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  1. #1
    Join Date
    Nov 2012
    Location
    Springfield IL
    Posts
    4

    Hmm Bryant 397H gas furnace gas valve or not?

    Hello everyone. I'm new here and any help is greatly appreciated!

    I'm pretty sure im on the right track, but I normally don't work on furnaces so I just want to make sure I'm not overlooking something obvious. I got a call from a friend whose furnace was acting up. When the t-stat calls for heat the inducer motor starts up, pressure switch proves, then the ignitor proceeds to try to light the pilot. It takes several tries for the pilot to stay lit. The pilot itself looks weak but eventually stays lit and the flame prover sends voltage the the main gas valve. The main burners fire up and either go out right away or they will stay on for less than a minute then go out. I suspected a dirty or bad flame sensor so i checked for voltage at the gas valve. The MGV remains energized for 2 to 3 seconds after the burners go out so i eliminated the flame prover as the problem. I think the gas valve is bad, but I want to be for sure before I start replacing parts. Should I check the gas pressure? I also noticed there was a pilot adjusting screw on the MGV . Then again I was thinking the main burners might need cleaned out or something. What do you guys think?

  2. #2
    Join Date
    May 2000
    Location
    Indianapolis, IN, USA
    Posts
    34,617
    Isn't that the old 3 wire pilot burner? Warp bar in the pilot switches to open main valve after the heat from the pilot causes it to switch. Have you taken the orifice out and cleaned it?

  3. #3
    Join Date
    Jul 2009
    Location
    Columbia, MD
    Posts
    4,488
    Make sure pilot orifice and pilot are good. OEM 3 wire pilot. This about raps it up

  4. #4
    Join Date
    Nov 2012
    Location
    Springfield IL
    Posts
    4
    No, I haven't cleaned the orifice, but I will in the morning. I will probably go ahead and clean the main burners also. Here's where I'm a little confused. After the main burners go out I am still getting 26 volts at the gas valve for about 2 seconds. If the flame prover sends power to the gas valve wouldn't the voltage drop off right away? If there is still voltage at the coil that means the gas valve should still be energized. So whats causing the main burners to go out? If the pilot goes out shouldn't the main burners continue to prove the flame or do the main burners go out along with the pilot. My goodman furnace has a HSI and the flame prover is proven by the main burners.This seems to me to be a better more practical design. I guess I just need to read up on these type of furnaces

  5. #5
    Join Date
    May 2000
    Location
    Indianapolis, IN, USA
    Posts
    34,617
    The "flame prover" is a bi-metal switch in the pilot. After the pilot goes out, the switch has to cool before it reports pilot out. Nothing is sensing flame on the main burners.

  6. #6
    Join Date
    Jul 2009
    Location
    Columbia, MD
    Posts
    4,488
    Voltage may be present. But if there is no amps the coil won't energize. The switch may open just enough that you see voltage. Use a oscilloscope type meter and you will prob see the voltage drop. We all know what the problem is. You will soon figure it out

  7. #7
    Join Date
    Feb 2008
    Location
    Altmar, New York, United States
    Posts
    5,182
    when was it last serviced?

  8. #8
    Join Date
    Nov 2012
    Location
    Springfield IL
    Posts
    4
    I removed the pilot assembly cleaned the flame prover and the pilot orifice. Simple as that!

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