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  1. #1
    Join Date
    Oct 2012
    Location
    CT
    Posts
    59

    Spark Ignition safety for Gas

    Hey Everyone,

    A little background on me, I'm new out of tech school, still looking for an apprentice job. In the mean time I'm trying to learn all I can by visiting this site (I've applied for pro, just waiting now) and watching videos on youtube, etc...

    This afternoon I was watching one video on YouTube where a guy has a furnace with a spark ignition that won't light. The spark is sparking but not lighting the gas. Eventually he takes a lighter and lights the gas. Watching this has me curious about what safety is in place for no flame on a spark ignition gas furnace? In class we dealt with a standing pilot and hot surface ignitor when we worked with gas, and they both had thermocouples to cut off gas supply if no flame was present, but we didn't learn much about spark ignition. Does a spark ignition use the same safety? If so, why was this guy able to light the furnace with a lighter after no ignition from spark? If anyone has a little time to provide me with some additional knowledge, I'd appreciate it.

    THanks!

  2. #2
    Join Date
    Jul 2007
    Location
    Virginia
    Posts
    5,915
    Quote Originally Posted by Joe7cri View Post
    I was watching one video on YouTube where a guy has a furnace with a spark ignition that won't light. The spark is sparking but not lighting the gas. Eventually he takes a lighter and lights the gas.

    Does a spark ignition use the same safety? If so, why was this guy able to light the furnace with a lighter after no ignition from spark? If anyone has a little time to provide me with some additional knowledge, I'd appreciate it.
    IID (intermittent ignition device) systems light a pilot, and then the main burners only after the pilot has been proven to be lit.

    This is usually done through flame rectification (possibly thermal detection). Some flame rectification systems use a separate sensor and ignitor, while others use a combination ignitor/sensor, which is referred to as "local sensing".

    DSI (direct spark ignition) systems light the main burners with a spark ignitor. Flame rectification is also used on DSI systems.

    All systems have a short time (only a few seconds) to prove that the pilot/burners are lit. If they do not light within this short safety period, the control will shut off the system automatically.

    In order to prevent nuisance lockouts, systems will try to relight a predetermined number of times.

    So all the guy is doing on the video is bypassing the spark ignitor and manually lighting the flame (pilot or main burner) with a lighter. If the flame isn't proven shortly there after, the gas valve will still close.

    Flame rectification is the SAFETY, regardless how the burner is lit.

    Just a note - view cautiously what you see on YouTube. There is some bad info on HVAC topics being uploaded.

    Hope this helps, and welcome to the world of HVAC.
    Instead of learning the tricks of the trade, learn the trade.

  3. #3
    Join Date
    Oct 2010
    Location
    oklahoma city
    Posts
    61
    Great reply, rundawg. Just to add a little, the DSI also has purge times before, and between ignition attempts. This is to purge any gas from a failed ignition cycle. It also allows the combustion/draft motor to prove its operation to the ignition control board.

  4. #4
    Join Date
    Oct 2012
    Location
    CT
    Posts
    59
    Thanks for the info! I was thinking that after the initial failure to light, it would lock out the gas. Great explanation, Thanks again!

  5. #5
    Join Date
    Oct 2002
    Posts
    1,394
    Be warned. Flames do conduct electricity! Manual ligting could give you a shock if the spark ignitor is also trying to light.

  6. #6
    Join Date
    Nov 2012
    Location
    Barrie, ON
    Posts
    29
    Quote Originally Posted by timjimbob View Post
    Be warned. Flames do conduct electricity! Manual ligting could give you a shock if the spark ignitor is also trying to light.
    ...or ground to the fuel source in someone's hand....good times

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