up to 60% on your heating bills by replacing it with a new high-efficiency furnace. So the cost to replace your old, inefficient furnace is paid back through lower utility bills.

If you live in a cold climate, you could see a payback in a few short years. If you live in a moderate climate, it might make more sense to purchase a mid-efficiency furnace. Your dealer can use heating data from your area to help you determine about how long it would take you to recover the additional cost of a high-efficiency model in energy savings. (Of course, after the payback, you continue to save on your energy bills for the life of the system.)

Heat pumps and air conditioners. Cooling efficiency for air conditioners and heat pumps is indicated by a SEER (Seasonal Energy Efficiency Ratio) rating, which tells you how efficiently a unit uses electricity. The higher the number, the greater the efficiency.

The typical SEER rating of units manufactured prior to 1992 is about 6.0. In 1992, the government established the minimum cooling efficiency standard for units installed in new homes at 10.0 SEER. High-efficiency units have a SEER of at least 12.0; the maximum available is about 17.

Heat pumps also have heating efficiency ratings, indicated as an HSPF (Heating Seasonal Performance Factor). In general, the higher the HSPF rating, the less electricity the unit will use to heat your home.

The 1992 government minimum heating efficiency standards for new heat pumps is 6.8 HSPF. Most heat pumps manufactured before 1992 have HSPF ratings below 5.0. Today, an HSPF of 7.5 or higher is considered “high-efficiency”; the maximum available is 10.0. (If you want to get real technical, the actual heating efficiency of air-source heat pumps is well over 100%, because they “steal” heat energy from the outside air — instead of using just electricity — to heat your home. So you get much more out of them than you put in.)

As with furnaces, higher efficiency in heat pumps and air conditioners usually means higher cost but lower utility bills. If you live in a warm and/or humid climate, you will probably see the higher cost of a high-efficiency air conditioner or heat pump paid back (through lower utility bills) in a few short years. Ask your dealer to help you determine about how long it would take you to recover the additional cost in energy savings. Of course, after the payback, you continue to save on your energy bills.

There’s one other factor that affects the efficiency of your air conditioning or heat pump system: the indoor coil. (Your heat pump or air conditioner is a “split system,” which means that there is an outdoor unit, or condenser, and an indoor unit, or evaporator coil.) If your condensing unit is not matched with the proper indoor coil, it may not give you the stated SEER and/or HSPF ratings and could even develop performance problems. (It’s kind of like putting two new tires on one side of your car and leaving the old, worn-out ones on the other side. You’d probably be disappointed with both the performance and the miles per gallon you get.) When you’re replacing an existing system, make sure you replace both units so your new condensing unit will give you optimal performance, efficiency and comfort.