Electronic Ignition Gas Furnaces

High Efficiency Gas Furnace Utilizing Electronic Ignition

Gas furnaces, boilers, and water heaters have evolved from the basic standing pilot ignition system to the state of the art electronic ignition models. Standing pilot gas furnaces relied on keeping a pilot flame lit for the furnace, boiler, or water heater to function. The advent of electronic ignition has made it possible to prove a flame without keeping a pilot light lit 24/7 and thereby saving money because electronic ignition only needs to have a flame when there is a call for heat from the thermostat.

Typically, electronic ignition is comprised of a manufacturers OEM circuit board or an after-market ignition control module located near the gas valve of the furnace, boiler, or water heater. The gas valve is controlled directly by this circuit board or ignition module whereby the standing pilot gas valve is usually controlled by the thermostat. There are different versions of gas furnace electronic ignition systems but they all serve the same purpose and function and that is to safely heat your dwelling. Among these types of gas furnace electronic ignition systems there are:

• Direct Spark Ignition - this version has no pilot light at all. It lights the main burners on a call for heat and a state of the art flame sensor detects a flame. If no flame is detected on a try for ignition the gas valve closes. After a specified amount of time, usually seconds, the ignition module will try ignition again. Again, if the flame sensor detects no flame a delay before trying again will occur. Typically after three failed trials for ignition with the flame sensor not detecting a flame the ignition module will go into a lock-out mode for at least an hour (most models). This is a safety measure to prevent a dangerous build up of gas. If the module endlessly tried for ignition the un-ignited gas could build up to dangerous levels and an explosion could occur. *The spark coming from the gas furnace circuit board or ignition control module can exceed 10,000 volts so caution is advised.
• Intermittent Pilot Ignition - this version has a pilot light which only lights on a call for heat. The pilot light is located near the main burner and on a call for heat the ignition module or gas furnace circuit board opens the pilot valve inside the gas valve and initiates a spark at the head of the pilot assembly to light the pilot light. The flame sensor is also located at the pilot assembly and as soon as the flame is detected the gas furnace circuit board or ignition control module opens the main valve inside the gas valve. The pilot light then lights the main burners. When the thermostat is satisfied all gas control components& shuts off until the next call for heat. If a flame is not sensed then a trial for ignition will repeat itself for up to four trials and then the gas controls will be locked out for a specified period of time (a minimum of an hour depending the gas furnace or gas controls manufacturer)when a new sequence will start over again. *The spark coming from the gas furnace circuit board or ignition control module can exceed 10,000 volts so caution is advised.
• Hot Surface Ignition - hot surface ignition can be either direct fire or indirect fire to a pilot (Honeywell Smart Valve is an example of indirect fire to a pilot). The igniter is a silicon carbon element which glows red hot when voltage is applied to it. On a call for heat, the gas furnace circuit board or ignition module sends a specified amount of voltage to the igniter. After a small timed delay and after the igniter is glowing red hot, the main valve opens in the gas valve. The gas strikes the silicon carbon element and ignition occurs. A flame sensor located near the burners detects the flame and the main burners remain lit and the voltage is taken away from the hot surface igniter. If a flame is not sensed then a trial for ignition will repeat itself for up to four trials and then the gas controls will& be locked out for a specified period of time (a minimum of an hour depending on the gas furnace or gas control manufacturer) when a new sequence will start over again.
• Mercury Bulb - this system uses a mercury switch to prove there is a flame on a pilot light. Mercury bulb systems usually use spark ignition to light a pilot and the mercury switch proves the flame. Once the switch proves the flame the main valve in the gas valve opens and the pilot lights the main burners. Mercury is very hazardous to the environment and not many manufacturers use this type of electronic ignition.

It is important to note that all these systems work in a furnace sequence of operation as determined by the controller. If the furnace safety controls detects a problem with any of the safeties, including the furnace flame sensor, the furnace sequence of operation will be interrupted on a lock-out. Furnace sequence of operation varies from furnace to furnace and it is important to know the furnace sequence of operation to troubleshoot the system if a problem exists and the furnace is locking out the ignition of the furnace.

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