Overview
The electronic ignition system in a gas furnace is a modern development that allows more reliable performance than standing pilot furnaces provides energy savings and contributes to better furnace efficiency (AFUE). With a standing pilot, found most commonly on older low efficiency furnaces (55% to 65% AFUE is not uncommon), a small gas flame is always burning and is known in the lexicon of American home repair as a "pilot light". The problem with this type of "analog" ignition is that it wastes energy by constantly burning gas and can sometimes be unreliable.
These issues have led to the development of electronic ignition systems for mid to high efficiency furnaces that exceed the U.S. government’s established minimum AFUE rating of 78%. The electronic ignition occurs typically in one of two ways:
• Intermittent Pilot, or
• Hot Surface Ignition
The intermittent pilot system uses an electronically controlled high voltage electrical spark to ignite the gas pilot and then subsequently the main burners, when the thermostat calls for heat.
The hot surface ignition system uses an electronically controlled resistance heating element not unlike a light bulb filament (and shown in the photo above), to ignite the gas burner.
It is important to understand some of the other components of a modern furnace that you will encounter depending on the type of high efficiency furnace you have. Why? Because they can also come into play in repairing an electronic ignition furnace when it won't run properly. Let's take a quick review of the types of furnace designs and components found in high efficiency furnaces using electronic
ignition.

Types of Gas Furnaces
There are 3 basic types of gas furnace designs:
• Conventional Warm Air Furnace,
• Induced Draft Furnace, and
• Condensing Furnace
Conventional Warm Air Furnace
Of these three types, the conventional warm air furnace is the least efficient (lowest AFUE) with AFUE ratings of units manufactured before 1992 often below the minimum of 78% and include some older furnaces with an AFUE as low as 55% to 65%. These older low efficient furnaces have a standing pilot.
As you enter the world of mid to high efficiency furnaces of 80% to 90% and higher, you'll find electronic ignition systems and either the induced draft furnace for an AFUE of 80% to 82% or a condensing furnace with efficiencies of between 90% to about 95%.
Mid Efficiency Induced Draft Fan Furnace
Induced draft furnaces improve upon the conventional furnace by using an electronic ignition, either intermittent pilot or hot surface ignition (HSI) instead of a standing pilot. Unlike a conventional furnace that will create a natural draft by drawing air in from an opening in the front of the furnace, the induced draft furnace will use a small fan to draw the combustion gasses into the flue. This combination of electronic controls, electronic ignition and artificially created draft increases the efficiency of the furnace from about 60% to between 80% to 82% AFUE.
High Efficiency Condensing Furnace
The condensing furnace gets its name because by using a second heat exchanger, hot flue gasses are cooled to the point where the water vapor condenses. This allows the furnace to extract even more heat from the combustion process. And since the resulting flue gases are cool, not hot, they can be vented outside horizontally with a plastic PVC pipe. The condensed water is run to a floor drain. Condensing furnaces are high efficiency with an AFUE of 90% and above and use hot surface ignition.

Troubleshooting a Home Furnace Thermostat
PROBLEM: Furnace Produces No Heat

Furnace Produces No Heat
Possible Causes
• Circuit breaker tripped or fuse is blown.
• Thermostat components dirty.
• Loose wires or open connection.
• Bad battery on digital electronic thermostats.

Possible Repairs
• Reset circuit breaker or replace fuse.
• Remove thermostat cover and gently clean components especially the bimetallic coil and the switch contact surfaces if the thermostat does not have a mercury switch.
• Set the thermostat to its lowest setting and clean the bimetallic coil with a soft brush or compressed air. Set the thermostat to its highest setting and clean the coil again. Reset the thermostat to its desired setting.
• Tighten any loose screw terminals. If a wire is loose from its terminal, place wire back on terminal clamp and tighten screw.
• Check for loose connection or disconnected wire on low-voltage transformer usually mounted on furnace
• On electronic models, check to see if the display is working indicating power. If it is not working, replace the batteries in the electronic thermostat.

Integrated Circuit Control Board

Integrated Ignition Control System
The brain of most modern furnace electronic ignition systems uses an integrated circuit board to control many sophisticated processes. These integrated controllers can also perform self-diagnostics providing codes similar to the diagnostic computer in your car. The integrated circuits continuously monitor the furnace's operation and the operation of the integrated control module itself. If a failure occurs, LED's can indicate a failure code. The codes are listed in the Owner's manual and also likely on the furnace door.
You will often find the control board in the blower compartment of the furnace with a 3-5 Amp fuse to protect the board from a short.
These integrated controllers are always used on higher efficiency furnaces that rely on many sophisticated design features to reach their high AFUE ratings of over 90%. However, they still provide the same basic function as their less intelligent cousin, the non-integrated electronic control module. That is, to control sequences of operation for the furnace.

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