Residential furnaces can be classified into four general categories, based on efficiency and design.

Natural draft

The first category would be natural draft, atmospheric burner furnaces. These furnaces consisted of cast-iron or riveted-steel heat exchangers built within an outer shell of brick, masonry, or steel. The heat exchangers were vented through brick or masonry chimneys. Air circulation depended on large, upwardly pitched pipes constructed of wood or metal The pipes would channel the warm air into floor or wall vents inside the home. This method of heating worked because warm air rises. The system was simple, had few controls, a single automatic gas valve, and no blower. These furnaces could be made to work with any fuel simply by adapting the burner area. They have been operated with wood, coke, coal, trash, paper, natural gas, and fuel oil. Furnaces that used solid fuels required daily maintenance to remove ash that accumulated in the bottom of the burner area. In later years, these furnaces were adapted with electric blowers to aid air distribution and speed moving heat into the home. Gas and oil-fired systems were usually controlled by a thermostat inside the home, while most wood and coal-fired furnaces were controlled by the amount of fuel in the burner and position of the fresh-air damper on the burner access door.

Forced-air

The second category of residential furnace is the forced-air, atmospheric burner style with a cast-iron or sectional steel heat exchanger. This style furnace was used to replace the big, natural draft systems, and was sometimes installed on the existing gravity duct work. The heated air was moved by blowers which were belt driven and designed for a wide range of speeds. These furnaces were still big and bulky compared to modern furnaces, and had heavy-steel exteriors with bolt-on removable panels. Energy efficiency would range anywhere from just over 50% to upward of 65% AFUE. This style furnace still used large, masonry or brick chimneys for flues and was eventually designed to accommodate air-conditioning systems.

Forced draft

The third category of furnace is the forced draft, mid-efficiency furnace with a steel heat exchanger and multi-speed blower. These furnaces were physically much more compact than the previous styles. They were equipped with combustion air blowers that would pull air through the heat exchanger which greatly increased fuel efficiency while allowing the heat exchangers to become smaller. These furnace have multi-speed blowers and were designed to work with central air-conditioning systems.

Condensing

The fourth category of furnace is the high-efficiency, or condensing furnace. High-efficiency furnaces can achieve from 89% to 98% fuel efficiency. This style of furnace includes a sealed combustion area, combustion draft inducer and a secondary heat exchanger. Because the heat exchanger removes most of the heat from the exhaust gas, it actually condenses water vapor and other chemicals (which form a mild acid) as it operates. The vent pipes are normally installed with PVC pipe versus metal vent pipe to prevent corrosion. The draft inducer allows for the exhaust piping to be routed vertically or horizontally as it exits the structure. The most efficient arrangement for high-efficiency furnaces include PVC piping that brings fresh combustion air from the outside of the home directly to the furnace. Normally the combustion air (fresh air) PVC is routed alongside the exhaust PVC during installation and the pipes exit through a sidewall of the home in the same location. High efficiency furnaces typically deliver a 25% to 35% fuel savings over a 60% AFUE furnace.

What is a Two-Stage Furnace?

When someone refers to a one-stage (aka single stage) or a two-stage furnace, they are actually talking about the burner section inside of the furnace, or more specifically the valve that controls it.  Your average, every day, old fashioned furnace is a single-stage or one-stage furnace.  What this means is that the valve that controls the amount of fuel that enters the burner in your furnace has two different positions: open and closed.  When this valve is open, fuel (usually natural gas or propane) travels through the valve to the pilot light where it ignites, and burns inside the burner section of the furnace.  The now heated gases then heat up the heat exchanger, which is where the air from your home is actually heated.  When the valve is in the closed position, the heat is off because no fuel is being provided to the burners and heat exchanger.

A two stage furnace actually has three positions to this valve: full open, partially open and closed.  What this allows your furnace to do when it turns on is move this valve to one of it�s two open positions (hence the term, �two-stage�).  This allows your furnace to operate at full blast, like your old fashioned furnace, or in a low position that only provides about 60 to 65% of the heating power of the full-open position.  Therefore a two stage furnace has two heat settings that are used automatically, high heat and low heat.

How Does a Two-Stage Furnace Work?

A two-stage furnace works by using a different set of programming than a normal one-stage furnace.  Simply put, when it is only moderately cold inside, then the furnace�s control board opens the fuel control valve to the partially open position, and heats the house to the desired temperature before turning off.  If the house is cooling a lot quicker than normal, for instance on a miserably cold day in the dead of winter, then the control panel opens the valve to the full open position, where the furnace runs full blast to heat your home.

What are the Advantages of a Two Stage Furnace?

There are three advantages to having a two stage furnace: it saves energy, it heats your home more completely and it filters your air better.

How a Two Stage Furnace Saves Energy

This part is probably pretty obvious.  Your two stage furnace saves on energy costs because it doesn�t burn as much fuel. Your two-stage furnace saves gas because it doesn�t dump as much fuel into the heat exchanger when it�s only moderately cold outside.  If it�s only 40 or 50 degrees out, then a two stage furnace will run in the partially open position and only use about 60% of the energy of it�s single stage counterparts.  A one stage furnace, however, can only run in the full open position, even if it�s only 50 degrees outside.

A Two Stage Furnace Heats Your Home More Evenly and Removes Cold Spots

The reason your home gets cold spots is one of three reasons: improperly positioned duct-work and air registers, a furnace that is too large for your house, or if it is a one-stage furnace.  Improper duct-work means that heat is not being evenly distributed throughout the extremities of your house.  An improperly sized furnace will constantly turn on and off because it is so powerful that it heats up your house in a matter of seconds and then kicks off again, allowing your house to cool before this heat is evenly distributed.  The third reason is because of a one stage furnace.  Just as with the example of using a furnace that is too big for your house, a single stage furnace only has one setting; full blast.  As such, it will heat your house rapidly and then turn off allowing your house to cool down, causing cold spots before it kicks on again.

Where a two stage furnace helps you out is that on a moderately cold day, the furnace kicks into low heat and runs continuously for several minutes and for it�s whole heating cycle.  Furnaces are made to operate for a specific period of time each time that they turn on.  This is how modern furnaces are so efficient.  If a furnace is too large, it kicks off before it is allowed to run through it�s whole cycle, which means that air is not evenly distributed and circulated throughout your house and your insulation is not properly warmed up, causing cold spots.  

Two Stage Furnaces Filter Your Air Better

Because a two stage furnace will run more often, it will be circulating more air through your duct-work and thus increase the filtration of your house.  If a furnace is one stage or is too big for your house, again it will kick off more often and spend less time filtering the air in your house.  More air moved means more air filtration.