Welcome! Here are the website rules, as well as some tips for using this forum.
Need to contact us? Visit https://heatinghelp.com/contact-us/.
Click here to Find a Contractor in your area.

Negative DC Voltage?

Rizz861 Member Posts: 52
When I’m measuring flame current with an intermittent ignition system with DC micro amps or on a standing pilot with DC millivolts I’ve noticed I’ve gotten negative voltage before. Why is this? My leads are in the right spot. Does the polarity of how you hook them up matter? For example, a thermocouple I use an alligator clip on red lead and put it at the very end (part that goes in gas valve) and the other gator clip on the wire. For intermittent ignition I go red going to igniter and black gator clip to ignition control module. Am I doing something wrong to get these negative volt readings?


  • retiredguy
    retiredguy Member Posts: 915
    You did not say what the type of control, type or model of the burner, or any other pertinent information. On larger burners using a flame rod as the flame sensor, anytime the DC micro amp reading would go from a positive reading to a negative reading and the burner still fired, the flame safeguard control was compromised and needed to be replaced. This is what I was told by an employee of Honeywell who designed a lot of their controls.
  • ethicalpaul
    ethicalpaul Member Posts: 5,735
    If you swap the leads of your meter you should see the polarity reverse, yes
    NJ Steam Homeowner. See my sight glass boiler videos: https://bit.ly/3sZW1el
  • 109A_5
    109A_5 Member Posts: 1,387
    Hello @Rizz861,
    Rizz861 said:

    Does the polarity of how you hook them up matter?

    It is DC, so absolutely the polarity does matter, if you want to see a positive value.

    As @ethicalpaul stated just reverse the leads.

    However in these cases you are probably more interested in the measures value(s) than the polarity.

    In the case of the thermocouple the solenoid coil in the gas valve may not care (but I bet they are built consistently the same anyway).
    With Flame rectification, I don't think you are going to find the laws of physics will change. A DC current of enough proper magnitude will be present when the flame is present and correct. If no flame or poor flame, no current or poor (low) current.

    Gas furnace will not light. Thermocouple test with the multimeter

    National - U.S. Gas Boiler 45+ Years Old
    Steam 300 SQ. FT. - EDR 347
    One Pipe System
  • Rizz861
    Rizz861 Member Posts: 52
    @retiredguy I remember this happened on a Honeywell control, an 8610 to be exact. We ended up replacing it, but from what I remember is was -1DC micro amps. So what you are saying according to Honeywell the safeguard of the ignition control module is compromised with a negative reading? That’s good to know. How about a standing pilot ignition system and a thermocouple? When measuring DC millivolts I’ve seen Negative readings here too. 
  • Rizz861
    Rizz861 Member Posts: 52
    Hey thanks for all the info everyone. I’m doing Tim’s class in RI next week so I’ll pick his brain on this topic too.