In the area I work in, Chicago and the surrounding counties, the most common heat source for a home is a forced air furnace.  The term ‘forced air’ refers to the furnace being equipped with a blower fan which moves the house’s air through the furnace and distribute the heated (or cooled, from the A/C system) air through the house.

In the ‘old days’, before forced air technology was developed, the heated air would move through the house simply by the concept of hot air rising and cold air falling.  These furnaces, technically known as ‘Gravity Hot Air’ furnaces were nick-named ‘Octopus’ furnaces because of the large number of ducts that radiated out of them (left).  As the air was heated, it flowed upward through the ducts and out the duct registers in the upper house.  When this heated air cooled it was heavier and fell through return registers that were located in the floor.  We see many older houses in this area that have large wooden grate registers in the floor, an indication that there was once an old Octopus furnace in the house.

A modern forced air furnace has a blower fan that forces the heated air (hence, the forced air designation) through the ducts and sucks the return air back through the return registers.  This not only moves the heat around the house more efficiently, but also provides for proper ventilation and air movement.  Without such a fan, the air would tend to stagnate in the other seasons when there was little need for heat.

Forced air furnaces can have a variety of different fuel sources.  Most common in the Chicago area is the natural gas furnace, but we also see furnaces that use electricity and even geothermal heat differences  Furnaces nowadays also tend to serve the air conditioning system as well as the heating system, with the cooling coil installed above the heat exchanger and the furnaces forced air blower moving the cooled air as well.

Given recent technology advances, and attempts to lower energy use and waste, there are now two major types of gas forced air furnaces.  To understand these advances, a little explanation is needed.

When you burn gas (methane or propane) with air, you get heat as well as a number of chemical reactions.  The following equation shows most of the created products:

Gas + Air (oxygen, nitrogen, carbon monoxide) = Heat + carbon dioxide + carbon monoxide + water + carbonic acid + nitric acid. 

So, the exhaust gases coming out of the furnace contain a mixture of things in the combustion gasses that must be exhausted out of the house.  Traditionally, these gases were taken up the chimney just like a wood fireplace. These chimneys were lined either with brick or clay tile liner.  This still works for older furnaces but can cause confusion and expensive damage for the newer, more energy efficient units.

Conventional Furnaces – In the older designs the combustion gases from the furnace were quickly taken up the chimney flue.  These gases would tend to flow upwards because they were hot (hot air tends to rise).  As they did so, they would also draw air from the house, called draft air or ‘make up’ air.  Older houses were usually very ‘leaky’, allowing outside air to come in through the walls, older windows and other exterior penetrations.  This was OK because this extra air was used by the furnace to support combustion (gas + air = fire).  The leaky air ‘made up’ to the air that the furnace was sending up the chimney.

These furnaces rely on drafting, the sucking effect of the heated gasses, to get the combustion gases up the chimney quickly before the moisture in the gases has a chance to condense out.  Remember our equation, one of the products of combustion is water.  If the combustion gasses linger too long in the chimney, the carbonic and nitric acid mix with the moisture, forming corrosive acids.  These acids eat through the masonry causing  damage to the flue.  Additionally, water portion of the combustion gasses, not yet condensed, and still retained a significant amount of latent heat which was just being wasted.

Condensing Furnaces – Newer furnace designs have the ability to capture this extra latent heat by causing the water portion of the combustion gases to condense back to the liquid phase in the furnace capturing this extra heat, releasing what was wasted heat back into thee house.  The downside of this new technology is that the exhaust gas flues had to be changed so as to avoid damage from the acidic moisture.  In mid-efficiency furnaces (Category I and III) the brick or clay chimney liners must be replaced with a stainless steel liner.  In high-efficiency furnaces (Category VI) the chimney is not even used and the combustion gases are vented through PVC tubing, usually out the side of the house.  If the newer furnaces are vented like the older ones, the chimney would just fall apart due to the acidic moisture eating the chimney from the inside out.

Furnace Categories –  The heating and cooling engineers have defined different furnaces by category, depending upon how the deal with this condensation issue.  The main factors considered are a) the air pressure inside the furnace flue, compared to the air pressure outside the flue, and b) the amount of water condensation that occurs within the flue as the combustion gasses are exhausted.  This also determines the energy efficiency of the furnace, the ratio of the amount of natural gas burned to the actual amount of heat used to heat the house.  This is measured by a calculation called the Annual Fuel Utilization Efficiency (AFUE).  Furnaces are rated based upon this calculation:

• 55 to 72 percent AFUE low-efficiency furnaces: Now obsolete, these were the old gravity hot-air furnaces and their like, They are still found in some houses. 
• 78 percent AFUE low-efficiency furnace: Conventional natural  draft hood type furnaces. They also, usually, have a standing flame pilot light that wastes energy, a single stage gas valve and a single speed blower fan.
• 80% AFUE mid-efficiency gas furnaces: Also called category I or III furnaces.This group includes furnaces with 80 percent (up to 83 percent) AFUE ratings.   These furnaces usually have a separate induction fan which draws air and the gas flame into the heat exchanger combustion chamber.  They can be easily identified by looking for a round wheel inside the furnace cabinet (right). The difference between the two categories is determined by the air pressure in the flue.  A category I furnace has a flue pressure that is lower than the surrounding air in the house.  This is caused by a small orifice installed just downstream from the induction fan.  If the furnace is connected to the same flue as a conventionally drafted water heater, a positive flue pressure from the furnace can cause back drafting of carbon monoxide.  A category III furnace has a flue pressure that is higher than the surrounding air and should be connected to the exhaust flue separately.  All mid-efficiency furnaces are also equipped with an electric spark igniter system rather than an always-on pilot light flame and some also have multi-stage gas valves and blower fans for additional efficiency.
• High-efficiency gas furnace: These category IV furnaces are Energy Star-certified with a 90 to 98 percent AFUE ratings.  There are several differences in these furnaces:
• There is a separate, secondary heat exchanger that is installed at the point where the house’s return air is entering the furnace (right).  This causes the colder return air to cool the partially cooled combustion gasses providing the greatest temperature difference between two.  The greater the difference in these two temperatures, the more efficient the heat transfer will be.  This is where the condensation occurs, sucking every last BTU of heat from the condensed latent heat of the combustion gasses.
• Because of this condensation, the moisture in the combustion gasses condenses out as slightly acidic water.  That is why category IV furnaces are sometimes referred to as condensing furnaces.  This water can be seen draining out of the furnace, usually through the same drainage system used by the air conditioning condensate water.  This moisture would pose a problem for metal exhaust flues, causing them to rust and corrode.  For this reason, category IV furnaces use PVC vent flues.  The combustion gases are usually around 80° F, so the PVC melting is not a concern.
• To add more efficiency, category IV furnaces are also usually directly vented.  This means that there is a combustion air intake vent as well as a combustion gas exhaust connected to the furnace.  This can be seen as two larger PVC pipes connected to the furnace (right) and two PVC vents on the outside of the house.  One pipe brings air in from the outside so the furnace has air for combustion.  The other one takes the combustion gasses outside, eliminating the need to use the chimney.  A common defect that home inspector’s see is when the furnace was installed by a non-professional who does not run the combustion air intake pipe.  in such a situation, the furnace will still operate properly, but will not be as efficient overall because it is using air that it has already heated (air from the inside of the house) and wasting it outside.  High-efficiency furnaces can also have additional energy saving features like multiple stage gas valves, multiple state induction and blower fans.  These allow the furnace to only use as much natural gas and electricity as is needed, not having the furnace run at full speed all the time.  They also always have an electric spark ignition system.  It is important to note that high-efficiency furnaces are more complex than regular furnaces.  This means that it is very important that they be services regularly and that the HVAC technician be properly trained to do the servicing.  I always recommend that the HVAC servicing company use only factory trained and certified technicians.
• A note of caution:  Houses usually outlive their gas furnaces and the furnace has to be

  replaced.  This can cause problems and confusion, even if well trained HVAC contractors do the replacement.  If you replace a conventionally drafted furnace with a newer mid-efficiency furnace, you MUST have the brick or clay tile chimney lined with a metal flue, preferably one made of stainless steel (much longer life than aluminum or galvanized steel liners).  The new mid-efficiency furnace will produce acidic combustion gasses which will eat your chimney from the inside.  Not all HVAC technicians will know this and some, not wanting to mention the added expanse of having the chimney lined,  will ignore it.  The chimney MUST be lined (in this area, it is a code requirement). If no liner is installed the chimney will slowly destroy itself (right).  Installing the chimney liner is best done by a certified chimney sweep.

• Additionally, if you install a high efficiency furnace and have it exhaust out with PVC piping at  the side or roof of the house, but you still have a conventionally vented water heater as the only thing using the chimney, a hazardous condition could occur.  Conventionally drafted water heaters do no generate enough heat to properly draft out of a brick or clay lined chimney by themselves.  In the colder months, the water heater exhaust gasses will travel part way up the chimney, get cold and fall back down the flue.  This will cause carbon monoxide to back draft at the water heater vent hood and can lead to death.  If you are changing your furnace to a high efficiency unit, you should also change your water heater as well.  This is a common safety problem that I see.