This section will focus on the most common type of new furnace, the 90%+ efficient condensing furnace.

90%+ efficient refers to the AFUE (Annual Fuel Utilization Efficiency) for gas furnaces.  Today’s furnaces are between 90% and 95% efficient (some of the modulating furnaces can be slightly higher) which means that they extract almost all of the available heat from the fuel.  If a 75,000 BTU furnace is 95% efficient, it is delivering 71,250 BTU to the air (75,000 x 0.95 = 71,250).  This is truly remarkable compared to the standard 60 – 80% models that they replaced.  In fact, because these furnaces are so efficient, they are sometimes called “condensing” furnaces because the heat exchanger extracts so much heat from the combustion gases that the water vapor in the air “condenses” out of solution and has to be carried away through a condensate drain line.  It is for this reason that these furnaces do not have metal flues – they are piped with plastic because the exhaust gas is so cool.

Most of these furnaces now have sealed combustion chambers.  Some are under positive pressure, some are negative, but both employ a combustion air blower.  This blower forces the combustion gases through the heat exchanger and out the flue.  There is no open flame exposed directly to the atmosphere in the home (provided the combustion air intake is plumbed directly outside like the exhaust).  This ensures that no combustion air is taken from the occupied space, thus reducing the possibility of carbon monoxide poisoning.

The sequence of operation begins at the thermostat.  Here there is a 24-volt control signal coming from the transformer in the furnace.  When the thermostat senses a call for heat, the contacts close and send the 24-volt control signal back to the furnace control board to tell it to begin.  Now the safety circuit takes over.  At this point, the combustion air blower will energize.  The safety circuit includes a pressure switch to check to see if the combustion air blower is operating correctly.  If the pressure switch detects there is not sufficient air pressure in the combustion chamber, meaning perhaps the flue is blocked or the blower is not working, the switch opens the safety circuit and the furnace shuts down.  Once the furnace is satisfied that there is adequate combustion air flow, it signals the ignitor to light.  This is a ceramic material that you may see glow orange inside the fire box.  If the ignitor is on and working correctly, the main gas valve will open.  When the gas meets the hot ignitor, it should light.  If it does, the flame sensor (basically a thermocouple) will sense that a flame is present and the ignitor will shut off.  If no flame is detected, the gas valve will close and the furnace will shut down.  After a predetermined delay (to give the heat exchanger time to get hot), the main blower motor will be energized, thus providing warm air to the space.  When the thermostat is satisfied, the gas valve will close.  After a predetermined delay, the combustion air blower and main blower will both de-energize.

It is important to keep the air filters clean and the registers open in the heated space.  The furnace needs a certain amount of air to remain cool.  If the high limit switch detects that the furnace is overheating (possibly due to inadequate air flow), it will shut off the gas valve and the main blower will continue to operate to cool down the furnace.  The blower will run, but there will be no flame.