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Flame Rectification Help!

Nhaggard07 Member Posts: 4
edited March 23 in Gas Heating
My first post here on Heating Help. I've read through a few of Dan's books and I'm thirsty for knowledge. I'm new to service and have one year in the industry. I have a furnace question that I hope is appropriate to post here on The Wall. 

I'm dealing with a system that has intermittent fault with flame sensor. Customer states every few months (since install 3 years ago!) furnace throws code and needs reset. The company who installed has replaced the board, the flame sensor, and the ground wire. Here's what I've documented so far- (system requires 3.7 ųa dc signal) I'm reading 1.5. Manufacturers literature states this reading is right at the threshold for a fault. This reading taken when unit running. My ac voltage to sensor is reading at 87v, manufactures requirement is between 90-110v. My gas pressure is at 3.55wc. 

Here's what I'm wondering- gas pressure is good, but is it possible under full load it's dropping at the furnace and causing my flame to be low, thus throwing code? But I'm not leaning this way since I'm only getting 1.5 DC when the gas pressure is correct.

What I'm leaning towards is that low ac voltage. It's only slightly below spec though. Is 3v below spec enough to drop my DC rectification that low? I don't know how to calculate that and verify. It's all I can come up with at the moment.

Any and all help appreciated! 



    EBEBRATT-Ed Member Posts: 15,517
    Somehow you have to get the signal up. And some manufacturers when they tell you the minimum it really needs to be higher for reliable operation.

    Make sure to check the grounding from the sensor to the board...clean and tight. Clean the sensor. Try adjusting. The gas pressure slightly to see of this helps.

    Also check the line voltage to the boiler or furnace. If that is below 120 it could make the signal low.

    Locking out months apart is difficult to find but you know you have a marginal signal so work on that.
  • Tim McElwain
    Tim McElwain Member Posts: 4,621
    What is your location in the country? The reason I ask is Imay be nearby.
    Mad Dog_2Nhaggard07
  • HomerJSmith
    HomerJSmith Member Posts: 2,441
    Make, model, code etc., if you want accurate help.
  • Bob Harper
    Bob Harper Member Posts: 1,034
    What is the minimum rated inlet gas pressure, static inlet and inlet under full house load? Is the line voltage about 111vac? 111-24= 87vac. I doubt the 87 is a problem. The phasing is correct. If the marginal rect. signal is the problem, yes, look to distortion of the flame. Low inlet pressure, improper venting, misaligned burners, etc. Verify the ground path is intact with no corrosion on contact points. Must have ground mass 4x the flame rod. Check the rod position in the flame. It should cut across the outer mantel with as much surface contact as possible- not into the center of the flame or near the top. Check the terminals for solid connection and no corrosion. Use DeOxyit as needed. If need be, put a data logger on it to correlate what times or conditions it drops out.
  • 109A_5
    109A_5 Member Posts: 1,385
    edited March 24
    Hello @Nhaggard07,
    Maybe the information found here may help.

    Try the 87 Volt reading with a different meter. If still low find out why. I don't think the 24 VAC system Voltage has any thing to do with it in this case. The internal Voltage drop (control board) to get the expected 90 - 110 Volts (flame probe to ground) is probably the nominal drop of the control board circuitry that senses the Micro Amps value of the pulsating DC when the internal resistance of the multi-meter loads it down. Different model meters have may different internal impedance's in AC Voltage mode.

    National - U.S. Gas Boiler 45+ Years Old
    Steam 300 SQ. FT. - EDR 347
    One Pipe System
  • Mad Dog_2
    Mad Dog_2 Member Posts: 6,918
    edited March 24
    NHaggard 07......You need Timmie Mc Elwain's help here. I know of no MORE knowledge man in this specialty.  Even if he walks you through it over the phone (which I done several times) you can mail him a check.  MAD DOG 🐕 
  • Nhaggard07
    Nhaggard07 Member Posts: 4
    I work in South Bend, IN area.

    I had a convo with customer and explained there are a few things I can try, but I can't offer him a guarantee. He doesn't want to throw money on a gamble, so he's going to keep pushing the installing company to figure it out. I don't blame him. But, he may end up calling me back and I'm going to build a game plan just in case. 

    I don't have access to model number at the moment. After I close out job, I can't see history. It was a York 40k, 80% furnace. The ground wire was replaced and looked good. New, tight bond, etc. It was bonded to a large metal plate right above the burners. I don't believe mass or connection was an issue. The flame rod placement was just off to the side of center of flame. Mainly in the flame outer mantle as Bob mentioned above. Flame sensor and ground wire were both placed in their factory default positions. 

    A data logger sounds like a great idea. I'm going to look into one of those. I've never seen or used one yet. Sounds like a God send for intermittent issues.
  • HomerJSmith
    HomerJSmith Member Posts: 2,441
    edited March 24
    Perhaps, grounding the burner tubes directly would improve the rectification values. Specifically, the tube that houses the pilot assembly.