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Overview of the Electronic Ignition Gas Furnace
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 75% 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 80%.
- 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.Continue to 2 of 10 below.
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Types of Electronic Ignition Furnaces
There are 3 basic types of gas furnace designs:
- Conventional Warm Air Furnace,
- Induced Draft Furnace, and
- Condensing 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 80% 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%.
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.
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.Continue to 3 of 10 below.
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Furnace Intermittent Pilot (IP)
Most usually found on some induced draft furnaces, this pilot ignites with a high voltage spark only when the thermostat calls for heat. Once the intermittent pilot (IP) is lit and the main burner senses the pilot flame (flame sensing rod), the main burner will ignite.
The furnace gas valve for this type of furnace is identifiable with its solenoid designations PV PV/MV MV (where MV=main valve, PV=pilot valve and PV/MV=common).Continue to 4 of 10 below.
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Furnace Hot Surface Ignition ( HSI)
The hot surface igniter (HSI) is the most commonly used electronic ignition system used today. The reason is they are reliable, they eliminate the need for a pilot, and they are inexpensive. These igniters directly ignite the burner and all they require is a flame sensor to detect the successful ignition of gas and proper flame occurring in the burners and turn off the hot surface igniter once proper ignition is achieved.
The hot surface igniter works like a light bulb filament, except here, we... want the glowing red hot heat from the element, not the light. The hot surface igniter is placed in the flow of gas entering the burners. The hot surface igniter is usually made from a material like silicon nitride or silicon carbide (more fragile) and as electricity passes through the igniter, it will glow red hot.
When the thermostat calls for heat 24 volts are sent to the electronic ignition control module (furnace brains) which energizes the furnace blower and other systems and sends 120 volts to the HSI. The HSI heats up to around 1,800°F to 2,500°F. Once the proper HSI operating temperature is achieved, the gas valve is opened introducing gas to the burner which is ignited by the red-hot element of the HSI. Once a proper flame is sensed by the flame sensor, the HSI is shut down. NOTE: Some furnace designs have the HSI also act as a flame sensor (local sensing) instead of having a remote sensing rod in the burner flame (remote sensing).
Under normal conditions, the HSI should last for 3-5 years. However, it will eventually crack and need to be replaced, quicker if the oils from your skin get on the element by improper handling.
These hot surface igniters are between $25 to $50 depending upon the model of furnace you have. I strongly recommend you keep a spare on hand. Just like a light bulb, the filament does not last forever and these igniters fail. As Murphy's Law will have it, the igniter will fail when it is at night or on a weekend and you can't easily get a spare.
We now need to take a look at some of the associated parts of the electronic ignition furnace.
Possible Reasons for HSI Failure
Continue to 5 of 10 below.
- Premature failure due to improper handling (oil from skin got onto element)
- Improper igniter
- End of normal usage life cycle (keep replacement on hand)
- Electric current in the home is too high (over 125 VAC)
- Severely dirty air filter causing high temperature limit switch to cycle furnace ignition on and off frequently.
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Induced Draft Furnace - Draft Fan Inducer and Vent
In this photo, you can see how the draft fan inducer motor relates to the furnace vent. If your furnace has a chimney vent and motor right under it, then you do not have a condensing furnace but rather an induced draft furnace.
Next, let's look at some detail of the components that make up the induced draft design.Continue to 6 of 10 below.
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Induced Draft Furnace - Component Detail
With this type of furnace you'll see the following components "under the hood":
- Draft Inducer Motor
- Pressure Switch
- Pressure Switch Tube
- Electronic Control Center or Pilot Control Module
How It Works
Continue to 7 of 10 below.
- The thermostat calls for heat,
- Draft inducer motor starts up,
- Pressure switch attached to the draft motor by a small tube will sense the negative pressure created by the draft inducer,
- The draft inducer motor runs for 30 to 60 seconds,
- Flame sensor senses heat from the pilot or HSI and allows gas to flow to... burners,
- Gas burner is ignited by the hot surface igniter or the intermittent pilot,
- These processes are controlled by the electronic control center module.
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Non-Integrated Circuit Control Module
The center of the electronic ignition system is the control center module. However, units such as the one above do not have integrated circuit control boards and are limited as to their capabilities. For example, they cannot perform self diagnostics or handle variable speed drives and multiple heat exchangers which are handled easily by integrated circuit control systems.Continue to 8 of 10 below.
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Integrated Circuit 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.Continue to 9 of 10 below.
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Troubleshooting a Hot Surface Ignition (HSI) System
Furnaces with electronic ignition systems may be much more efficient than their early counterparts, but they do it largely through electronics. However, there is still some troubleshooting you can do, but fixing the problems can involve replacement of parts more so than repair or adjustment.Continue to 10 of 10 below.
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Troubleshooting an Intermittent Pilot Ignition System
Because this type of system only lights during heating cycles, it's much more energy efficient than a standing pilot light. However, when problems occur, the ignition might not spark and the furnace doesn't work. Other times, the spark may be present, but the pilot won't light. If the pilot does light, but the main burner doesn't come on, you have another problem. A tricky problem to troubleshoot is when the burners do ignite but then turn off after a few seconds.