Types of Furnaces

There are two basic "families" of furnaces available today.

80% efficient models and 90% models (sometimes called condensing furnaces)

The 80% furnaces are further divided into three categories.

1. Single stage 80% furnace  This is a basic furnace that burns the same way every time it fires.  It has one speed, (or stage), full blast on.  A minute or two after the burner starts, the fan comes on and blows the heat into the home through the ductwork.  After the thermostat is satisfied, the burner shuts down while the fan continues to run for another couple of minutes until it also turns off.  Since furnaces are usually sized to make enough heat to handle a very cold winter day, on milder days a single stage furnace may make more heat than is needed.  This can be somewhat uncomfortable and wasteful as the temperature fluctuates inside the home.
2. Two Stage 80% Furnace  This furnace is usually built in the same type cabinet and heat exchanger as the single stage unit.  The difference is that when the thermostat calls for heat the burners first fire up at about half speed.  This is the first stage.  If the weather outside is mild on that day that may be as far as it needs to go.  This is very common with our beautiful Denver weather.  On colder days, when the first stage can't make enough heat, the second stage will come on and the furnace acts more like a single stage unit.  The major advantages are less fuel used as well as a more comfortable home.
3. Two Stage Variable Speed 80% furnace  These units operate like a two stage unit with one important difference.  The other units all use a standard blower moter system.  A standard motor only runs basically one of a couple of choices of RPMs.  Pick the RPM (usually at the factory or during installation) and that's how much air you get into your duct.  That's assuming that several other conditions are met.  Dirty filters, undersized ducts, altitude and other factors can reduce the actual amount of air delivered with standard motors.  A variable speed motor on the other hand will "vary" it's speed until a certain actual airflow is achieved.  This usually results in about 25 -35% more total airflow in our high altitude in Denver.

In the 90%+ furnace family there is one important difference.  Usually a factory will make all of the same three models as the 80% furnaces with an extra, secondary, heat exchanger.  This allows the furnace to remove even more heat from the products of combustion before exhausting to the outside.  These furnaces can be as high as 96% efficient and are available in all of the same types as the 80% units described above.

What's the right unit for you?  That will depend on several factors.  We're happy to meet with you and discuss your various options.