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  1. #1

    Intermittent problem with Ruud gas furnace

    I have a Ruud gas furnace that I am having an intermittent problem with. I cannot find any model number on the furnace but it was new in 1991 when my home was built. The furnace does not have a gas pilot light. It does have an electric igniter and does have a Robert Shaw Model HS780 – 34NL – 306A Ignition Control Unit. The part # on the Robert Shaw Unit is 100-00812.

    The problem is that sometimes when the thermostat calls for heat, the electric igniter heats up and glows adjacent to the burner but the main gas valve does not open to allow gas to flow to the burner. This problem first presented itself in the spring of 2006. During the last few days of that spring when I needed heat I observed that the igniter was illuminated but the gas valve did not open, but then the need for heat was over for that spring. In the fall of 2006 I expected to have to face the problem with the furnace but when I tested the furnace, it fired up and worked flawlessly until the spring of 2007. Again, for the last few days of spring 2007 when it would have been nice to have a little heat, the previously noted problem reoccurred. Now, in the fall of 2007, I am testing the furnace for the coming heating season and the furnace has fired up twice out of perhaps eight attempts. But the ignited is still glowing brightly.

    Can any one tell me what it is exactly that is supposed to open the gas valve? I expect that there is a sensor that will verify that the igniter is if fact hot enough to ignite the gas before the gas valve is allowed to open but what is the nature if that sensor? Does is need cleaning?

    Just what is the function of the Robert Shaw Ignition Control Unit?

    Is any diagram available, which will show how this system is supposed to work?

    Thanks. Rich

  2. #2
    Join Date
    Dec 2002
    Location
    Houston,Tx.
    Posts
    15,831
    Quote Originally Posted by RichMiller View Post
    II expect that there is a sensor that will verify that the igniter is if fact hot enough to ignite the gas before the gas valve is allowed to open
    Thanks. Rich


    Rich you need to call a service professional, trust me this is something that only a service
    professional can resolve.
    __________________________________________________ _______________________
    “Experience is a hard teacher because she gives the test first, the lesson afterwards". - Vernon Law

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  3. #3
    Join Date
    Jun 2004
    Location
    Howell, Michigan
    Posts
    16,168
    10-4 on that Bill. You need a service tech to properly diagnose your problem. Sorry but we are not allowed to dispense DIY help in this forum.

  4. #4
    Join Date
    Sep 2001
    Location
    indiana
    Posts
    232

    ignitor

    replace your ignitor
    2 hot 2 cool

  5. #5
    Join Date
    May 2000
    Location
    BENTON, KY 42025
    Posts
    339
    How will changing the ingnitor going to open the gas valve?

    Call a professional.

  6. #6
    Join Date
    Mar 2005
    Location
    burlington county n.j.
    Posts
    9,706
    Quote Originally Posted by dennis663 View Post
    replace your ignitor







    WHAT !!!

  7. #7
    Join Date
    Sep 2001
    Location
    indiana
    Posts
    232

    ignitor

    o.k., i'm not going to school you on how a gas valve with a glow bar ignitor works, but if you did your homework, you would know that a poor resitive value going through an ignitor will keep it from operating, if the gas valve doesn't get the right value from the ignitor, it will not energize....
    2 hot 2 cool

  8. #8
    Join Date
    May 2000
    Location
    BENTON, KY 42025
    Posts
    339
    Quote Originally Posted by dennis663 View Post
    o.k., i'm not going to school you on how a gas valve with a glow bar ignitor works, but if you did your homework, you would know that a poor resitive value going through an ignitor will keep it from operating, if the gas valve doesn't get the right value from the ignitor, it will not energize....
    You might want to check into another school.........

  9. #9
    Join Date
    Sep 2001
    Location
    indiana
    Posts
    232

    ignitor

    i didn't mean to offend you, i know you wasn't taught things like this back in the stone age, i've changed quit a few ignitors with this symptom, it always works, + it's about the cheapest thing you can buy to replace, if it's wrong, your not out much !!!
    2 hot 2 cool

  10. #10
    Join Date
    May 2000
    Location
    BENTON, KY 42025
    Posts
    339
    I 've changed dozens of bad HSI recently on systems with no pilot, and the valve will still open, whether the ignitor is burned into or not. How can that happen according to your post?
    People taught in the stone age need more information than was given by the OP to just tell them to change a part, no matter how cheap it is.
    There are techs and there are part changers, big difference.

  11. #11
    Join Date
    Sep 2001
    Location
    indiana
    Posts
    232

    ignitor

    o.k., then tell me how the gas valve knows to open ? because the ignitor get's hot or because an elecrical value is put across the coil ?

    yes, it's a trick question.
    2 hot 2 cool

  12. #12
    Join Date
    May 2000
    Location
    BENTON, KY 42025
    Posts
    339
    Timed on through the board, not through the HSI.

  13. #13
    Join Date
    Sep 2001
    Location
    indiana
    Posts
    232

    ignitor

    VERY GOOD!!!, now what tells the board to open the gas valve ?, is it because the ignitor get's hot or because it send an electrical value to the board ? & what happens if the value the board is looking for is not strong enough ?
    2 hot 2 cool

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